erichkeane added a comment.
In D146178#4200821 <https://reviews.llvm.org/D146178#4200821>, @rsmith wrote:
> The approach of attempting to adjust the depth here is not correct, and we
> should rip this whole mechanism out and reimplement it. Consider this
> closely related testcase:
>
> template<typename T, typename U>
> concept Concept = true;
>
> template<typename T>
> struct A
> {
> template<Concept<T> V>
> void C(V&& t);
> };
>
> template<>
> template<Concept<int> V>
> void A<int>::C(V&& t)
> {}
>
> Clang rejects this because it's trying to compare `Concept<T>` from the
> declaration against `Concept<int>` from the explicit specialization.
>
> The right way to fix that and the issue being addressed here is that, rather
> than adjusting the depths, we should substitute the outer template arguments
> from the scope specifier (`A<int>::`) into the constraint before performing
> the comparison. (In the special case where none of the outer template
> parameters are used by the inner template, that does effectively just adjust
> the depths of any inner template parameters.)
>
> The relevant language rule was introduced by the resolution of CA104 in
> P2103R0
> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2103r0.html#ca104>
> (which Clang's status page incorrectly marks as complete in Clang 10). In
> particular, see temp.constr.decl/4
> <https://eel.is/c++draft/temp.constr.decl#4> for the rule that we should
> implement, but don't:
>
>> When determining whether a given introduced constraint-expression `C1` of a
>> declaration in an instantiated specialization of a templated class is
>> equivalent ([temp.over.link]) to the corresponding constraint-expression
>> `C2` of a declaration outside the class body, `C1` is instantiated. If the
>> instantiation results in an invalid expression, the constraint-expressions
>> are not equivalent.
>
> So, in this case, we're supposed to instantiate the constraint expression
> `Concept<T>`, substituting the template arguments of `A` into it (in a SFINAE
> context).
IIRC, the original motivation for this attempt was that we frequently compare
template arguments, even in cases where that isn't permitted? I had sent you a
couple of emails when I was attempting this, but you'd not gotten a chance to
respond. I think I came up on this as the only way to do this without
instantiating the constraint (which I recall you'd said was not the right way
to compare constraints at one point?).
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