Make view lookups last-wins and chained access safe

* Lookups (operator[], at, find, contains, count, value, JSON pointers)
  return the last member of a duplicate key, as materialize() and
  parse() keep it.
* operator[] on a discarded view returns a discarded view instead of
  throwing, so v["a"]["b"] is safe for a missing "a".
* operator[] and at() take any integer type (not only int and size_t),
  fixing ambiguous calls with unsigned, long, std::int64_t, ...
* Fix the operator[] documentation, which claimed a discarded view for
  a type mismatch where type_error.305 is thrown.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/at.md

@@ -8,15 +8,18 @@ basic_json_view at(const string_t& key) const;
 
 // (2)
 basic_json_view at(size_type idx) const;
-basic_json_view at(int idx) const;
+template<typename IntegerType>
+basic_json_view at(IntegerType idx) const;
 
 // (3)
 basic_json_view at(const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
    [Notes on duplicate keys](operator[].md#notes)).
-2. Returns the array element at index `idx`.
+2. Returns the array element at index `idx`. The template accepts every integer type except `#!cpp bool` and
+   `#!cpp std::size_t` and forwards to the `size_type` overload, as for [`operator[]`](operator[].md); a negative
+   `idx` is out of range.
 3. Returns the value a JSON pointer `ptr` refers to, starting at this value.
 
 ## Parameters
@@ -32,7 +35,7 @@ basic_json_view at(const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the first member with key `key`
+1. the value of the last member with key `key`
 2. the element at index `idx`
 3. the value `ptr` resolves to, starting at this value
 
@@ -72,8 +75,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, stopping at the first match. Each comparison first checks the key's length --
-   already known from the index, without reading the key bytes -- before comparing its content.
+   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at

docs/mkdocs/docs/api/basic_json_view/begin.md

@@ -24,7 +24,7 @@ Constant.
 
 For an object, iteration visits **every** member, including all occurrences of a duplicate key -- unlike
 [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md), [`contains`](contains.md), and [`count`](count.md),
-which all resolve to the *first* member with a given key. See the
+which all resolve to the *last* member with a given key. See the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]`.
 
 Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what

docs/mkdocs/docs/api/basic_json_view/contains.md

@@ -33,8 +33,8 @@ No-throw guarantee: this function never throws exceptions.
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
-   known from the index, without reading the key bytes -- before comparing its content.
+   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) with a JSON pointer.

docs/mkdocs/docs/api/basic_json_view/count.md

@@ -23,9 +23,9 @@ No-throw guarantee: this function never throws exceptions.
 
 ## Complexity
 
-Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
-known from the index, without reading the key bytes -- before comparing its content.
+Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
+in document order, scanning all of them, since the last match is wanted. Each comparison first checks the key's length
+-- already known from the index, without reading the key bytes -- before comparing its content.
 
 ## Notes
 
@@ -36,7 +36,7 @@ Unlike [`BasicJsonType::count()`](../basic_json/count.md), whose return value ca
 an `ObjectType` that allows multiple entries per key, `count()` here never does: it is exactly
 [`contains()`](contains.md) as `#!cpp 0`/`#!cpp 1`. This holds even if the source text has a duplicate key -- see the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]` -- because a `#!cpp count() > 1` result would require
-counting every member with a matching key, not just finding the first one.
+counting every member with a matching key (the lookup functions resolve to the *last* one).
 
