[CodeCompletion] For types with type parameters, their constraint extensions are considered unusable. rdar://23173692

Author: nkcsgexi

lib/Sema/TypeCheckProtocol.cpp

@@ -4198,6 +4198,11 @@ bool TypeChecker::isProtocolExtensionUsable(DeclContext *dc, Type type,
   if (!protocolExtension->isConstrainedExtension())
     return true;
 
+  // If the type still has parameters, the constrained extension is considered
+  // unusable.
+  if (type->hasTypeParameter())
+    return false;
+
   // Set up a constraint system where we open the generic parameters of the
   // protocol extension.
   ConstraintSystem cs(*this, dc, None);

test/IDE/complete_crashes.swift

@@ -63,7 +63,7 @@ struct CustomGenericCollection<Key> : DictionaryLiteralConvertible {
   // GENERIC_PARAM_AND_ASSOC_TYPE-DAG: Decl[TypeAlias]/CurrNominal:        Key[#Key#]; name=Key
   // GENERIC_PARAM_AND_ASSOC_TYPE-DAG: Decl[TypeAlias]/CurrNominal:        Value[#Value#]; name=Value
   // GENERIC_PARAM_AND_ASSOC_TYPE: End completions
-  
+
   var count: Int { #^GENERIC_PARAM_AND_ASSOC_TYPE^# }
 }
 
@@ -159,3 +159,9 @@ func rdar22834017() {
 }
 // FIXME: We could provide a useful completion here. rdar://problem/22846558
 // INVALID_TYPE_INIT-NOT: Begin completions
+
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=RDAR_23173692 | FileCheck %s -check-prefix=RDAR_23173692
+func rdar23173692() {
+  return IndexingGenerator(#^RDAR_23173692^#)
+}
+// RDAR_23173692: Begin completions