Simplify `nodiscard` macros by StephanTLavavej · Pull Request #6061 · microsoft/STL · GitHub
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Simplify nodiscard macros#6061

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StephanTLavavej merged 8 commits into
microsoft:mainfrom
StephanTLavavej:know-this-card
Feb 11, 2026
Merged

Simplify nodiscard macros#6061
StephanTLavavej merged 8 commits into
microsoft:mainfrom
StephanTLavavej:know-this-card

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@StephanTLavavej

@StephanTLavavej StephanTLavavej commented Jan 31, 2026 •
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This is a prelude to using [[nodiscard]] literally everywhere, which will be a huge mechanical change.

Commits

  • Simplify nodiscard macro definitions.
    • MSVC, Clang, and EDG always support the most advanced form. I'm removing the control macro _HAS_NODISCARD from <vcruntime.h>.
  • _NODISCARD_CTOR_MSG => _NODISCARD_MSG
  • _NODISCARD_MSG(_Msg) => [[nodiscard(_Msg)]]
  • Improve message wrapping.
  • Replace _NODISCARD_CTOR with slightly more informative messages.
  • Reported VSO-2696854 "C1XX should support [[fallthrough]] in C++14 mode".
    • Clang and EDG already do.
  • Replace _NODISCARD_CTOR with _NODISCARD.
    • If compiler warnings aren't sufficiently clear that they're complaining about unused temporary objects, the compiler should be improved.
  • _NODISCARD_CTOR_LOCK is now identical to _NODISCARD_LOCK.

__has_cpp_attribute(nodiscard) Investigation

Click to expand: 
C:\Temp>type meow.cpp

#ifndef __has_cpp_attribute
static_assert(false, "#ifndef __has_cpp_attribute, this is not C++.");
#elif __has_cpp_attribute(nodiscard) >= 201907L
static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
#elif __has_cpp_attribute(nodiscard) >= 201603L
static_assert(false, "__has_cpp_attribute(nodiscard) >= 201603L, only nodiscard-msg available.");
#else
static_assert(false, "C++ but really old, nodiscard-msg/ctor unavailable.");
#endif
C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (cl /EHsc /nologo /W4 /c %I meow.cpp)

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++14 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++17 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++20 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (clang-cl /EHsc /nologo /W4 /c %I meow.cpp)

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++14 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++17 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++20 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (cl /EHsc /nologo /W4 /c /BE %I meow.cpp)

C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++14 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^


C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++17 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^


C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++20 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^

@StephanTLavavej StephanTLavavej requested a review from a team as a code owner January 31, 2026 12:52
@StephanTLavavej StephanTLavavej added the enhancement Something can be improved label Jan 31, 2026
@github-project-automation github-project-automation Bot moved this to Initial Review in STL Code Reviews Jan 31, 2026
@StephanTLavavej StephanTLavavej moved this from Initial Review to Final Review in STL Code Reviews Jan 31, 2026
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Comment thread stl/inc/yvals_core.h Outdated
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Comment thread stl/inc/yvals_core.h Outdated
davidmrdavid
davidmrdavid approved these changes Feb 5, 2026
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Just one non-blocking question, approved

Comment thread stl/inc/yvals_core.h
@StephanTLavavej StephanTLavavej moved this from Final Review to Ready To Merge in STL Code Reviews Feb 5, 2026
@StephanTLavavej StephanTLavavej moved this from Ready To Merge to Merging in STL Code Reviews Feb 9, 2026
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StephanTLavavej commented Feb 9, 2026

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@StephanTLavavej StephanTLavavej merged commit 41b2771 into microsoft:main Feb 11, 2026
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@github-project-automation github-project-automation Bot moved this from Merging to Done in STL Code Reviews Feb 11, 2026
@StephanTLavavej StephanTLavavej deleted the know-this-card branch February 11, 2026 13:52
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@davidmrdavid davidmrdavid davidmrdavid approved these changes
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