 ## Examples
 

docs/mkdocs/docs/api/basic_json_view/find.md

@@ -6,7 +6,7 @@ iterator find(const char* key) const;
 iterator find(const string_t& key) const;
 ```
 
-Finds a member with key `key` -- the first one, should the key occur more than once (see
+Finds a member with key `key` -- the last one, should the key occur more than once (see
 [Notes on duplicate keys](operator[].md#notes)). If the value is not an object, or no member has this key,
 [`end()`](end.md) is returned.
 
@@ -25,9 +25,9 @@ No-throw guarantee: this function never throws exceptions.
 
 ## Complexity
 
-Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
-known from the index, without reading the key bytes -- before comparing its content.
+Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
+in document order, scanning all of them, since the last match is wanted. Each comparison first checks the key's length
+-- already known from the index, without reading the key bytes -- before comparing its content.
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json_view/get.md

@@ -87,9 +87,9 @@ exception thrown while converting through `materialize()` (the last bullet) is d
 !!! info "Duplicate keys"
 
     `#!cpp std::map`/`#!cpp std::unordered_map` conversions keep the *last* value of a repeated key, like
-    [`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the opposite
-    of [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md), which resolve to the
-    *first* occurrence (see the [Notes on duplicate keys](operator[].md#notes)).
+    [`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the member
+    [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md) resolve to, too (see the
+    [Notes on duplicate keys](operator[].md#notes)).
 
 !!! info "No pointers, references, or implicit conversion"
 

docs/mkdocs/docs/api/basic_json_view/is_discarded.md

@@ -9,6 +9,10 @@ view (see [(constructor)](basic_json_view.md)), and for [`root()`](../basic_json
 that is itself [discarded](../basic_json_document/is_discarded.md) -- in particular, the root of a failed
 [`parse()`](../basic_json_document/parse.md) with `allow_exceptions` set to `#!cpp false`.
 
+A discarded view is also what [`operator[]`](operator[].md) returns for a missing key, an index out of range, or a
+JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as
+`#!cpp v["a"]["b"]` is safe). [`at`](at.md) throws instead.
+
 ## Return value
 
 `#!cpp true` if the view is discarded, `#!cpp false` otherwise.
@@ -45,6 +49,7 @@ site.
 
 ## See also
 
+- [operator[]](operator[].md) - access specified element; yields a discarded view where an element is missing
 - [operator bool](operator_bool.md) - return whether the view refers to a value
 - [(constructor)](basic_json_view.md) - the default constructor creates a discarded view
 - [is_discarded (basic_json_document)](../basic_json_document/is_discarded.md) - return whether the last parse failed

docs/mkdocs/docs/api/basic_json_view/items.md

@@ -48,7 +48,7 @@ Constant.
 
 As for [`begin()`](begin.md)/[`end()`](end.md), `items()` visits **every** member of an object, including all
 occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md),
-[`contains`](contains.md), and [`count`](count.md), which resolve to the *first* member with a given key. See the
+[`contains`](contains.md), and [`count`](count.md), which resolve to the *last* member with a given key. See the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]`.
 
 !!! danger "Lifetime issues"
@@ -63,8 +63,8 @@ occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](a
 
     The example below shows a settings object whose source text records every update to a key as a duplicate
     member, in the order they happened. `items()` walks all of them, so the update history is visible, while
-    [`operator[]`](operator[].md) only ever sees the *first* one and [`materialize()`](materialize.md) -- like
-    [`BasicJsonType::parse()`](../basic_json/parse.md) -- keeps only the *last*.
+    [`operator[]`](operator[].md) sees the *last* one, and so does [`materialize()`](materialize.md) -- like
+    [`BasicJsonType::parse()`](../basic_json/parse.md).
 
     ```cpp
     --8<-- "examples/basic_json_view__items.cpp"

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -8,16 +8,19 @@ basic_json_view operator[](const string_t& key) const;
 
 // (2)
 basic_json_view operator[](size_type idx) const;
-basic_json_view operator[](int idx) const;
+template<typename IntegerType>
+basic_json_view operator[](IntegerType idx) const;
 
 // (3)
 basic_json_view operator[](const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
    the [Notes](#notes) below) -- or a [discarded](is_discarded.md) view if there is no such member.
-2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. (The
-   `#!cpp int` overload only exists so that an integer literal is not ambiguous between this overload and 1.)
