std::popcount false non-constexpr with IntelliSense

[adamant-pwn]

Environment

• OS and Version: Arch Linux
• VS Code Version: 1.107.0-insider
• C/C++ Extension Version: 1.29.3
• If using SSH remote, specify OS of remote machine: -

Bug Summary and Steps to Reproduce

Bug Summary:

expression must have a constant valueC/C++(28)
main.cpp(5, 23): cannot call non-constexpr function "__builtin_popcountg" (declared implicitly)
main.cpp(5, 23): called from:
main.cpp(5, 23): called from:

Steps to reproduce:

constexpr auto test = []() {
    return std::popcount(42u);
}();

Expected behavior:
Preview for test=3.

Configuration and Logs

c_cpp_properties.json:


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C/C++: Log Diagnostics:


-------- Diagnostics - 12/11/2025, 1:33:36 PM
Version: 1.29.3
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    Process ID: 207406
    Memory Usage: 34 MB
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------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 92029
Number of files parsed: 14077


Language server logging:

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Shutting down IntelliSense server: /home/adamant/Desktop/code/main.cpp
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Done expanding recursive includes for: /home/adamant/Desktop/code/main.cpp
Reducing recursive includes for: /home/adamant/Desktop/code/main.cpp
Done reducing recursive includes for: /home/adamant/Desktop/code/main.cpp
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  define: __DEC64_MANT_DIG__=16
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  define: __SIG_ATOMIC_WIDTH__=32
  define: __INT_LEAST64_TYPE__=long int
  define: __INT16_TYPE__=short int
  define: __INT_LEAST8_TYPE__=signed char
  define: __FLT16_MAX__=6.55040000000000000000000000000000000e+4F16
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  define: __SIZEOF_INT__=4
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  define: __INTPTR_MAX__=0x7fffffffffffffffL
  define: __FLT64_HAS_QUIET_NAN__=1
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  define: __EXCEPTIONS=1
  define: __UINT16_C(c)=c
  define: __PTRDIFF_WIDTH__=64
  define: __INT_FAST16_WIDTH__=64
  define: __FLT64_HAS_INFINITY__=1
  define: __FLT64X_MAX__=1.18973149535723176502126385303097021e+4932F64x
  define: __FLT16_HAS_INFINITY__=1
  define: __STDCPP_DEFAULT_NEW_ALIGNMENT__=16
  define: __SIG_ATOMIC_MIN__=(-__SIG_ATOMIC_MAX__ - 1)
  define: __code_model_small__=1
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  define: __k8__=1
  define: __INTPTR_TYPE__=long int
  define: __UINT16_TYPE__=short unsigned int
  define: __WCHAR_TYPE__=int
  define: __pic__=2
  define: __STDCPP_FLOAT16_T__=1
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  define: __INT_FAST64_WIDTH__=64
  define: __INT_FAST64_MAX__=0x7fffffffffffffffL
  define: __GCC_ATOMIC_TEST_AND_SET_TRUEVAL=1
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  define: __linux__=1
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  define: __UINT32_C(c)=c ## U
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  define: _GNU_SOURCE=1
  define: __ATOMIC_ACQ_REL=4
  define: __ATOMIC_RELEASE=3
  other: --g++
  other: --gnu_version=150201
  stdver: c++23
  intelliSenseMode: linux-gcc-x64
Update IntelliSense time (sec): 0.128
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Additional context

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[sean-mcmanus]

@adamant-pwn How are you including the definition for popcount? I assume you're doing a #include <bit>

[sean-mcmanus]

@adamant-pwn I don't repro the issue with g++-14...it might be particular to the g++-15 headers.

[sean-mcmanus]

@adamant-pwn I repro the issue with clang++ 20 headers.

[sean-mcmanus]

Simplest repro is constexpr int i = __builtin_popcountg(0u);.

[adamant-pwn]

Another similar thing that I recently discovered:

#include <numeric>

constexpr auto test = []() {
    return std::gcd(42, 1);
}();

Same setup otherwise.

expression must have a constant valueC/C++(28)
main.cpp(3, 23): cannot call non-constexpr function "__builtin_ctzg" (declared implicitly)
main.cpp(3, 23): called from:
main.cpp(3, 23): called from:
main.cpp(3, 23): called from:

[sean-mcmanus]

@adamant-pwn Yeah, it's a bug with our EDG 6.7 compiler, as can be verified using https://godbolt.org/ with that compiler and args --c++23 --g++

constexpr int i = __builtin_popcountg(0u);
constexpr int j = __builtin_ctzg(0u);

I'll file a bug on them and ask them to fix any other cases with other __builtin functions.

We've had this issue in the past with other compiler builtins.