[Clang] Fix finding instantiated decls for class template specializations during instantiation (#72346)

This change aims to fix
#70375

It appears to me that the logic here should be handling specializations
in general, not just partial specialization. It also seems that both the
comment before the block and the `isInstantiationOf(ClassTemplate,
SpecTemplate)` below agree with my judgement.

The issue might just be a mistake that someone mistaken specialization
as a special case of partial specializations, while it's actually the
other way around.

Needs some experts to comment here if this is the right fix. 

The code that caused clang ICE is added as a test case.

Author: yuxuanchen1997

clang/docs/ReleaseNotes.rst

@@ -608,6 +608,8 @@ Bug Fixes in This Version
 
 - Fixed an issue that a benign assertion might hit when instantiating a pack expansion
   inside a lambda. (`#61460 <https://github.com/llvm/llvm-project/issues/61460>`_)
+- Fix crash during instantiation of some class template specializations within class
+  templates. Fixes (`#70375 <https://github.com/llvm/llvm-project/issues/70375>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -6207,13 +6207,13 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
       return D;
 
     // Determine whether this record is the "templated" declaration describing
-    // a class template or class template partial specialization.
+    // a class template or class template specialization.
     ClassTemplateDecl *ClassTemplate = Record->getDescribedClassTemplate();
     if (ClassTemplate)
       ClassTemplate = ClassTemplate->getCanonicalDecl();
-    else if (ClassTemplatePartialSpecializationDecl *PartialSpec
-               = dyn_cast<ClassTemplatePartialSpecializationDecl>(Record))
-      ClassTemplate = PartialSpec->getSpecializedTemplate()->getCanonicalDecl();
+    else if (ClassTemplateSpecializationDecl *Spec =
+                 dyn_cast<ClassTemplateSpecializationDecl>(Record))
+      ClassTemplate = Spec->getSpecializedTemplate()->getCanonicalDecl();
 
     // Walk the current context to find either the record or an instantiation of
     // it.

clang/test/SemaCXX/template-specialization.cpp

@@ -19,3 +19,36 @@ int main() {
   B::foo<4>(); // expected-note {{in instantiation of function template specialization 'B::foo<4>'}}
   return 0;
 }
+
+namespace GH70375 {
+
+template <typename Ty>
+struct S {
+  static void bar() {
+    Ty t;
+    t.foo();
+  }
+
+  static void take(Ty&) {}
+};
+
+template <typename P>
+struct Outer {
+  template <typename C>
+  struct Inner;
+
+  using U = S<Inner<P>>;
+
+  template <>
+  struct Inner<void> {
+    void foo() {
+      U::take(*this);
+    }
+  };
+};
+
+void instantiate() {
+  Outer<void>::U::bar();
+}
+
+}