+2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. The
+   template accepts every integer type except `#!cpp bool` and `#!cpp std::size_t` (`#!cpp int`, `#!cpp unsigned`,
+   `#!cpp long`, `#!cpp std::int64_t`, ...) and forwards to the `size_type` overload, so that an integer argument is
+   not ambiguous between that overload and 1; a negative `idx` is out of range.
 3. Returns the value a JSON pointer `ptr` refers to, starting at this value, or a [discarded](is_discarded.md) view
    wherever resolving it further is not possible without inserting into or extending the document (see
    [Return value](#return-value) and [Exceptions](#exceptions) below).
@@ -35,10 +38,12 @@ basic_json_view operator[](const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the first member with key `key`, or a discarded view if `#!cpp is_object()` is `#!cpp false` or no
-   member has this key
-2. the element at index `idx`, or a discarded view if `#!cpp is_array()` is `#!cpp false` or `#!cpp idx >= size()`
-3. the value `ptr` resolves to, starting at this value, or a discarded view for exactly the reference tokens where the
+1. the value of the last member with key `key`, or a discarded view if no member has this key (or if this view is
+   [discarded](is_discarded.md))
+2. the element at index `idx`, or a discarded view if `#!cpp idx >= size()` or `idx` is negative (or if this view is
+   [discarded](is_discarded.md))
+3. the value `ptr` resolves to, starting at this value, or a discarded view (also if this view is
+   [discarded](is_discarded.md)) for exactly the reference tokens where the
    **const** overload of [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) invokes undefined behavior for
    the same pointer and the same document: an object member that does not exist, or an array index that is out of
    range
@@ -49,10 +54,12 @@ Strong exception safety: if an exception is thrown, there are no changes to the
 
 ## Exceptions
 
-1. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an object --
+1. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an object and
+   not [discarded](is_discarded.md) --
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a string argument on a non-object value.
-2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array --
+2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array and
+   not [discarded](is_discarded.md) --
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a numeric argument on a non-array value.
 3. Throws the same exceptions, with the same messages, that the **const** overload of
@@ -73,9 +80,9 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, stopping at the first match. Each comparison first checks the key's length --
-   already known from the index, without reading the key bytes -- before comparing its content, so a key of a
-   different length than `key` is rejected without touching the source text.
+   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
+   key of a different length than `key` is rejected without touching the source text.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
@@ -84,20 +91,29 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Notes
 
 Unlike `BasicJsonType::operator[]`, which is undefined behavior (guarded by a
-[runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator always
-returns a safe, testable result: a [discarded](is_discarded.md) view, which is `#!cpp false` in a boolean context.
+[runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator returns a
+safe, testable result for a missing key or an index out of range: a [discarded](is_discarded.md) view, which is
+`#!cpp false` in a boolean context.
 There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the
 document.
 
+!!! info "Chained access"
+
+    `#!cpp operator[]` on a [discarded](is_discarded.md) view returns a discarded view and does not throw, so a chain
+    like `#!cpp v["a"]["b"][0]` is safe even if `"a"` or `"b"` is missing: the first missing step makes the whole
+    result discarded, which is tested once at the end. Type errors on values that are *not* discarded still throw: a
+    key on an array or a primitive, or an index on an object or a primitive, is `type_error.305` as for
+    `BasicJsonType`. [`at`](at.md) still throws for a discarded view, as it does for a missing key.
+
 !!! info "Duplicate keys"
 
     If the source text has an object with a duplicate key, `#!cpp operator[]` (and [`at`](at.md), [`find`](find.md),
-    [`contains`](contains.md), [`count`](count.md)) all resolve to the *first* member with that key, because a
-    lookup can stop as soon as it finds a match. This is different from
-    [`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which replay every
-    member in order and so end up keeping the *last* value for a repeated key -- there is no reason for them to stop
