[TypeChecker] NFC: Add tests for type inference from default expression (when enabled)

Author: xedin

test/Constraints/Inputs/type_inference_via_defaults_other_module.swift

@@ -0,0 +1,3 @@
+public func with_defaults<T>(_ v: T = 42.0) {
+  print("with_defaults: \(v); type = \(type(of: v))")
+}

test/Constraints/type_inference_from_default_exprs.swift

@@ -0,0 +1,81 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend-emit-module -emit-module-path %t/InferViaDefaults.swiftmodule -enable-experimental-type-inference-from-defaults -module-name InferViaDefaults %S/Inputs/type_inference_via_defaults_other_module.swift
+// RUN: %target-swift-frontend -enable-experimental-type-inference-from-defaults -module-name main -typecheck -verify -I %t %s %S/Inputs/type_inference_via_defaults_other_module.swift
+
+func testInferFromResult<T>(_: T = 42) -> T { fatalError() }
+// expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from result type}}
+
+func testInferFromOtherPos1<T>(_: T = 42, _: [T]) {}
+// expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}
+
+func testInferFromOtherPos2<T>(_: T = 42, _: T = 0.0) {}
+// expected-error@-1 2 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}
+
+protocol P {
+  associatedtype X
+}
+
+func testInferFromSameType<T, U: P>(_: T = 42, _: [U]) where T == U.X {}
+// expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable through same-type requirement: T == U.X}}
+
+func test1<T>(_: T = 42) {} // Ok
+
+struct S : P {
+  typealias X = Int
+}
+
+func test2<T: P>(_: T = S()) {} // Ok
+
+struct A : P {
+  typealias X = Double
+}
+
+class B : P {
+  typealias X = String
+
+  init() {}
+}
+
+func test2<T: P & AnyObject>(_: T = B()) {} // Ok
+
+func test2NonClassDefault<T: P & AnyObject>(_: T = S()) {}
+// expected-error@-1 {{global function 'test2NonClassDefault' requires that 'S' be a class type}}
+// expected-note@-2 {{where 'T' = 'S'}}
+
+func test2NonConformingDefault<T: P>(_: T = 42.0) {}
+// expected-error@-1 {{global function 'test2NonConformingDefault' requires that 'Double' conform to 'P'}}
+// expected-note@-2 {{where 'T' = 'Double'}}
+
+func testMultiple<T, U>(a: T = 42.0, b: U = "") {} // Ok
+
+// Subscripts
+
+extension S {
+  subscript<T: P>(a: T = S()) -> Int {
+    get { return 42 }
+  }
+
+  subscript<T: P, U: AnyObject>(a: T = S(), b: U = B()) -> Int {
+    get { return 42 }
+  }
+}
+
+
+func main() {
+  test1() // Ok
+
+  test2() // Ok
+  test2(A()) // Ok as well
+
+  testMultiple()                // Ok (T = Double, U = String)
+  testMultiple(a: 0)            // Ok (T = Int, U = String)
+  testMultiple(b: S())          // Ok (T = Double, U = S)
+  testMultiple(a: 0.0, b: "a")  // Ok
+
+  // From a different module
+  with_defaults() // Ok
+  with_defaults("") // Ok
+
+  _ = S()[] // Ok
+  _ = S()[B()] // Ok
+}