[Runtime] Fix gatherWrittenGenericArgs crashing when description is NULL

stdlib/public/runtime/MetadataLookup.cpp

@@ -2011,6 +2011,8 @@ void swift::gatherWrittenGenericArgs(
                              const TypeContextDescriptor *description,
                              SmallVectorImpl<const Metadata *> &allGenericArgs,
                              Demangler &BorrowFrom) {
+  if (!description)
+    return;
   auto generics = description->getGenericContext();
   if (!generics)
     return;

test/Generics/Inputs/generic-nested-in-extension-on-objc-class.swift

@@ -0,0 +1,6 @@
+import Foundation
+
+extension NSString {
+  class _Inner1<T> where T: NSObject {}
+}
+

test/Generics/generic-nested-in-extension-on-objc-class.swift

@@ -0,0 +1,40 @@
+// RUN: %empty-directory(%t)
+// RUN: cp %s %t/main.swift
+// RUN: %target-build-swift -module-name test %t/main.swift %S/inputs/generic-nested-in-extension-on-objc-class.swift -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+// Test the fix for a crash when instantiating the metadata for a generic type
+// nested in an extension on an ObjC class in a different file.
+// https://bugs.swift.org/browse/SR-11374
+
+extension NSString {
+  class _Inner2<T> where T: NSObject {}
+}
+
+extension NSArray {
+  class Inner1: NSString._Inner1<NSArray> {
+    override init() {
+      super.init()
+      print("Inner1")
+    }
+  }
+  class Inner2: NSString._Inner2<NSArray> {
+    override init() {
+      super.init()
+      print("Inner2")
+    }
+  }
+}
+
+// CHECK: Inner1
+_ = NSArray.Inner1()
+
+// CHECK: Inner2
+_ = NSArray.Inner2()
+
+