Fix crash in constraining partial specialization on nested template.

[usaxena95]

Summary

Fixes: https://github.com/llvm/llvm-project/issues/53354

[usaxena95]

Added test.

[ilya-biryukov]

clang/lib/Sema/SemaTemplateDeduction.cpp

5809

NIT: maybe keep a single if statement and use dyn_cast_or_null?

clang/test/SemaTemplate/concepts-GH53354.cpp

20

why not && true!?

[usaxena95]

Addressed comments.

[ilya-biryukov]

LGTM, thanks!

clang/lib/Sema/SemaTemplateDeduction.cpp

struct MarkUsedTemplateParameterVisitor :
 
   bool VisitTemplateTypeParmType(TemplateTypeParmType *T) {
     if (T->getDepth() == Depth)
       Used[T->getIndex()] = true;
     return true;
   }
 
   bool TraverseTemplateName(TemplateName Template) {
-    if (auto *TTP =
-            dyn_cast<TemplateTemplateParmDecl>(Template.getAsTemplateDecl()))
+    if (auto *TTP = llvm::dyn_cast_or_null<TemplateTemplateParmDecl>(
+            Template.getAsTemplateDecl()))
       if (TTP->getDepth() == Depth)
         Used[TTP->getIndex()] = true;
     RecursiveASTVisitor<MarkUsedTemplateParameterVisitor>::
         TraverseTemplateName(Template);
     return true;
   }
 
   bool VisitDeclRefExpr(DeclRefExpr *E) {
     if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(E->getDecl()))
       if (NTTP->getDepth() == Depth)

clang/test/SemaTemplate/concepts-GH53354.cpp

+// RUN: %clang_cc1 -std=c++20 -verify %s
+// expected-no-diagnostics
+
+template <template <class> class>
+struct S
+{};
+
+template <class T>
+concept C1 = requires
+{
+  typename S<T::template value_types>;
+};
+
+template <class T>
+requires C1<T>
+struct A {};
+
+template <class T>
+requires C1<T> && true
+struct A<T> {};