Merge pull request #25489 from xedin/rdar-50512161

[TypeChecker] NFC: Add a test-case for rdar://problem/50512161

Author: xedin

test/Constraints/protocols.swift

@@ -401,3 +401,20 @@ func rdar_50099849() {
   let _ = C<A, B>() // expected-error {{generic parameter 'C' could not be inferred}}
   // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}} {{17-17=<<#C: Trivial#>>}}
 }
+
+// rdar://problem/50512161 - improve diagnostic when generic parameter cannot be deduced
+func rdar_50512161() {
+  struct Item {}
+
+  struct TrivialItem : Trivial {
+    typealias T = Item?
+  }
+
+  func foo<I>(_: I.Type = I.self, item: I.T) where I : Trivial { // expected-note {{in call to function 'foo(_:item:)'}}
+    fatalError()
+  }
+
+  func bar(_ item: Item) {
+    foo(item: item) // expected-error {{generic parameter 'I' could not be inferred}}
+  }
+}