Concepts 'more constrained' not detected for member functions

[royjacobson]

https://godbolt.org/z/bvKGhY4Yz

template <int N> concept C0 = (N == 0);
template <int N, int N2> concept C0x = C0<N>;
template <int N1, int N2> concept C00 = C0x<N1, N2> && C0<N2>;

template<int N1, int N2>
struct A {
    void f() requires C00<N1, N2>;
    void f() requires C0x<N1, N2>;
};

template<int N1, int N2>
void f() requires C00<N1, N2>;

template<int N1, int N2>
void f() requires C0x<N1, N2>;

int main() {
    f<0, 0>(); // Works

    A<0, 0> a;
    a.f();  // Works in GCC only.
}

This should compile, I think, but it doesn't. For some reason MSVC also say that a.f() is ambiguous.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[yuanfang-chen]

Clang incorrectly thinks C0x subsumes C00

[royjacobson]

Ah, that would make sense. If we compare the instantiated constraints after substitution of template params then they really are equivalent (to 0 == 0).

So it might be just be solved when we merge the deferred concepts PR.

[yuanfang-chen]

Just did a search and found the very interesting PR (https://reviews.llvm.org/D119544) you mentioned. Yeah, I think that should fix this bug.

[royjacobson]

So, after some debugging, this looks like a bug in NormalizedConstraint::fromConstraintExpr. When normalizing the constraint it replaces the template argument with DeclRefExpr ... 'int' NonTypeTemplateParm ... 'N' 'int' that doesn't preserve the arguments substitution chain and then we think C0x subsumes C00.

But the normalization code is pretty dense so I haven't made progress on how to fix it.

[usx95]

On the similar lines:
Redefinition of function is accepted but this looks like an ODR violation. f<0,0> must be ambiguous.
EDIT: I was wrong here. C00<0,0> subsumes C0x<0,0> and it is not ambiguous.

template <int N> concept C0 = (N == 0);
template <int N, int N2> concept C0x = C0<N>;
template <int N1, int N2> concept C00 = C0x<N1, N2> && C0<N2>;

template<int N1, int N2>
int f() requires C00<N1, N2> { return 0; }

template<int N1, int N2>
int f() requires C0x<N1, N2> { return 1; }

int main() {
    return f<0, 0>(); 
}

https://godbolt.org/z/KMrnWzx56