CWG2975 [temp.constr.normal] Effect of concept _template-head_s on parameter mappings

[hubert-reinterpretcast]

Full name of submitter (unless configured in github; will be published with the issue): Hubert Tong

Reference (section label): temp.constr.normal

Link to reflector thread (if any): N/A

Issue description

Consider:

template <auto> constexpr bool B = true;

template <unsigned X0> concept C = B<X0>;
template <unsigned short X1> concept C2 = C<X1>;

template <unsigned X> void f() requires C<X>;  // #1
template <unsigned X> int f() requires C2<X> && true;  // #2

void g() {
  return f<65536>();  // should probably call #1
}
void h() {
  f<0>();  // ambiguous?
}

When forming the normalized constraints, it is unclear how the type of X1 in the definition of C2 affects the parameter mapping. https://wg21.link/temp.constr.normal#1.4 does not seem to address this at all.

There is implementation divergence:
GCC and MSVC reject the call to f from g (resolving to #2) and similarly resolves the call to f from h to #2.
Clang and EDG accept the call to f from g (resolving to #1) and rejects the call to f from h as ambiguous.

Online compiler link: https://godbolt.org/z/hEKaosbWe

Adopting the GCC/MSVC behaviour, which seems to ignore the type of X1 in the definition of C2, seems objectively wrong: 65536u is not a value that C2 accepts.

The Clang and EDG behaviour suggests that the parameter mapping for the B<X0> atomic constraints are different between #1 and #2. Presumably when substituting X from #2 into the normalized constraint for C2, some decoration was included.

Suggested resolution

Introducing some sort of "shadow constraint" that X is a valid template argument for a template parameter declared as unsigned short when normalizing the use of C2 from #2 would reject the call from g but accept the call from h as resolving to #2. This seems to be a sensible result, but leaves none of the surveyed implementations unscathed.

[jensmaurer]

CWG2975

[hubert-reinterpretcast]

CWG2975

Thanks Jens.

Suggested minor issue write-up adjustment:

and select #‌2 as more constraint constrained

[jensmaurer]

Fixed