Do not implicitly instantiate the definition of a class template specialization

that has been explicitly specialized!

We assume in various places that we can tell the template specialization kind
of a class type by looking at the declaration produced by TagType::getDecl.
That was previously not quite true: for an explicit specialization, we could
have first seen a template-id denoting the specialization (with a use that does
not trigger an implicit instantiation of the defintiion) and then seen the
first explicit specialization declaration. TagType::getDecl would previously
return an arbitrary declaration when called on a not-yet-defined class; it
now consistently returns the most recent declaration in that case.

llvm-svn: 295118

clang/lib/AST/Type.cpp

@@ -3023,8 +3023,10 @@ static TagDecl *getInterestingTagDecl(TagDecl *decl) {
     if (I->isCompleteDefinition() || I->isBeingDefined())
       return I;
   }
-  // If there's no definition (not even in progress), return what we have.
-  return decl;
+  // If there's no definition (not even in progress), return the most recent
+  // declaration. This is important for template specializations, in order to
+  // pick the declaration with the most complete TemplateSpecializationKind.
+  return decl->getMostRecentDecl();
 }
 
 TagDecl *TagType::getDecl() const {

clang/test/SemaTemplate/explicit-specialization-member.cpp

@@ -57,3 +57,14 @@ template<typename T> struct Helper {
 template<typename T> void Helper<T>::func<2>() {} // expected-error {{cannot specialize a member}} \
                                                   // expected-error {{no function template matches}}
 }
+
+namespace b35070233 {
+  template <typename T> struct Cls {
+    static void f() {}
+  };
+
+  void g(Cls<int>);
+
+  template<> struct Cls<int>; // expected-note {{forward declaration}}
+  template<> void Cls<int>::f(); // expected-error {{incomplete type}}
+}