MetadataLookup: Use extension's generic context for non-nominal extensions

In protocol extensions, and in the future parameterized extensions, have their own generic arguments
independent of an originating nominal type's formal generic parameters. Instead of crashing, handle
this gracefully. rdar://problem/50038754

Author: jckarter

stdlib/public/runtime/MetadataLookup.cpp

@@ -247,7 +247,9 @@ _findExtendedTypeContextDescriptor(const ExtensionContextDescriptor *extension,
       return nullptr;
     node = node->getChild(0);
   }
-  node = Demangle::getUnspecialized(node, demangler);
+  if (Demangle::isSpecialized(node)) {
+    node = Demangle::getUnspecialized(node, demangler);
+  }
 
   return _findNominalTypeDescriptor(node, demangler);
 }
@@ -823,15 +825,17 @@ bool swift::_gatherGenericParameterCounts(
                                  const ContextDescriptor *descriptor,
                                  SmallVectorImpl<unsigned> &genericParamCounts,
                                  Demangler &BorrowFrom) {
-  // If we have an extension descriptor, extract the extended type and use
-  // that.
   DemanglerForRuntimeTypeResolution<> demangler;
   demangler.providePreallocatedMemory(BorrowFrom);
 
   if (auto extension = dyn_cast<ExtensionContextDescriptor>(descriptor)) {
+    // If we have an nominal type extension descriptor, extract the extended type
+    // and use that. If the extension is not nominal, then we can use the
+    // extension's own signature.
     if (auto extendedType =
-            _findExtendedTypeContextDescriptor(extension, demangler))
+          _findExtendedTypeContextDescriptor(extension, demangler)) {
       descriptor = extendedType;
+    }
   }
 
   // Once we hit a non-generic descriptor, we're done.

test/Interpreter/opaque_return_type_protocol_ext.swift

@@ -0,0 +1,22 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+protocol P {
+  associatedtype AT
+  func foo() -> AT
+}
+
+extension P {
+  func foo() -> some P {
+    return self
+  }
+}
+
+func getPAT<T: P>(_: T.Type) -> Any.Type {
+  return T.AT.self
+}
+
+extension Int: P { }
+
+// CHECK: Int
+print(getPAT(Int.self))