Sema: Add a test that passes with -enable-experimental-associated-type-inference

Author: slavapestov

test/decl/protocol/req/rdar118998138.swift

@@ -0,0 +1,54 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+// RUN: not %target-typecheck-verify-swift
+
+public protocol FakeCopyable {}
+
+extension Int: FakeCopyable {}
+
+// Remove both 'FakeCopyable' requirements below and 
+// inference for Element in Hello succeeds.
+
+public protocol MyIteratorProtocol<Element> {
+  associatedtype Element : FakeCopyable
+
+  mutating func next() -> Element?
+}
+
+public protocol MySequence<Element> {
+  associatedtype Element : FakeCopyable
+
+  associatedtype Iterator: MyIteratorProtocol where Iterator.Element == Element
+
+  __consuming func makeIterator() -> Iterator
+
+  func _customContainsEquatableElement(
+    _ element: Element
+  ) -> Bool?
+}
+
+extension MySequence {
+  @usableFromInline
+  func _customContainsEquatableElement(
+    _ element: Iterator.Element
+  ) -> Bool? { return nil }
+}
+
+extension MySequence where Self.Iterator == Self {
+  @inlinable
+  public __consuming func makeIterator() -> Self {
+    return self
+  }
+}
+
+// ------------
+
+internal struct Hello {}
+
+extension Hello: MySequence, MyIteratorProtocol {
+  // Inferred:
+  // typealias Element = Int
+
+  internal mutating func next() -> Int? {
+    return nil
+  }
+}