Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp
+++ lib/Sema/SemaExprCXX.cpp
@@ -189,12 +189,15 @@
     // have one) and, if that fails to find a match, in the scope (if
     // we're allowed to look there).
     Found.clear();
-    if (Step == 0 && LookupCtx)
+    if (Step == 0 && LookupCtx) {
+      if (RequireCompleteDeclContext(SS, LookupCtx))
+        return nullptr;
       LookupQualifiedName(Found, LookupCtx);
-    else if (Step == 1 && LookInScope && S)
+    } else if (Step == 1 && LookInScope && S) {
       LookupName(Found, S);
-    else
+    } else {
       continue;
+    }
 
     // FIXME: Should we be suppressing ambiguities here?
     if (Found.isAmbiguous())
Index: test/SemaCXX/nested-name-spec.cpp
===================================================================
--- test/SemaCXX/nested-name-spec.cpp
+++ test/SemaCXX/nested-name-spec.cpp
@@ -169,6 +169,13 @@
 struct Y;  // expected-note{{forward declaration of 'Y'}}
 Y::foo y; // expected-error{{incomplete type 'Y' named in nested name specifier}}
 
+namespace PR25156 {
+struct Y;  // expected-note{{forward declaration of 'PR25156::Y'}}
+void foo() {
+  Y::~Y(); // expected-error{{incomplete type 'PR25156::Y' named in nested name specifier}}
+}
+}
+
 X::X() : a(5) { } // expected-error{{use of undeclared identifier 'X'}}
 
 struct foo_S {