[ConstraintSystem] De-prioritize solutions with Swift -> C pointer conversions

Just like other implicit conversions - always prefer solutions with
the lowest possible number of them.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -11385,6 +11385,10 @@ ConstraintSystem::SolutionKind
 ConstraintSystem::simplifyPointerToCPointerRestriction(
     Type type1, Type type2, TypeMatchOptions flags,
     ConstraintLocatorBuilder locator) {
+  // Make sure that solutions with implicit pointer conversions
+  // are always worse than the ones without them.
+  increaseScore(SK_ImplicitValueConversion);
+
   auto &ctx = getASTContext();
 
   PointerTypeKind swiftPtrKind, cPtrKind;

test/Constraints/swift_to_c_pointer_conversions.swift.gyb

@@ -1,5 +1,3 @@
-// %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -import-objc-header %S/Inputs/overloaded_async_method.h %s -typecheck -verify
-
 // RUN: %empty-directory(%t)
 
 // RUN: %gyb %s -o %t/swift_to_c_pointer_conversions.swift
@@ -236,3 +234,12 @@ func test_raw_ptr_optional_to_optional_conversion(
     % end
   % end
 }
+
+// Check that implicit conversions don't make expressions ambiguous
+func test_overloaded_ref_is_not_ambiguous() {
+  func overloaded_func() -> UnsafeRawPointer { fatalError() }
+  func overloaded_func() -> UnsafePointer<Int8> { fatalError() }
+
+  const_char_ptr_func(overloaded_func()) // Ok
+  const_opt_char_ptr_func(overloaded_func()) // Ok
+}

test/Constraints/swift_to_c_pointer_conversions_sil.swift

@@ -0,0 +1,36 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -import-objc-header %S/Inputs/c_pointer_conversions.h %s -emit-sil -verify | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+
+// Check that implicit conversions don't make expressions ambiguous
+func test_overloaded_ref_is_not_ambiguous() {
+  func overloaded_func() -> UnsafeRawPointer { fatalError() }
+  func overloaded_func() -> UnsafePointer<Int8> { fatalError() }
+
+  func overloaded_mutable_func() -> UnsafeMutableRawPointer { fatalError() }
+  func overloaded_mutable_func() -> UnsafeMutablePointer<Int8> { fatalError() }
+
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G5_funcL0_SPys4Int8VGyF
+  // CHECK-NEXT: apply
+  const_char_ptr_func(overloaded_func()) // Ok
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G5_funcL0_SPys4Int8VGyF
+  // CHECK-NEXT: apply
+  const_opt_char_ptr_func(overloaded_func()) // Ok
+
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G13_mutable_funcL0_Spys4Int8VGyF
+  // CHECK-NEXT: apply
+  char_ptr_func(overloaded_mutable_func()) // Ok
+
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G13_mutable_funcL0_Spys4Int8VGyF
+  // CHECK-NEXT: apply
+  const_char_ptr_func(overloaded_mutable_func()) // Ok (picks UnsafeMutablePointer using implicit conversion)
+
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G13_mutable_funcL0_Spys4Int8VGyF
+  // CHECK-NEXT: apply
+  opt_char_ptr_func(overloaded_mutable_func()) // Ok
+
+  // CHECK: function_ref @$s34swift_to_c_pointer_conversions_sil36test_overloaded_ref_is_not_ambiguousyyF0G13_mutable_funcL0_Spys4Int8VGyF
+  // CHECK-NEXT: apply
+  const_opt_char_ptr_func(overloaded_mutable_func()) // Ok
+}