[Clang] Fix finding instantiated decls for class template specializations during instantiation

clang/docs/ReleaseNotes.rst

@@ -591,6 +591,8 @@ 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();
+}
+
+}