Sema: Add a test I wrote a few days ago and forgot to check in

Author: slavapestov

test/decl/nested/type_in_extension.swift

@@ -0,0 +1,19 @@
+// RUN: %target-parse-verify-swift
+
+struct G<T> {}
+
+extension G {
+  struct H<U> { } // expected-error {{type 'H' cannot be nested in extension of generic type 'G'}}
+}
+
+extension { // expected-error {{expected type name in extension declaration}}
+  struct S<T> {
+    func foo(t: T) {} // expected-error {{use of undeclared type 'T'}}
+  }
+
+  class M : S {} // expected-error {{use of undeclared type 'S'}}
+
+  protocol P { // expected-error {{protocol 'P' cannot be nested inside another declaration}}
+    associatedtype A
+  }
+}