Clang complains about non-constant expressions in static constinit initialization when such code is not executed

[randombit]

Sorry about the title I'm not really sure how to describe this.

Testcase:

class Inner {
   public:
      constexpr bool is_constant_evaluated() const {
         if(__builtin_is_constant_evaluated()) {
            return true;
         } else {
            volatile int x = 0;
            return (x == 1);
         }
      }

      constexpr Inner() {}
};

class Outer {
   public:
      static const constexpr auto A = Inner();

      // If constexpr, no warning
      // If constinit, warns
      static const constinit bool A_is_constant_evaluated = A.is_constant_evaluated();
};

static_assert(Outer::A_is_constant_evaluated == true);

Compiling this with -Wpedantic -std=c++20, clang 17.0.6 complains

eval.cpp:9:21: warning: in-class initializer for static data member is not a constant expression; folding it to a constant is a GNU extension [-Wgnu-folding-constant]
    9 |             return (x == 1);

GCC and MSVC both accept this without any warnings.

This warning makes no sense because first this code is in a else block where the if is guarded by __builtin_is_constant_evaluated. (std::is_constant_evaluated and if constexpr seem to have the same result.) Secondly it doesn't make sense since if Clang was invoking the code with the else block then A_is_constant_evaluated would be false. But the static_assert succeeds.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Jack Lloyd (randombit)

Sorry about the title I'm not really sure how to describe this.

Testcase:

class Inner {
   public:
      constexpr bool is_constant_evaluated() const {
         if(__builtin_is_constant_evaluated()) {
            return true;
         } else {
            volatile int x = 0;
            return (x == 1);
         }
      }

      constexpr Inner() {}
};

class Outer {
   public:
      static const constexpr auto A = Inner();

      // If constexpr, no warning
      // If constinit, warns
      static const constinit bool A_is_constant_evaluated = A.is_constant_evaluated();
};

static_assert(Outer::A_is_constant_evaluated == true);

Compiling this with -Wpedantic -std=c++20, clang 17.0.6 complains

eval.cpp:9:21: warning: in-class initializer for static data member is not a constant expression; folding it to a constant is a GNU extension [-Wgnu-folding-constant]
    9 |             return (x == 1);

GCC and MSVC both accept this without any warnings.

This warning makes no sense because first this code is in a else block where the if is guarded by __builtin_is_constant_evaluated. (std::is_constant_evaluated and if constexpr seem to have the same result.) Secondly it doesn't make sense since if Clang was invoking the code with the else block then A_is_constant_evaluated would be false. But the static_assert succeeds.

[EugeneZelenko]

main and 18 seems working: https://godbolt.org/z/Mfx5Y1Pso

[randombit]

Clang 18 and whatver godbolt has for clang trunk also warn - you have to have -Wpedantic https://godbolt.org/z/3MzMcPWEc https://godbolt.org/z/PMbPWf3K8

Edit: Rather you have to have -Wgnu-folding-constant turned on, which is pulled in by -Wpedantic

[shafik]

Maybe related: #55890