Add test for https://bugs.swift.org/browse/SR-7422

test/decl/protocol/conforms/self.swift

@@ -35,3 +35,38 @@ class NonFinalClass : P {
     return self
   }
 }
+
+// Test for default implementation that comes from a constrained extension
+// - https://bugs.swift.org/browse/SR-7422
+
+// FIXME: Better error message here?
+
+class SillyClass {}
+
+protocol HasDefault {
+  func foo()
+  // expected-note@-1 {{protocol requires function 'foo()' with type '() -> ()'; do you want to add a stub?}}
+}
+
+extension HasDefault where Self == SillyClass {
+  func foo() {}
+  // expected-note@-1 {{candidate has non-matching type '<Self> () -> ()'}}
+}
+
+extension SillyClass : HasDefault {}
+// expected-error@-1 {{type 'SillyClass' does not conform to protocol 'HasDefault'}}
+
+// This is OK, though
+class SeriousClass {}
+
+extension HasDefault where Self : SeriousClass {
+  func foo() {}
+  // expected-note@-1 {{candidate has non-matching type '<Self> () -> ()'}}
+
+  // FIXME: the above diangostic is from trying to check conformance for
+  // 'SillyClass' and not 'SeriousClass'. Evidently name lookup finds members
+  // from all constrained extensions, and then if any don't have a matching
+  // generic signature, diagnostics doesn't really know what to do about it.
+}
+
+extension SeriousClass : HasDefault {}