Add testcase for @_implements

Author: graydon

test/attr/attr_implements.swift

@@ -0,0 +1,72 @@
+// RUN: %target-run-simple-swift %s
+// REQUIRES: executable_test
+
+protocol P {
+  func f0() -> Int;
+  func f(x:Int, y:Int) -> Int;
+}
+
+protocol Q {
+  func f(x:Int, y:Int) -> Int;
+}
+
+struct S : P, Q, Equatable {
+
+  // Test that it's possible to denote a zero-arg requirement
+  // (This involved extended the parser for unqualified DeclNames)
+  @_implements(P, f0())
+  func g0() -> Int {
+    return 10
+  }
+
+  // Test that it's possible to implement two different protocols with the
+  // same-named requirements.
+  @_implements(P, f(x:y:))
+  func g(x:Int, y:Int) -> Int {
+    return 5
+  }
+
+  @_implements(Q, f(x:y:))
+  func h(x:Int, y:Int) -> Int {
+    return 6
+  }
+
+  // Test that it's possible to denote an operator requirement
+  // (This involved extended the parser for unqualified DeclNames)
+  @_implements(Equatable, ==(_:_:))
+  public static func isEqual(_ lhs: S, _ rhs: S) -> Bool {
+    return false
+  }
+}
+
+func call_P_f_generic<T:P>(p:T, x: Int, y: Int) -> Int {
+  return p.f(x:x, y:y)
+}
+
+func call_P_f_existential(p:P, x: Int, y: Int) -> Int {
+  return p.f(x:x, y:y)
+}
+
+func call_Q_f_generic<T:Q>(q:T, x: Int, y: Int) -> Int {
+  return q.f(x:x, y:y)
+}
+
+func call_Q_f_existential(q:Q, x: Int, y: Int) -> Int {
+  return q.f(x:x, y:y)
+}
+
+let s = S()
+assert(call_P_f_generic(p:s, x:1, y:2) == 5)
+assert(call_P_f_existential(p:s, x:1, y:2) == 5)
+assert(call_Q_f_generic(q:s, x:1, y:2) == 6)
+assert(call_Q_f_existential(q:s, x:1, y:2) == 6)
+assert(!(s == s))
+
+// Note: at the moment directly calling the member 'f' on the concrete type 'S'
+// doesn't work, because it's considered ambiguous between the 'g' and 'h'
+// members (each of which provide an 'f' via the 'P' and 'Q' protocol
+// conformances), and adding a non-@_implements member 'f' to S makes it win
+// over _both_ the @_implements members. Unclear if this is correct; I think so?
+
+// print(s.f(x:1, y:2))
+