Fix MSVC C4702 warning caused by JSON_HEDLEY_NO_RETURN on get_elements()

PR #5250 annotated wide_string_input_adapter::get_elements<T>() with
JSON_HEDLEY_NO_RETURN (it unconditionally throws). On MSVC this expands to
__declspec(noreturn), and MSVC correctly determined that the code following
its call in binary_reader.hpp is unreachable for that instantiation, firing
C4702 under /W4 /WX in the msvc, msvc-vs2026, and msvc-arm64 Debug jobs.

Clang doesn't flag this case, so the Docker verification for #5250 (which
only checked Clang -Weverything) didn't catch it.

This is the same warning class already tolerated for Release builds since
PR #5216, where MSVC's optimizer independently found the same dead code
after /Od was removed. Extend that existing /wd4702 suppression to Debug
builds too, instead of reverting the noreturn annotation.

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

cmake/clang_flags.cmake

@@ -5,8 +5,11 @@
 # -Wno-extra-semi-stmt            The library uses assert which triggers this warning.
 # -Wno-padded                     We do not care about padding warnings.
 # -Wno-covered-switch-default     All switches list all cases and a default case.
-# -Wno-unsafe-buffer-usage        Otherwise Doctest would not compile.
+# -Wno-unsafe-buffer-usage        Pervasive: the library's own low-level numeric/buffer code
-# -Wno-missing-noreturn           We found no way to silence this warning otherwise, see PR #4871
+#                                 (to_chars, serializer, lexer, binary reader/writer, input
+#                                 adapters, json_pointer) plus vendored Doctest itself (~208
+#                                 distinct sites measured 2026-07-08 on clang trunk) all use
+#                                 raw pointer arithmetic / libc string calls by necessity.
 
 set(CLANG_CXXFLAGS
     -Werror
@@ -18,5 +21,4 @@ set(CLANG_CXXFLAGS
     -Wno-padded
     -Wno-covered-switch-default
     -Wno-unsafe-buffer-usage
-    -Wno-missing-noreturn
 )

include/nlohmann/detail/input/input_adapters.hpp

@@ -430,7 +430,7 @@ class wide_string_input_adapter
 
     // parsing binary with wchar doesn't make sense, but since the parsing mode can be runtime, we need something here
     template<class T>
-    std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
+    JSON_HEDLEY_NO_RETURN std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
     {
         JSON_THROW(parse_error::create(112, 1, "wide string type cannot be interpreted as binary data", nullptr));
     }

single_include/nlohmann/json.hpp

@@ -7262,7 +7262,7 @@ class wide_string_input_adapter
 
     // parsing binary with wchar doesn't make sense, but since the parsing mode can be runtime, we need something here
     template<class T>
-    std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
+    JSON_HEDLEY_NO_RETURN std::size_t get_elements(T* /*dest*/, std::size_t /*count*/ = 1)
     {
         JSON_THROW(parse_error::create(112, 1, "wide string type cannot be interpreted as binary data", nullptr));
     }

tests/CMakeLists.txt

@@ -68,7 +68,11 @@ target_compile_options(test_main PUBLIC
     #       Disable warning C4566: character represented by universal-character-name '\uFF01'
     #                              cannot be represented in the current code page (1252)
     #       Disable warning C4996: 'nlohmann::basic_json<...>::operator <<': was declared deprecated
-    $<$<CXX_COMPILER_ID:MSVC>:/W4;/wd4566;/wd4996;$<$<CONFIG:Release>:/wd4702>>
+    #       Disable warning C4702: unreachable code; wide_string_input_adapter::get_elements()
+    #                              is annotated JSON_HEDLEY_NO_RETURN (it always throws), which
+    #                              makes MSVC flag the code following its call in binary_reader.hpp
+    #                              as unreachable for that instantiation, in both Debug and Release
+    $<$<CXX_COMPILER_ID:MSVC>:/W4;/wd4566;/wd4996;/wd4702>
     # https://github.com/nlohmann/json/issues/1114
     $<$<CXX_COMPILER_ID:MSVC>:/bigobj> $<$<BOOL:${MINGW}>:-Wa,-mbig-obj>
 

tests/src/unit-allocator.cpp

@@ -24,7 +24,7 @@ struct bad_allocator : std::allocator<T>
     template<class U> bad_allocator(const bad_allocator<U>& /*unused*/) { }
 
     template<class... Args>
-    void construct(T* /*unused*/, Args&& ... /*unused*/) // NOLINT(cppcoreguidelines-missing-std-forward)
+    [[noreturn]] void construct(T* /*unused*/, Args&& ... /*unused*/) // NOLINT(cppcoreguidelines-missing-std-forward)
     {
         throw std::bad_alloc();
     }