[ConstraintSystem] Allow injecting callAsFunction after defaulted arguments

- Adjust argument matching to be less aggressive while matching misplaced trailing closures
   against defaulted parameters that do not accept trailing closures.

   For example:

   ```swift
   func test(a: Int? = 42, b: String? = nil) {}

   test {
     ...
   }
   ```

  Trailing closure in this case should be marked as "extraneous" instead of matched
   against parameter `a:`.

- If trailing closure is extraneous in a call to `.init` on callable type, let's not attempt to
   to fix it and allow solver to inject `.callAsFunction` first.

Resolves: rdar://problem/94959816

(cherry picked from commit ea4ac2c)
(cherry picked from commit 2750e69)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -467,6 +467,35 @@ static bool matchCallArgumentsImpl(
         return;
       }
 
+      // Let's consider current closure to be extraneous if:
+      //
+      // - current parameter has a default value and doesn't accept a trailing
+      //   closure; and
+      // - no other free parameter after this one accepts a trailing closure via
+      //   forward or backward scan. This check makes sure that it's safe to
+      //   reject and push it to the next parameter without affecting backward
+      //   scan logic.
+      //
+      // In other words - let's push the closure argument through defaulted
+      // parameters until it can be considered extraneous if no parameters
+      // could possibly match it.
+      if (!paramInfo.acceptsUnlabeledTrailingClosureArgument(paramIdx) &&
+          !parameterRequiresArgument(params, paramInfo, paramIdx)) {
+        if (llvm::none_of(
+                range(paramIdx + 1, params.size()), [&](unsigned idx) {
+                  return parameterBindings[idx].empty() &&
+                         (paramInfo.acceptsUnlabeledTrailingClosureArgument(
+                              idx) ||
+                          backwardScanAcceptsTrailingClosure(params[idx]));
+                })) {
+          haveUnfulfilledParams = true;
+          return;
+        }
+
+        // If one or more parameters can match the closure, let's check
+        // whether backward scan is applicable here.
+      }
+
       // If this parameter does not require an argument, consider applying a
       // backward-match rule that skips this parameter if doing so is the only
       // way to successfully match arguments to parameters.
@@ -1076,8 +1105,10 @@ constraints::getCompletionArgInfo(ASTNode anchor, ConstraintSystem &CS) {
 class ArgumentFailureTracker : public MatchCallArgumentListener {
 protected:
   ConstraintSystem &CS;
+  NullablePtr<ValueDecl> Callee;
   SmallVectorImpl<AnyFunctionType::Param> &Arguments;
   ArrayRef<AnyFunctionType::Param> Parameters;
+  Optional<unsigned> UnlabeledTrailingClosureArgIndex;
   ConstraintLocatorBuilder Locator;
 
 private:
@@ -1109,11 +1140,14 @@ class ArgumentFailureTracker : public MatchCallArgumentListener {
   }
 
 public:
-  ArgumentFailureTracker(ConstraintSystem &cs,
+  ArgumentFailureTracker(ConstraintSystem &cs, ValueDecl *callee,
                          SmallVectorImpl<AnyFunctionType::Param> &args,
                          ArrayRef<AnyFunctionType::Param> params,
+                         Optional<unsigned> unlabeledTrailingClosureArgIndex,
                          ConstraintLocatorBuilder locator)
-      : CS(cs), Arguments(args), Parameters(params), Locator(locator) {}
+      : CS(cs), Callee(callee), Arguments(args), Parameters(params),
+        UnlabeledTrailingClosureArgIndex(unlabeledTrailingClosureArgIndex),
+        Locator(locator) {}
 
   ~ArgumentFailureTracker() override {
     if (!MissingArguments.empty()) {
@@ -1143,6 +1177,19 @@ class ArgumentFailureTracker : public MatchCallArgumentListener {
     if (!CS.shouldAttemptFixes())
       return true;
 
+    // If this is a trailing closure, let's check if the call is
+    // to an init of a callable type. If so, let's not record it
+    // as extraneous since it would be matched against implicitly
+    // injected `.callAsFunction` call.
+    if (UnlabeledTrailingClosureArgIndex &&
+        argIdx == *UnlabeledTrailingClosureArgIndex && Callee) {
+      if (auto *ctor = dyn_cast<ConstructorDecl>(Callee.get())) {
+        auto resultTy = ctor->getResultInterfaceType();
+        if (resultTy->isCallableNominalType(CS.DC))
+          return true;
+      }
+    }
+
     ExtraArguments.push_back(std::make_pair(argIdx, Arguments[argIdx]));
     return false;
   }
@@ -1251,12 +1298,15 @@ class CompletionArgumentTracker : public ArgumentFailureTracker {
   struct CompletionArgInfo ArgInfo;
 
