Let to_json(std::optional<T>) propagate exceptions from T's to_json

The overload was marked noexcept even though its body assigns *opt to
the JSON value, which calls T's to_json (or allocates for std::string,
std::vector, or json). Any exception from there -- a user-defined
to_json reporting an error, or std::bad_alloc -- called std::terminate()
instead of propagating. The noexcept also made basic_json's converting
constructor noexcept(true) for std::optional<T>, so json j = opt; could
not report the error either.

Fixes #5642.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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

@@ -88,12 +88,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 ## Exceptions
 
 - Throws [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or
-  empty input like a null `FILE*` or `char*` pointer, or an `std::istream` without a stream buffer
-  (`#!cpp i.rdbuf() == nullptr`, for instance `#!cpp std::istream(nullptr)`).
-- If reading from an `std::istream` reaches the end of the input and `eofbit` is part of the stream's
-  [`exceptions()`](https://en.cppreference.com/w/cpp/io/basic_ios/exceptions) mask, the `std::ios_base::failure`
-  thrown by the stream itself propagates instead of a `parse_error`, the same as it would for the standard library's
-  own extraction operators.
+  empty input like a null `FILE*` or `char*` pointer.
 
 ## Complexity
 
@@ -259,8 +254,6 @@ outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-by
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
   it as end of input; planned to become the default in version 4.0.0.
-- Extended empty-input detection to also cover an `std::istream` without a stream buffer, and fixed a crash
-  (`std::terminate`) when parsing from an `std::istream` with `eofbit` in its exception mask, in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/operator_gtgt.md

@@ -20,12 +20,7 @@ the stream `i`
 
 ## Exceptions
 
-- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or if
-  `i` has no stream buffer (`#!cpp i.rdbuf() == nullptr`, for instance `#!cpp std::istream(nullptr)`).
-- If reading from `i` reaches the end of the input and `eofbit` is part of `i`'s
-  [`exceptions()`](https://en.cppreference.com/w/cpp/io/basic_ios/exceptions) mask, the `std::ios_base::failure`
-  thrown by `i` itself propagates instead of a `parse_error`, the same as it would for the standard library's own
-  extraction operators.
+- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
 
 ## Complexity
 
@@ -123,5 +118,3 @@ being read.
   it as end of input; planned to become the default in version 4.0.0.
 - `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave the character that terminates a number in
   the stream; planned to become the default in version 4.0.0.
-- Fixed a null pointer dereference for an `std::istream` without a stream buffer (now throws `parse_error.101`), and a
-  crash (`std::terminate`) when `i` has `eofbit` in its exception mask, in version 3.13.0.

include/nlohmann/detail/conversions/to_json.hpp

@@ -287,7 +287,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
 {
     if (opt.has_value())
     {

include/nlohmann/detail/input/input_adapters.hpp

@@ -106,13 +106,7 @@ class input_stream_adapter
             // was given back with release_lookahead()
             commit_lookahead();
 #endif
-            // only call clear() if there is something to clear: it throws
-            // std::ios_base::failure if the stream has exceptions() enabled
-            // for a state bit that remains set, and a destructor must not throw
-            if ((is->rdstate() & ~std::ios::eofbit) != 0)
-            {
-                is->clear(is->rdstate() & std::ios::eofbit);
-            }
+            is->clear(is->rdstate() & std::ios::eofbit);
         }
     }
 
@@ -817,16 +811,12 @@ inline file_input_adapter input_adapter(std::FILE* file)
 
 inline input_stream_adapter input_adapter(std::istream& stream)
 {
-    if (stream.rdbuf() == nullptr)
-    {
-        JSON_THROW(parse_error::create(101, 0, "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
-    }
     return input_stream_adapter(stream);
 }
 
 inline input_stream_adapter input_adapter(std::istream&& stream)
 {
-    return input_adapter(stream);
+    return input_stream_adapter(stream);
 }
 #endif  // JSON_NO_IO
 

single_include/nlohmann/json.hpp

@@ -6816,7 +6816,7 @@ struct external_constructor<value_t::object>
 #ifdef JSON_HAS_CPP_17
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
-void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
+void to_json(BasicJsonType& j, const std::optional<T>& opt)
 {
     if (opt.has_value())
     {
@@ -7650,13 +7650,7 @@ class input_stream_adapter
             // was given back with release_lookahead()
             commit_lookahead();
 #endif
-            // only call clear() if there is something to clear: it throws
-            // std::ios_base::failure if the stream has exceptions() enabled
-            // for a state bit that remains set, and a destructor must not throw
-            if ((is->rdstate() & ~std::ios::eofbit) != 0)
-            {
-                is->clear(is->rdstate() & std::ios::eofbit);
-            }
+            is->clear(is->rdstate() & std::ios::eofbit);
         }
     }
 
