Krystian Stasiowski da928c6d6e [Clang][Sema] ASTContext::getUnconstrainedType propagates dependence (#92425) ...

When the argument passed to `ASTContext::getUnconstrainedType` is an
unconstrained `AutoType`, will return the argument unchanged. However,
when called with a constrained `AutoType`, an unconstrained,
non-dependent `AutoType` will be returned even if the argument was
dependent. Consider the following:
```
template<typename T>
concept C = sizeof(T) == sizeof(int);

template<auto N>
struct A;

template<C auto N>
struct A<N>; // error: class template partial specialization is not more specialized than the primary template
```
When comparing the template parameters for equivalence,
`ASTContext::getUnconstrainedType` is used to remove the constraints per
[temp.over.link] p6 sentence 2. For the template
parameter `N` of the class template, it returns a dependent `AutoType`.
For the template parameter `N` of the class template partial
specialization, it returns a non-dependent `AutoType`. We subsequently
compare the adjusted types and find they are not equivalent, thus we
consider the partial specialization to not be more specialized than the
primary template per [temp.func.order] p6.2.2.

This patch changes `ASTContext::getUnconstrainedType` such that the
dependence of a constrained `AutoType` will propagate to the returned
unconstrained `AutoType`. This causes the above example to be correctly
accepted, fixing #77377.

2024-05-16 14:10:46 -04:00

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