Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp
+++ lib/Sema/SemaExprCXX.cpp
@@ -189,7 +189,7 @@
     // 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 && !RequireCompleteDeclContext(SS, LookupCtx))
       LookupQualifiedName(Found, LookupCtx);
     else if (Step == 1 && LookInScope && S)
       LookupName(Found, S);
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 {