[TypeChecker] Allow inference from default even when type could be inferred from result as well

This enables support for inference in initializers of generic types
and in `case`s of generic enums e.g.

```
struct S<T> {
  init(_: T = 42) {}
}

enum E<T: Collection> {
case test(_: T = [42])
}
```

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -6244,11 +6244,6 @@ ERROR(type_sequence_on_non_generic_param, none,
 // MARK: Type inference from default expressions
 //------------------------------------------------------------------------------
 
-ERROR(cannot_default_generic_parameter_inferrable_from_result, none,
-      "cannot use default expression for inference of %0 because it "
-      "is inferrable from result type",
-      (Type))
-
 ERROR(cannot_default_generic_parameter_inferrable_from_another_parameter, none,
       "cannot use default expression for inference of %0 because it "
       "is inferrable from parameters %1",

lib/Sema/TypeCheckConstraints.cpp

@@ -535,15 +535,6 @@ Type TypeChecker::typeCheckParameterDefault(Expr *&defaultValue,
   {
     auto anchorTy = anchor->getInterfaceType()->castTo<GenericFunctionType>();
 
-    // Reject if generic parameters could be inferred from result type.
-    if (containsTypes(anchorTy->getResult(), genericParameters)) {
-      ctx.Diags.diagnose(
-          defaultValue->getLoc(),
-          diag::cannot_default_generic_parameter_inferrable_from_result,
-          paramInterfaceTy);
-      return Type();
-    }
-
     // Reject if generic parameters are used in multiple different positions
     // in the parameter list.
 

test/Constraints/type_inference_from_default_exprs.swift

@@ -2,8 +2,11 @@
 // 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 testInferFromResult<T>(_: T = 42) -> T { fatalError() } // Ok
+
+enum ETest<T> {
+  case test(_: T = 42) // expected-note {{default value declared here}}
+}
 
 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}}
@@ -101,7 +104,27 @@ func testReq2<T, U>(_: (T, U) = (E(), B())) where T: GenClass<U>, U: AnyObject {
 func testReq3<T: P, U>(_: [T?] = [B()], _: U) where T.X == U {}
 // expected-error@-1 {{cannot use default expression for inference of '[T?]' because requirement 'U == T.X' refers to other generic parameters}}
 
+protocol Shape {
+}
+
+struct Circle : Shape {
+}
+
+struct Rectangle : Shape {
+}
+
+struct Figure<S: Shape> {
+  init(_: S = Circle()) {} // expected-note 2 {{default value declared here}}
+}
+
 func main() {
+  _ = testInferFromResult() // Ok T == Int
+  let _: Float = testInferFromResult() // expected-error {{cannot convert value of type 'Int' to specified type 'Float'}}
+
+  _ = ETest.test() // Ok
+
+  let _: ETest<String> = .test() // expected-error {{cannot convert default value of type 'String' to expected argument type 'Int' for parameter #0}}
+
   test1() // Ok
 
   test2() // Ok
@@ -129,4 +152,18 @@ func main() {
   testNested6() // Ok
 
   testReq2() // Ok
+
+  func takesFigure<T>(_: Figure<T>) {}
+  func takesCircle(_: Figure<Circle>) {}
+  func takesRectangle(_: Figure<Rectangle>) {}
+
+  _ = Figure.init() // Ok S == Circle
+  let _: Figure<Circle> = .init() // Ok (S == Circle)
+  let _: Figure<Rectangle> = .init()
+  // expected-error@-1 {{cannot convert default value of type 'Rectangle' to expected argument type 'Circle' for parameter #0}}
+
+  takesFigure(.init()) // Ok
+  takesCircle(.init()) // Ok
+  takesRectangle(.init())
+  // expected-error@-1 {{cannot convert default value of type 'Rectangle' to expected argument type 'Circle' for parameter #0}}
 }

test/Constraints/type_inference_from_default_exprs_executable_test.swift

@@ -99,6 +99,25 @@ struct OtherD : P {
   typealias X = OtherB
 }
 
+protocol Shape {
+}
+
+struct Circle : Shape {
+}
+
+struct Rectangle : Shape {
+}
+
+struct Figure<S: Shape> {
+  init(v: S = Circle()) {
+    print("Figure: \(v); type = \(type(of: v))")
+  }
+}
+
+enum TestE<T: Collection> {
+case a(_: T = [B()])
+}
+
 func run_tests() {
   test1()
   // CHECK: test1: 42; type = Int
@@ -159,6 +178,20 @@ func run_tests() {
 
   testReq1()
   // CHECK: testReq1: (main.E, main.B); type = (E, B)
+
+  _ = Figure.init()
+  // CHECK: Figure: Circle(); type = Circle
+  let _: Figure<Circle> = .init()
+  // CHECK: Figure: Circle(); type = Circle
+
+  func takesFigure<T>(_: Figure<T>) {}
+
+  takesFigure(.init())
+  // CHECK: Figure: Circle(); type = Circle
+
+  let e: TestE = .a()
+  print("ETEst: \(e); type = \(type(of: e))")
+  // CHECK: ETEst: a([main.B]); type = TestE<Array<B>>
 }
 
 @main struct Test {