@@ -8361,16 +8355,12 @@ inline file_input_adapter input_adapter(std::FILE* file)
 
 inline input_stream_adapter input_adapter(std::istream& stream)
 {
-    if (stream.rdbuf() == nullptr)
-    {
-        JSON_THROW(parse_error::create(101, 0, "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
-    }
     return input_stream_adapter(stream);
 }
 
 inline input_stream_adapter input_adapter(std::istream&& stream)
 {
-    return input_adapter(stream);
+    return input_stream_adapter(stream);
 }
 #endif  // JSON_NO_IO
 

tests/src/unit-conversions.cpp

@@ -1826,6 +1826,16 @@ TEST_CASE("std::u8string")
 #endif
 #endif
 
+// a type whose to_json reports an error by throwing, used below to check that
+// converting a std::optional<T> to JSON propagates an exception thrown while
+// converting its contained value instead of calling std::terminate (#5642)
+struct throwing_to_json_type {};
+
+void to_json(json&, const throwing_to_json_type&)
+{
+    throw std::runtime_error("cannot serialize throwing_to_json_type");
+}
+
 TEST_CASE("std::optional")
 {
     SECTION("null")
@@ -1908,6 +1918,19 @@ TEST_CASE("std::optional")
         CHECK(json(opt_object) == j_object);
         CHECK(std::map<std::string, std::optional<int>>(j_object) == opt_object);
     }
+
+#if !defined(JSON_NOEXCEPTION)
+    SECTION("exception from contained value's to_json propagates (#5642)")
+    {
+        // to_json(BasicJsonType&, const std::optional<T>&) must not be
+        // noexcept: it calls T's to_json, which may throw (a user-defined
+        // to_json that reports an error, or std::bad_alloc for T =
+        // std::string/vector/json). Before the fix, this called
+        // std::terminate() instead of letting the exception propagate.
+        const std::optional<throwing_to_json_type> opt = throwing_to_json_type{};
+        CHECK_THROWS_WITH_AS(json(opt), "cannot serialize throwing_to_json_type", std::runtime_error&);
+    }
+#endif
 }
 #endif
 

tests/src/unit-deserialization.cpp

@@ -388,37 +388,6 @@ TEST_CASE("deserialization")
             }));
         }
 
-        SECTION("stream with eofbit in its exception mask (issue #5646)")
-        {
-            // reaching EOF while parsing a value that fills the whole input
-            // (e.g., a number, or any value under strict parsing) makes
-            // get_character() call std::istream::clear() to record eofbit;
-            // with eofbit in the exception mask, that clear() itself throws
-            // std::ios_base::failure - it must propagate to the caller instead
-            // of ~input_stream_adapter() throwing a second exception while the
-            // first is still unwinding, which would call std::terminate
-            json _;
-
-            std::istringstream is1("1");
-            is1.exceptions(std::ios::eofbit);
-            CHECK_THROWS_AS(_ = json::parse(is1), std::ios_base::failure&);
-
-            // the same holds for the common std::ifstream::exceptions(failbit |
-            // badbit | eofbit) pattern, because only eofbit ends up set
-            std::istringstream is2("1");
-            is2.exceptions(std::ios::failbit | std::ios::badbit | std::ios::eofbit);
-            CHECK_THROWS_AS(_ = json::parse(is2), std::ios_base::failure&);
-        }
-
-        SECTION("stream without a streambuf (issue #5646)")
-        {
-            // std::istream(nullptr) has badbit set and rdbuf() == nullptr;
-            // get_character() must not dereference that null streambuf
-            std::istream is(nullptr);
-            json _;
-            CHECK_THROWS_WITH_AS(_ = json::parse(is), "[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON", json::parse_error&);
-        }
-
         SECTION("string")
         {
             json::string_t const s = R"(["foo",1,2,3,false,{"one":1})";

tests/src/unit-precise-stream-position.cpp

@@ -234,33 +234,4 @@ TEST_CASE("JSON_PRECISE_STREAM_POSITION")
         CHECK(j == json(1));
         CHECK(remaining(is) == "true");
     }
-
-    SECTION("stream with eofbit in its exception mask (issue #5646)")
-    {
-        // with JSON_PRECISE_STREAM_POSITION, get_character() peeks via
-        // sb->sgetc() rather than consuming via sb->sbumpc(), but it still
-        // calls std::istream::clear() to record eofbit once the streambuf is
-        // exhausted; with eofbit in the exception mask, that clear() itself
-        // throws std::ios_base::failure, which must propagate to the caller
-        // instead of ~input_stream_adapter() throwing a second exception
-        // while the first is still unwinding (which would call std::terminate)
-        json _;
-
-        std::istringstream is1("1");
-        is1.exceptions(std::ios::eofbit);
-        CHECK_THROWS_AS(_ = json::parse(is1), std::ios_base::failure&);
-
-        std::istringstream is2("1");
-        is2.exceptions(std::ios::failbit | std::ios::badbit | std::ios::eofbit);
-        CHECK_THROWS_AS(_ = json::parse(is2), std::ios_base::failure&);
-    }
-
-    SECTION("stream without a streambuf (issue #5646)")
-    {
-        // std::istream(nullptr) has badbit set and rdbuf() == nullptr;
-        // get_character() must not dereference that null streambuf
-        std::istream is(nullptr);
-        json _;
-        CHECK_THROWS_WITH_AS(_ = json::parse(is), "[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON", json::parse_error&);
-    }
 }