Default equality operator for an empty union cannot be used in constant expressions

[Fedr]

This program

union U {
    constexpr bool operator ==(const U&) const = default;
};

static_assert( U{} == U{} );

is accepted in GCC and EDG. But Clang fails with the error:

<source>:5:16: error: static assertion expression is not an integral constant expression
    5 | static_assert( U{} == U{} );
      |                ^~~~~~~~~~
<source>:5:16: note: pointer to temporary is not a constant expression
<source>:5:16: note: temporary created here

Online demo: https://gcc.godbolt.org/z/MMrbaaWae

Related discussion: https://stackoverflow.com/q/79793373/7325599

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Fedor Chelnokov (Fedr)

This program ```c++ union U { constexpr bool operator ==(const U&) const = default; };

static_assert( U{} == U{} );

is accepted in GCC and EDG. But Clang fails with the error:

<source>:5:16: error: static assertion expression is not an integral constant expression
5 | static_assert( U{} == U{} );
| ^~~~~~~~~~
<source>:5:16: note: pointer to temporary is not a constant expression
<source>:5:16: note: temporary created here

Online demo: https://gcc.godbolt.org/z/MMrbaaWae

Related discussion: https://stackoverflow.com/q/79793373/7325599
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