is_constant_evaluated can evaluate to false in static class member initializer

[efriedma-quic]

Testcase:

#include <type_traits>
struct X { static const int x = 1/(1-std::is_constant_evaluated()); };
int z[X::x];

gcc and MSVC correctly error; clang somehow evaluates it to 1. It looks like clang somehow falls back on non-constant-context evaluation in some cases.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eli Friedman (efriedma-quic)

Testcase:

#include <type_traits>
struct X { static const int x = 1/(1-std::is_constant_evaluated()); };
int z[X::x];

gcc and MSVC correctly error; clang somehow evaluates it to 1. It looks like clang somehow falls back on non-constant-context evaluation in some cases.

[cor3ntin]

This is a variation on #99684 - constant initialization edition
The interesting thing is that GCC behaves differently if the declaration is in the class our out of line.
I am not sure what the reason for that would be.

https://compiler-explorer.com/z/eeano1vGh

[efriedma-quic]

From the standard: "If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression." The initializer isn't required to be an integer constant expression for an inline definition, or an out-of-line definition. (I probably shouldn't have even included the array in the testcase here.)