-    early. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. See the example below and [`size()`](size.md#notes).
+    [`contains`](contains.md), [`count`](count.md), [`value`](value.md), and JSON pointer resolution) all resolve to
+    the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
+    [`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
+    agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
+    duplicates, in document order. A lookup scans all members for this: it cannot stop at the first match. See the
+    example below and [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"
 

docs/mkdocs/docs/api/basic_json_view/value.md

@@ -12,7 +12,7 @@ T value(const json_pointer& ptr, const T& default_value) const;
 string_t value(const json_pointer& ptr, const char* default_value) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
    [Notes on duplicate keys](operator[].md#notes)) -- converted to `T`, or `default_value` if there is no such member.
 2. Returns the value a JSON pointer `ptr` refers to, starting at this value, converted to `T`, or `default_value` if
    `ptr` cannot be resolved.
@@ -39,7 +39,7 @@ equivalent) deduce `string_t`, not `const char*`, for their return type and for
 
 ## Return value
 
-1. the first member with key `key`, converted to `T`, or `default_value`
+1. the last member with key `key`, converted to `T`, or `default_value`
 2. the value `ptr` resolves to, converted to `T`, or `default_value`
 
 ## Exception safety
@@ -68,8 +68,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`operator[]`](operator[].md#complexity), members are compared one after
-   another, in document order, stopping at the first match. Plus the complexity of converting the found member to
-   `T` (see [`get`](get.md)).
+   another, in document order, scanning all of them, since the last match is wanted. Plus the complexity of converting
+   the found member to `T` (see [`get`](get.md)).
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value

docs/mkdocs/docs/examples/basic_json_view__items.cpp

@@ -7,8 +7,8 @@ int main()
 {
     // a settings object whose source text records every update to a key as
     // a duplicate member. items() visits all of them, in document order, so
-    // the update history is visible; operator[] only ever sees the first
-    // one, and materialize() -- like basic_json::parse() -- keeps the last
+    // the update history is visible; operator[] and materialize() -- like
+    // basic_json::parse() -- see the last one
     json_document updates = json_document::parse(R"({"retries": 1, "timeout": 30, "retries": 5})");
     const auto settings = updates.root();
 
@@ -17,6 +17,6 @@ int main()
         std::cout << item.key() << '=' << item.value().materialize().dump() << '\n';
     }
 
-    std::cout << "first \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
+    std::cout << "last \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
     std::cout << "last \"retries\" kept by materialize(): " << settings.materialize()["retries"].dump() << '\n';
 }

docs/mkdocs/docs/examples/basic_json_view__items.output

@@ -1,5 +1,5 @@
 retries=1
 timeout=30
 retries=5
-first "retries" seen by operator[]: 1
+last "retries" seen by operator[]: 5
 last "retries" kept by materialize(): 5

docs/mkdocs/docs/features/json_view.md

@@ -126,14 +126,20 @@ whenever any of the other conditions above was not met.
   members in the order they appear in the source text. `basic_json`'s default `object_t` is a `std::map`, which
   sorts by key, so iterating a [`materialize()`](../api/basic_json_view/materialize.md)d value can print members in
   a different order than iterating the view they came from.
+- **Chained access is safe.** [`operator[]`](../api/basic_json_view/operator%5B%5D.md) with a missing key, an index
+  out of range, or an unresolvable JSON pointer returns a [discarded](../api/basic_json_view/is_discarded.md) view, and
+  `operator[]` on a discarded view returns a discarded view without throwing: `#!cpp v["a"]["b"][0]` can be tested
+  once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and
+  [`at`](../api/basic_json_view/at.md) throws for every missing value.
 - **Duplicate keys are visible.** If an object in the source text repeats a key,
   [`begin()`](../api/basic_json_view/begin.md)/[`end()`](../api/basic_json_view/end.md) and
   [`items()`](../api/basic_json_view/items.md) visit *every* occurrence (and [`size()`](../api/basic_json_view/size.md)
   counts all of them), while [`operator[]`](../api/basic_json_view/operator%5B%5D.md),
   [`at`](../api/basic_json_view/at.md), [`find`](../api/basic_json_view/find.md),
   [`contains`](../api/basic_json_view/contains.md), and [`count`](../api/basic_json_view/count.md) resolve to the
