Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -4573,12 +4573,14 @@
         RequireCompleteDeclContext(D.getCXXScopeSpec(), DC))
       return nullptr;
 
+    // If a class is incomplete, do not parse entities inside it.
     if (isa<CXXRecordDecl>(DC) && !cast<CXXRecordDecl>(DC)->hasDefinition()) {
       Diag(D.getIdentifierLoc(),
            diag::err_member_def_undefined_record)
         << Name << DC << D.getCXXScopeSpec().getRange();
-      D.setInvalidType();
-    } else if (!D.getDeclSpec().isFriendSpecified()) {
+      return nullptr;
+    }
+    if (!D.getDeclSpec().isFriendSpecified()) {
       if (diagnoseQualifiedDeclaration(D.getCXXScopeSpec(), DC,
                                       Name, D.getIdentifierLoc())) {
         if (DC->isRecord())
Index: test/SemaCXX/incomplete-call.cpp
===================================================================
--- test/SemaCXX/incomplete-call.cpp
+++ test/SemaCXX/incomplete-call.cpp
@@ -47,3 +47,15 @@
 void test_incomplete_object_call(C& c) {
   c(); // expected-error{{incomplete type in call to object of type}}
 }
+
+namespace pr18542 {
+  struct X {
+    int count;
+    template<typename CharT> class basic_istream;
+    template<typename CharT>
+      void basic_istream<CharT>::read() { // expected-error{{out-of-line definition of 'read' from class 'basic_istream<CharT>' without definition}}
+        count = 0;
+      }
+  };
+}
+