[clang] [Clang] prevent errors for deduction guides using deduced type aliases (PR #117450)

[via cfe-commits]

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@@ -11451,7 +11451,11 @@ bool Sema::CheckDeductionGuideDeclarator(Declarator &D, QualType &R,
     bool MightInstantiateToSpecialization = false;
     if (auto RetTST =
             TSI->getTypeLoc().getAsAdjusted<TemplateSpecializationTypeLoc>()) {
-      TemplateName SpecifiedName = RetTST.getTypePtr()->getTemplateName();
+      const TemplateSpecializationType *TST = RetTST.getTypePtr();
+      while (TST && TST->isTypeAlias())
+        TST = TST->getAliasedType()->getAs<TemplateSpecializationType>();
+
+      TemplateName SpecifiedName = TST->getTemplateName();
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antangelo wrote:

If there is some test case that would trip the assert (i.e. some input for which the alias type is not a template specialization type), then we should handle it more gracefully. If it (conceivably) can't happen, then an assert is fine.

I thought about it for a bit and came up with this (somewhat contrived) invalid code that might fall into this path:
```c++
template<class T> struct A {};

struct B {};

template<typename T>
using X = B;

template<typename T>
A() -> X<T>;
```
https://godbolt.org/z/shzK9eccz

https://github.com/llvm/llvm-project/pull/117450