-  *first* occurrence, since a lookup can stop as soon as it finds a match. `basic_json::parse()` (and so
-  [`materialize()`](../api/basic_json_view/materialize.md)) instead keeps only the *last* value for a repeated key.
+  *last* occurrence -- the one `basic_json::parse()` (and so
+  [`materialize()`](../api/basic_json_view/materialize.md)) keeps for a repeated key -- which makes a lookup scan all
+  members instead of stopping at a match.
   See the [Notes on duplicate keys](../api/basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
 - **No [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) path.** Exceptions thrown by `basic_json_view`'s own
   element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the

include/nlohmann/detail/view/lookup.hpp

@@ -11,6 +11,8 @@
 #include <cstddef> // size_t
 #include <cstdint> // uint16_t, uint32_t, uint64_t
 #include <cstring> // memcmp, memcpy
+#include <limits> // numeric_limits
+#include <type_traits> // integral_constant, is_integral, is_same
 
 #include <nlohmann/json.hpp>
 #include <nlohmann/detail/view/document_data.hpp>
@@ -81,12 +83,14 @@ class short_key
     std::uint64_t m_b = 0;
 };
 
-/// the key node of the first member of an object with the given key, or
-/// nullptr; most keys are rejected by their length, from the index alone
+/// the key node of the last member of an object with the given key, or
+/// nullptr (the last one, as materialize() and parse() keep it); most keys are
+/// rejected by their length, from the index alone
 inline const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
     const node* const end = document_data::child_end(object);
     const auto* const k = reinterpret_cast<const unsigned char*>(key); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
+    const node* last = nullptr;
     if (NLOHMANN_VIEW_LIKELY(n <= 16))
     {
         const short_key probe(k, n);
@@ -94,19 +98,37 @@ inline const node* find_member(const document_data& d, const node* object, const
         {
             if (m->len == n && probe.matches(reinterpret_cast<const unsigned char*>(d.str(*m)))) // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
             {
-                return m;
+                last = m;
             }
         }
-        return nullptr;
+        return last;
     }
     for (const node* m = document_data::first_child(object); m != end; m = document_data::after(m + 1))
     {
         if (m->len == n && std::memcmp(d.str(*m), key, n) == 0)
         {
-            return m;
+            last = m;
         }
     }
-    return nullptr;
+    return last;
+}
+
+/// whether an integer type is accepted as an array index by the view's
+/// operator[] and at(): every integer type but bool and size_t, which has its
+/// own overload
+template<typename T>
+struct is_index_type : std::integral_constant < bool,
+    std::is_integral<T>::value && !std::is_same<T, bool>::value && !std::is_same<T, std::size_t>::value >
+{};
+
+/// an integer as an index: negative values, and values that do not fit a
+/// size_t, map to the largest size_t (out of range for every array)
+template<typename SizeType, typename IntegerType>
+SizeType to_index(IntegerType idx) noexcept
+{
+    const IntegerType zero = 0;
+    const auto result = static_cast<SizeType>(idx);
+    return (idx < zero || static_cast<IntegerType>(result) != idx) ? (std::numeric_limits<SizeType>::max)() : result;
 }
 
 /// the element of an array at an index below its size

include/nlohmann/json_view.hpp

@@ -234,13 +234,18 @@ class basic_json_view
     // element access //
     ////////////////////
 
-    /// the value of the member with this key (the first one, should the key
-    /// occur more than once); a discarded view if there is none. Throws
-    /// type_error.305 if this is not an object.
+    /// the value of the member with this key (the last one, should the key
+    /// occur more than once); a discarded view if there is none, or if this
+    /// is a discarded view (so that v["a"]["b"] is safe). Throws type_error.305
+    /// if this is any other value but an object.
     NLOHMANN_VIEW_ALWAYS_INLINE basic_json_view operator[](string_view_t key) const
     {
         if (NLOHMANN_VIEW_UNLIKELY(!is_object()))
         {
+            if (is_discarded())
+            {
+                return basic_json_view();
+            }
             detail::view::throw_type_error(305, "cannot use operator[] with a string argument with ", type_name());
         }
         return lookup(key);
@@ -257,31 +262,43 @@ class basic_json_view
     }
 
     /// the element at this index; a discarded view if the index is out of
-    /// range. Throws type_error.305 if this is not an array.
+    /// range, or if this is a discarded view. Throws type_error.305 if this is
+    /// any other value but an array.
     basic_json_view operator[](size_type idx) const
     {
