[ConstraintSystem] Record trailing choice match choice when arguments/result are equivalent to applied function

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -8802,14 +8802,6 @@ ConstraintSystem::simplifyApplicableFnConstraint(
     return false;
   };
 
-  // If the types are obviously equivalent, we're done. This optimization
-  // is not valid for operators though, where an inout parameter does not
-  // have an explicit inout argument.
-  if (type1.getPointer() == desugar2) {
-    if (!isOperator || !hasInOut())
-      return SolutionKind::Solved;
-  }
-
   // Local function to form an unsolved result.
   auto formUnsolved = [&] {
     if (flags.contains(TMF_GenerateConstraints)) {
@@ -8838,6 +8830,19 @@ ConstraintSystem::simplifyApplicableFnConstraint(
   ConstraintLocatorBuilder outerLocator =
     getConstraintLocator(anchor, parts, locator.getSummaryFlags());
 
+  // If the types are obviously equivalent, we're done. This optimization
+  // is not valid for operators though, where an inout parameter does not
+  // have an explicit inout argument.
+  if (type1.getPointer() == desugar2) {
+    if (!isOperator || !hasInOut()) {
+      recordTrailingClosureMatch(
+          getConstraintLocator(
+              outerLocator.withPathElement(ConstraintLocator::ApplyArgument)),
+          TrailingClosureMatching::Forward);
+      return SolutionKind::Solved;
+    }
+  }
+
   // Handle applications of types with `callAsFunction` methods.
   // Do this before stripping optional types below, when `shouldAttemptFixes()`
   // is true.

unittests/Sema/CMakeLists.txt

@@ -1,7 +1,8 @@
 
 add_swift_unittest(swiftSemaTests
   SemaFixture.cpp
-  BindingInferenceTests.cpp)
+  BindingInferenceTests.cpp
+  ConstraintSimplificationTests.cpp)
 
 target_link_libraries(swiftSemaTests
   PRIVATE

unittests/Sema/ConstraintSimplificationTests.cpp

@@ -0,0 +1,43 @@
+//===--- ConstraintSimplificationTests.cpp --------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "SemaFixture.h"
+#include "swift/Sema/ConstraintSystem.h"
+
+using namespace swift;
+using namespace swift::unittest;
+using namespace swift::constraints;
+
+TEST_F(SemaTest, TestTrailingClosureMatchRecordingForIdenticalFunctions) {
+  ConstraintSystem cs(DC, ConstraintSystemOptions());
+
+  auto intType = getStdlibType("Int");
+  auto floatType = getStdlibType("Float");
+
+  auto func = FunctionType::get({FunctionType::Param(intType)}, floatType);
+
+  cs.addConstraint(
+      ConstraintKind::ApplicableFunction, func, func,
+      cs.getConstraintLocator({}, ConstraintLocator::ApplyFunction));
+
+  SmallVector<Solution, 2> solutions;
+  cs.solve(solutions);
+
+  ASSERT_EQ(solutions.size(), (unsigned)1);
+
+  const auto &solution = solutions.front();
+
+  auto *locator = cs.getConstraintLocator({}, ConstraintLocator::ApplyArgument);
+  auto choice = solution.trailingClosureMatchingChoices.find(locator);
+  ASSERT_TRUE(choice != solution.trailingClosureMatchingChoices.end());
+  ASSERT_EQ(choice->second, TrailingClosureMatching::Forward);
+}