Index: lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiate.cpp
+++ lib/Sema/SemaTemplateInstantiate.cpp
@@ -2796,6 +2796,13 @@
       isa<TemplateTemplateParmDecl>(D))
     return nullptr;
 
+  // If the declaration we are looking for is an incomplete or forward type
+  // declaration, we may not have a chance to instantiate it.
+  if (const TagDecl *TD = dyn_cast<TagDecl>(D))
+    if (TD->getDefinition() ||
+        (!TD->isThisDeclarationADefinition() && !TD->isFreeStanding()))
+      return nullptr;
+
   // If we didn't find the decl, then we either have a sema bug, or we have a
   // forward reference to a label declaration.  Return null to indicate that
   // we have an uninstantiated label.
Index: lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4399,6 +4399,21 @@
         isa<TemplateTemplateParmDecl>(D))
       return D;
 
+    // If the declaration we are looking for is an incomplete or forward type
+    // declaration, we may not have a chance to instantiate it.
+    if (const TagDecl *TD = dyn_cast<TagDecl>(D)) {
+      const TagDecl *DeclToInstantiate = nullptr;
+      if (TagDecl *Def = TD->getDefinition())
+        DeclToInstantiate = Def;
+      else if (!TD->isThisDeclarationADefinition() && !TD->isFreeStanding())
+        DeclToInstantiate = TD;
+      if (DeclToInstantiate) {
+        Decl *Inst = SubstDecl(D, CurContext, TemplateArgs);
+        CurrentInstantiationScope->InstantiatedLocal(D, Inst);
+        return cast<TypeDecl>(Inst);
+      }
+    }
+
     if (D->isInvalidDecl())
       return nullptr;
 
Index: test/SemaTemplate/instantiate-local-class.cpp
===================================================================
--- test/SemaTemplate/instantiate-local-class.cpp
+++ test/SemaTemplate/instantiate-local-class.cpp
@@ -194,3 +194,22 @@
   void f() { F<int>(); }
 };
 }
+
+namespace PR18653 {
+  template <class T> void f();
+  template <class T> struct str {
+    void m() {
+      f<class newclass>();
+    }
+  };
+  template struct str<int>;
+
+  template<typename T> void f() { void g(struct x); }
+  template void f<int>();
+
+  template<typename T> void f2() {
+    void g2(struct x2);
+    struct x2 {};
+  }
+  template void f2<int>();
+}