         if (NLOHMANN_VIEW_UNLIKELY(!is_array()))
         {
+            if (is_discarded())
+            {
+                return basic_json_view();
+            }
             detail::view::throw_type_error(305, "cannot use operator[] with a numeric argument with ", type_name());
         }
         return idx < m_node->len ? basic_json_view(m_doc, detail::view::element_at(m_node, idx)) : basic_json_view();
     }
 
-    /// (an int argument would be ambiguous between size_type and const char*)
-    basic_json_view operator[](int idx) const
+    /// any other integer type (int, unsigned, long, std::int64_t, ...; a
+    /// single overload for size_type alone would be ambiguous for all of them
+    /// and for const char*); negative values are out of range
+    template < typename IntegerType, typename std::enable_if < detail::view::is_index_type<IntegerType>::value, int >::type = 0 >
+    basic_json_view operator[](IntegerType idx) const
     {
-        return operator[](static_cast<size_type>(idx));
+        return operator[](detail::view::to_index<size_type>(idx));
     }
 
     /// the value a JSON pointer refers to; a discarded view if a key is
-    /// missing or an index is out of range. Other errors throw what const
-    /// basic_json::operator[] throws.
+    /// missing or an index is out of range, or if this is a discarded view.
+    /// Other errors throw what const basic_json::operator[] throws.
     basic_json_view operator[](const json_pointer& ptr) const
     {
+        if (NLOHMANN_VIEW_UNLIKELY(is_discarded()))
+        {
+            return basic_json_view();
+        }
         return detail::view::resolve_pointer(*this, detail::json_pointer_access::reference_tokens(ptr), detail::view::pointer_mode::unchecked);
     }
 
-    /// the value of the member with this key (the first one, should the key
+    /// the value of the member with this key (the last one, should the key
     /// occur more than once). Throws type_error.304 if this is not an object,
     /// and out_of_range.403 if there is no such member.
     basic_json_view at(string_view_t key) const
@@ -323,9 +340,12 @@ class basic_json_view
         return basic_json_view(m_doc, detail::view::element_at(m_node, idx));
     }
 
-    basic_json_view at(int idx) const
+    /// any other integer type, see operator[]; negative values are out of
+    /// range
+    template < typename IntegerType, typename std::enable_if < detail::view::is_index_type<IntegerType>::value, int >::type = 0 >
+    basic_json_view at(IntegerType idx) const
     {
-        return at(static_cast<size_type>(idx));
+        return at(detail::view::to_index<size_type>(idx));
     }
 
     /// the value a JSON pointer refers to; throws what basic_json::at()
@@ -336,7 +356,7 @@ class basic_json_view
     }
 
     /// the member with this key converted to T, or the default value if there
-    /// is no such member (the first one, should the key occur more than
+    /// is no such member (the last one, should the key occur more than
     /// once). Throws type_error.306 if this is not an object.
     template < typename T, typename std::enable_if < !std::is_same<typename std::decay<T>::type, const char*>::value, int >::type = 0 >
     T value(string_view_t key, const T& default_value) const
@@ -401,7 +421,7 @@ class basic_json_view
     // lookup //
     ////////////
 
-    /// an iterator to the member with this key (the first one, should the
+    /// an iterator to the member with this key (the last one, should the
     /// key occur more than once), or end(); end() also for non-objects
     iterator find(string_view_t key) const
     {
@@ -579,7 +599,7 @@ class basic_json_view
         : m_doc(d), m_node(n)
     {}
 
-    /// the value of the first member with this key, or a discarded view
+    /// the value of the last member with this key, or a discarded view
     /// (object required)
     NLOHMANN_VIEW_ALWAYS_INLINE basic_json_view lookup(string_view_t key) const noexcept
     {

tests/src/unit-json_view.cpp

@@ -23,6 +23,7 @@ using nlohmann::ordered_json_view;
 #include <cstdio>
 #include <cstring>
 #include <iterator>
+#include <limits>
 #include <list>
 #include <map>
 #include <random>
@@ -442,7 +443,7 @@ std::string exception_of(F f)
 
 // compares a view with the ordered_json value materialize() gives for it:
 // types, sizes, elements and members (by index, key, and iteration), in
-// document order; duplicate keys are found as their first occurrence
+// document order; duplicate keys are found as their last occurrence
 void check_access(const ordered_json_view& v, const ordered_json& j)
 {
     REQUIRE(v.type() == j.type());
@@ -483,11 +484,23 @@ void check_access(const ordered_json_view& v, const ordered_json& j)
             const std::string key(it.key().data(), it.key().size());
             CHECK(v.contains(key));
             CHECK(v.count(key) == 1);
