Sema: Add test case for https://bugs.swift.org/browse/SR-11392

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr11392.swift

@@ -0,0 +1,28 @@
+// RUN: %target-swift-frontend -verify -emit-ir %s
+
+// Ideally this would type check successfully; we should be able to
+// infer that X == Int using the same-type constraint 'A.X == X'.
+
+protocol P1 {
+    associatedtype X
+    // expected-note@-1 {{protocol requires nested type 'X'; do you want to add it?}}
+    associatedtype A: P2 where A.X == X
+}
+
+protocol P2 {
+    associatedtype X
+}
+
+struct S {}
+
+extension S {
+    struct A: P2 {
+        typealias X = Int
+    }
+}
+
+
+extension S: P1 {}
+// expected-error@-1 {{type 'S' does not conform to protocol 'P1'}}
+
+print(S.X.self)