Extended Description

$ clang++.exe --version
clang version 11.0.0 (https://github.com/llvm/llvm-project.git fc3c80c)

$ cat test.cpp
#ifdef BUG
template
#endif
struct test_t {
template
int exec(U) {
if constexpr (requires { this->is_memmovable(); })
return 1;
else
return 0;
}

template
requires false
auto is_memmovable() -> void;
};

int test() {
#ifdef BUG
using test_type = test_t;
#else
using test_type = test_t;
#endif

return test_type{}.exec(0);
}

$ clang++.exe -std=c++2a -O3 -c -S -o - test.cpp -DBUG
"?test@@yahxz": # @"?test@@yahxz"

%bb.0:

    movl    $1, %eax
    retl

$ clang++.exe -std=c++2a -O3 -c -S -o - test.cpp
"?test@@yahxz": # @"?test@@yahxz"

%bb.0:

    xorl    %eax, %eax
    retl

The requires expression only gives the wrong result if test_t is a template. Otherwise it correctly evaluates to false.