-            if (std::find(keys.begin(), keys.end(), key) != keys.end())
+            if (std::find(keys.begin(), keys.end(), key) == keys.end())
             {
-                continue; // a duplicate: lookups find the first one
+                keys.push_back(key);
+            }
+            // lookups find the last member with the key, which is this one if
+            // there is no later one
+            auto next = it;
+            ++next;
+            bool is_last = true;
+            for (; next != v.end(); ++next)
+            {
+                is_last = is_last && next.key() != it.key();
+            }
+            if (!is_last)
+            {
+                continue;
             }
-            keys.push_back(key);
             CHECK(v.find(key) == it);
             CHECK(v[key].materialize() == it->materialize());
             CHECK(v.at(key).materialize() == it.value().materialize());
@@ -578,15 +591,35 @@ TEST_CASE("json_view element access and iteration")
 #endif
     }
 
-    SECTION("duplicate keys: lookups find the first member, iteration all")
+    SECTION("duplicate keys: lookups find the last member, iteration all")
     {
         const json_document d = json_document::parse(R"({"a":1,"b":2,"a":3})");
         const json_view v = d.root();
         CHECK(v.size() == 3);
-        CHECK(v["a"].materialize() == 1);
-        CHECK(v.at("a").materialize() == 1);
-        CHECK(v.find("a") == v.begin());
+        CHECK(v["a"].materialize() == 3);
+        CHECK(v.at("a").materialize() == 3);
+        CHECK(v.find("a") == std::next(v.begin(), 2));
+        CHECK(v.find("a").value().materialize() == 3);
+        CHECK(v.find("b") == std::next(v.begin()));
         CHECK(v.count("a") == 1);
+        CHECK(v.contains("a"));
+        CHECK(v.value("a", 0) == 3);
+        CHECK(v["a"].materialize() == v.materialize()["a"]); // as materialize()
+        // keys of every length class (the 16-byte short compare and memcmp)
+        for (const std::size_t n :
+                {
+                    0u, 1u, 3u, 7u, 8u, 15u, 16u, 17u, 40u
+                })
+        {
+            const std::string key(n, 'k');
+            const json_document dk = json_document::parse("{\"" + key + "\":1,\"" + key + "x\":2,\"" + key + "\":3,\"" + key + "\":4}");
+            CAPTURE(n)
+            CHECK(dk.root()[key].materialize() == 4);
+            CHECK(dk.root().at(key).materialize() == 4);
+            CHECK(dk.root().find(key) == std::next(dk.root().begin(), 3));
+            CHECK(dk.root().value(key, 0) == 4);
+            CHECK(dk.root()[key + "x"].materialize() == 2);
+        }
         std::string order;
         for (auto it = v.begin(); it != v.end(); ++it)
         {
@@ -644,7 +677,124 @@ TEST_CASE("json_view element access and iteration")
         const json_view invalid{};
         CHECK(invalid.begin() == invalid.end());
         CHECK(std::string(invalid.type_name()) == "discarded");
-        CHECK_THROWS_WITH_AS(invalid["a"], "[json.exception.type_error.305] cannot use operator[] with a string argument with discarded", json::type_error&);
+    }
+
+    SECTION("chained access is safe: operator[] of a discarded view is discarded")
+    {
+        const json_document d = json_document::parse(R"({"a":{"b":[10,20]},"s":"str"})");
+        const json_view v = d.root();
+        // missing keys and indexes
+        CHECK(!v["x"]);
+        CHECK(!v["x"]["y"]);
+        CHECK(!v["x"]["y"]["z"]);
+        CHECK(!v["x"][0]);
+        CHECK(!v["x"][0u][1L]);
+        CHECK(!v["a"]["b"][2]);
+        CHECK(!v["a"]["b"][2]["c"]);
+        CHECK(!v["a"]["b"][2][json_view::json_pointer("/c")]);
+        CHECK(!v["x"][json_view::json_pointer("/a/b")]);
+        CHECK(!v["x"][json_view::json_pointer("")]);
+        CHECK(v["x"]["y"].is_discarded());
+        CHECK(!v["x"][std::string("y")]);
+        // a resolvable path still resolves
+        CHECK(v["a"]["b"][1].materialize() == 20);
+        CHECK(v[json_view::json_pointer("/a/b/1")].materialize() == 20);
+        // the discarded view of an unresolved pointer is discarded too
+        CHECK(!v[json_view::json_pointer("/x/y")]["z"]);
+        CHECK(!v[json_view::json_pointer("/a/b/5")][0]);
+#if !defined(JSON_NOEXCEPTION)
+        // type errors on values that are not discarded stay
+        CHECK_THROWS_WITH_AS(v[0], "[json.exception.type_error.305] cannot use operator[] with a numeric argument with object", json::type_error&);
+        CHECK_THROWS_WITH_AS(v["a"]["b"]["c"], "[json.exception.type_error.305] cannot use operator[] with a string argument with array", json::type_error&);
+        CHECK_THROWS_WITH_AS(v["s"]["c"], "[json.exception.type_error.305] cannot use operator[] with a string argument with string", json::type_error&);
