Fix container input_adapter SFINAE for lvalue-only ADL begin/end (#111)

The container overload of json::parse(c) / accept(c) / sax_parse(c, ...)
silently dropped from overload resolution for user types whose ADL
begin(T&) / end(T&) accepted only non-const lvalue references
(a legitimate pattern matching std::begin semantics). This was because
the detection code used std::declval<ContainerType>() which synthesized
an rvalue, and the rvalue failed to bind to lvalue-only ADL functions.

Fix by making both the outer input_adapter(ContainerType&&) and the
factory's create(ContainerType&&) forwarding references, preserving the
caller's value category and constness via reference collapsing. This
ensures detection (std::declval) and actual use (std::forward) always
match without needing decay/remove_reference.

- Rewrite input_adapters.hpp container overload with forwarding refs
- Add regression tests for lvalue-only non-const ADL begin/end
- Add regression test for rvalue containers (no breakage)
- Update API docs (parse, accept, sax_parse, from_*) to clarify
  that begin/end must match std::begin/std::end semantics
- Add version history notes for 3.13.0
- Regenerate amalgamation

Second-order effect: binary_reader.hpp's internal call to
input_adapter(number_vector) now deduces iterator vs const_iterator
based on the lvalue; functionally harmless (iterator_input_adapter is
iterator-type-agnostic), verified via unit-ubjson/unit-bjdata tests.

Closes remaining limitation from #4354 / PR #5218 (todo 106).

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

docs/mkdocs/docs/api/basic_json/accept.md

@@ -35,7 +35,8 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters (throws if null)
     - a `std::string`
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type, for instance.
@@ -110,6 +111,7 @@ A UTF-8 byte order mark is silently ignored.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
 - Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/from_bjdata.md

@@ -29,7 +29,8 @@ The exact mapping and its limitations are described on a [dedicated page](../../
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type
@@ -101,3 +102,4 @@ Linear in the size of the input.
 ## Version history
 
 - Added in version 3.11.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.

docs/mkdocs/docs/api/basic_json/from_bson.md

@@ -29,7 +29,8 @@ The exact mapping and its limitations are described on a [dedicated page](../../
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type
@@ -101,6 +102,7 @@ Linear in the size of the input.
 ## Version history
 
 - Added in version 3.4.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/from_cbor.md

@@ -32,7 +32,8 @@ The exact mapping and its limitations are described on a [dedicated page](../../
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type
@@ -111,6 +112,7 @@ Linear in the size of the input.
 - Changed to consume input adapters, removed `start_index` parameter, and added `strict` parameter in version 3.0.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
 - Added `tag_handler` parameter in version 3.9.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/from_msgpack.md

@@ -29,7 +29,8 @@ The exact mapping and its limitations are described on a [dedicated page](../../
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type
@@ -103,6 +104,7 @@ Linear in the size of the input.
 - Parameter `start_index` since version 2.1.1.
 - Changed to consume input adapters, removed `start_index` parameter, and added `strict` parameter in version 3.0.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/from_ubjson.md

@@ -29,7 +29,8 @@ The exact mapping and its limitations are described on a [dedicated page](../../
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type
@@ -102,6 +103,7 @@ Linear in the size of the input.
 
 - Added in version 3.1.0.
 - Added `allow_exceptions` parameter in version 3.2.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/parse.md

@@ -34,7 +34,8 @@ static basic_json parse(IteratorType first, IteratorType last,
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters (throws if null)
     - a `std::string`
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type, for instance.
@@ -236,6 +237,7 @@ Invalid Unicode escapes and unpaired surrogates in the input are reported as
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Changed [runtime assertion](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0.
 - Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/sax_parse.md

@@ -39,8 +39,8 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
     - a `FILE` pointer
     - a C-style array of characters
     - a pointer to a null-terminated string of single byte characters
-    - an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of
-      iterators.
+    - a container `obj` for which `begin(obj)` and `end(obj)` produce a valid pair of iterators
+      (as found via ADL or member functions, with semantics compatible to `std::begin` and `std::end`)
 
 `IteratorType`
 :   a compatible iterator type for overload (2); a pair of character iterators whose `value_type` is an integral type
@@ -127,6 +127,7 @@ A UTF-8 byte order mark is silently ignored.
 - Added in version 3.2.0.
 - Ignoring comments via `ignore_comments` added in version 3.9.0.
 - Added `ignore_trailing_commas` in version 3.13.0.
+- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
 
 !!! warning "Deprecation"