decltype of a class-type NTTP used as a requires-expression parameter #462

Issue Summary

For a class-type non-type template parameter F, decltype(F) (unparenthesized, note) should denote the declared type of the template parameter, and thus not be const-qualified (only the template parameter object that F names is const). So a requires-expression parameter declared as decltype(F) f should allow calling a non-const member function on f.

Instead, we treat f as const, and the requirement f() below is not satisfied. GCC 15.2, Clang 21.1 and MSVC accept the code.

Reproducing Source Code

struct NoConst {
  int operator()() { return 0; }
};
template<auto F>
inline constexpr bool const_callable_v = requires { F(); };
// decltype(F) is non-const NoConst
template<auto F>
inline constexpr bool non_const_callable_v =
    requires(decltype(F) f) { f(); };
static_assert(!const_callable_v<NoConst{}>);
static_assert(non_const_callable_v<NoConst{}>); // Incorrectly rejected.

Command-line Options

cpfe --c++20 repro.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

We diagnose this with:

"repro.cpp", line 14: error: static assertion failed
static_assert(non_const_callable_v<NoConst{}>); // Incorrectly rejected.
^

The same happens with --gnu_version=150200, --clang_version=210000, and --ms_c++20. The first static_assert (F itself is a const lvalue, so F() is ill-formed) is handled correctly.

This appears related to, but distinct from, #414 (fixed by #422): The failure persists with that fix.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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