+        CHECK_THROWS_WITH_AS(v["s"][0], "[json.exception.type_error.305] cannot use operator[] with a numeric argument with string", json::type_error&);
+        CHECK_THROWS_AS(v["a"]["b"][0]["c"], json::type_error&);
+        CHECK_THROWS_AS(v["s"][json_view::json_pointer("/x")], json::out_of_range&);
+        // at() keeps throwing on a discarded view
+        const json_view invalid{};
+        CHECK(!invalid["a"]);
+        CHECK(!invalid[0]);
+        CHECK(!invalid[json_view::json_pointer("/a")]);
+        CHECK_THROWS_WITH_AS(invalid.at("a"), "[json.exception.type_error.304] cannot use at() with discarded", json::type_error&);
+        CHECK_THROWS_WITH_AS(invalid.at(0), "[json.exception.type_error.304] cannot use at() with discarded", json::type_error&);
+        CHECK_THROWS_AS(v.at("x").at("y"), json::out_of_range&);
+        CHECK_THROWS_AS(v["x"].at("y"), json::type_error&);
+        CHECK_THROWS_AS(v.at(json_view::json_pointer("/x/y")), json::out_of_range&);
+        CHECK_THROWS_AS(v["x"].at(json_view::json_pointer("/y")), json::out_of_range&);
+#endif
+    }
+
+    SECTION("integer types as array indexes")
+    {
+        const json_document d = json_document::parse("[10,20,30]");
+        const json_view v = d.root();
+        const json j = v.materialize();
+        // (compile-time: no overload is ambiguous)
+        CHECK(v[0].materialize() == 10);
+        CHECK(v[1].materialize() == 20);
+        CHECK(v[0u].materialize() == 10);
+        CHECK(v[1u].materialize() == 20);
+        CHECK(v[1L].materialize() == 20);
+        CHECK(v[2UL].materialize() == 30);
+        CHECK(v[1LL].materialize() == 20);
+        CHECK(v[2ULL].materialize() == 30);
+        CHECK(v[static_cast<short>(1)].materialize() == 20);
+        CHECK(v[static_cast<unsigned short>(2)].materialize() == 30);
+        CHECK(v[static_cast<signed char>(1)].materialize() == 20);
+        CHECK(v[static_cast<unsigned char>(2)].materialize() == 30);
+        CHECK(v[std::int8_t(1)].materialize() == 20);
+        CHECK(v[std::int16_t(2)].materialize() == 30);
+        CHECK(v[std::int32_t(1)].materialize() == 20);
+        CHECK(v[std::int64_t(2)].materialize() == 30);
+        CHECK(v[std::uint32_t(0)].materialize() == 10);
+        CHECK(v[std::uint64_t(1)].materialize() == 20);
+        CHECK(v[std::size_t(2)].materialize() == 30);
+        CHECK(v[std::ptrdiff_t(1)].materialize() == 20);
+        CHECK(j[0u] == 10); // as basic_json
+
+        CHECK(v.at(0).materialize() == 10);
+        CHECK(v.at(1u).materialize() == 20);
+        CHECK(v.at(1L).materialize() == 20);
+        CHECK(v.at(2LL).materialize() == 30);
+        CHECK(v.at(2ULL).materialize() == 30);
+        CHECK(v.at(static_cast<short>(1)).materialize() == 20);
+        CHECK(v.at(static_cast<unsigned short>(2)).materialize() == 30);
+        CHECK(v.at(std::int32_t(0)).materialize() == 10);
+        CHECK(v.at(std::uint32_t(0)).materialize() == 10);
+        CHECK(v.at(std::int64_t(0)).materialize() == 10);
+        CHECK(v.at(std::uint64_t(1)).materialize() == 20);
+        CHECK(v.at(std::size_t(2)).materialize() == 30);
+        CHECK(j.at(std::uint32_t(0)) == 10); // as basic_json
+
+        // out of range, including negative values (no wrap-around)
+        CHECK(!v[3]);
+        CHECK(!v[3u]);
+        CHECK(!v[3L]);
+        CHECK(!v[-1]);
+        CHECK(!v[-1L]);
+        CHECK(!v[-1LL]);
+        CHECK(!v[static_cast<short>(-1)]);
+        CHECK(!v[std::int64_t(-3)]);
+        CHECK(!v[(std::numeric_limits<std::int64_t>::min)()]);
+        CHECK(!v[(std::numeric_limits<std::uint64_t>::max)()]);
+        CHECK(!v[(std::numeric_limits<std::size_t>::max)()]);
+        CHECK(!v[std::numeric_limits<int>::max()]);
+#if !defined(JSON_NOEXCEPTION)
+        CHECK_THROWS_WITH_AS(v.at(3), "[json.exception.out_of_range.401] array index 3 is out of range", json::out_of_range&);
+        CHECK_THROWS_WITH_AS(v.at(3u), "[json.exception.out_of_range.401] array index 3 is out of range", json::out_of_range&);
+        CHECK_THROWS_WITH_AS(v.at(std::int64_t(3)), "[json.exception.out_of_range.401] array index 3 is out of range", json::out_of_range&);
+        CHECK_THROWS_AS(v.at(-1), json::out_of_range&);
+        CHECK_THROWS_AS(v.at(-1L), json::out_of_range&);
+        CHECK_THROWS_AS(v.at(std::int64_t(-1)), json::out_of_range&);
+        CHECK_THROWS_AS(v.at((std::numeric_limits<std::int64_t>::min)()), json::out_of_range&);
+        CHECK_THROWS_AS(v.at((std::numeric_limits<std::uint64_t>::max)()), json::out_of_range&);
+        // not an array
+        const json_document o = json_document::parse("{}");
+        CHECK_THROWS_AS(o.root()[0u], json::type_error&);
+        CHECK_THROWS_AS(o.root()[1L], json::type_error&);
+        CHECK_THROWS_AS(o.root().at(std::uint32_t(0)), json::type_error&);
+        CHECK_THROWS_AS(o.root().at(std::int64_t(0)), json::type_error&);
+#endif
     }
 
     SECTION("iterators")