 public:
-  CompletionArgumentTracker(ConstraintSystem &cs,
+  CompletionArgumentTracker(ConstraintSystem &cs, ValueDecl *callee,
                             SmallVectorImpl<AnyFunctionType::Param> &args,
                             ArrayRef<AnyFunctionType::Param> params,
+                            Optional<unsigned> unlabeledTrailingClosureArgIndex,
                             ConstraintLocatorBuilder locator,
                             struct CompletionArgInfo ArgInfo)
-      : ArgumentFailureTracker(cs, args, params, locator), ArgInfo(ArgInfo) {}
+      : ArgumentFailureTracker(cs, callee, args, params,
+                               unlabeledTrailingClosureArgIndex, locator),
+        ArgInfo(ArgInfo) {}
 
   Optional<unsigned> missingArgument(unsigned paramIdx,
                                      unsigned argInsertIdx) override {
@@ -1666,14 +1716,16 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
     if (cs.isForCodeCompletion()) {
       if (auto completionInfo = getCompletionArgInfo(locator.getAnchor(), cs)) {
         listener = std::make_unique<CompletionArgumentTracker>(
-            cs, argsWithLabels, params, locator, *completionInfo);
+            cs, callee, argsWithLabels, params,
+            argList->getFirstTrailingClosureIndex(), locator, *completionInfo);
       }
     }
     if (!listener) {
       // We didn't create an argument tracker for code completion. Create a
       // normal one.
-      listener = std::make_unique<ArgumentFailureTracker>(cs, argsWithLabels,
-                                                          params, locator);
+      listener = std::make_unique<ArgumentFailureTracker>(
+          cs, callee, argsWithLabels, params,
+          argList->getFirstTrailingClosureIndex(), locator);
     }
     auto callArgumentMatch = constraints::matchCallArguments(
         argsWithLabels, params, paramInfo,

test/Constraints/callAsFunction.swift

@@ -8,15 +8,23 @@
 protocol View {}
 struct EmptyView: View {}
 
+enum Align {
+case top, center, bottom
+}
+
 struct MyLayout {
+  init(alignment: Align? = .center, spacing: Double? = 0.0) {}
+
   func callAsFunction<V: View>(content: () -> V) -> MyLayout { .init() }
+  // expected-note@-1 {{where 'V' = 'Int'}}
   func callAsFunction<V: View>(answer: () -> Int,
                                content: () -> V) -> MyLayout { .init() }
+  // expected-note@-2 {{where 'V' = 'Int'}}
 }
 
 struct Test {
   var body1: MyLayout {
-    MyLayout() {
+    MyLayout(spacing: 1.0) {
       EmptyView() // Ok
     }
   }
@@ -28,6 +36,58 @@ struct Test {
       EmptyView() // Ok
     }
   }
+
+  var body3: MyLayout {
+    MyLayout(alignment: .top) {
+      let x = 42
+      return x
+    } content: {
+      EmptyView() // Ok
+    }
+  }
+
+  var body4: MyLayout {
+    MyLayout(spacing: 1.0) {
+      let x = 42
+      return x
+    } content: {
+      _ = 42
+      return EmptyView() // Ok
+    }
+  }
+
+  var body5: MyLayout {
+    MyLayout(alignment: .bottom, spacing: 1.0) {
+      42
+    } content: {
+      EmptyView() // Ok
+    }
+  }
+
+  var body6: MyLayout {
+    MyLayout(spacing: 1.0) {
+      _ = EmptyView()
+      return 42
+    } // expected-error {{instance method 'callAsFunction(content:)' requires that 'Int' conform to 'View'}}
+  }
+
+  var body7: MyLayout {
+    MyLayout(alignment: .center) {
+      42
+    } content: {
+      _ = EmptyView()
+      return 42
+    } // expected-error {{instance method 'callAsFunction(answer:content:)' requires that 'Int' conform to 'View'}}
+  }
+
+  var body8: MyLayout {
+    MyLayout {
+      let x = ""
+      return x // expected-error {{cannot convert return expression of type 'String' to return type 'Int'}}
+    } content: {
+      EmptyView()
+    }
+  }
 }
 
 // rdar://92912878 - filtering prevents disambiguation of `.callAsFunction`
@@ -51,3 +111,14 @@ func test_no_filtering_of_overloads() {
     }
   }
 }
+
+func test_default_arguments_do_not_interfere() {
+  struct S {
+    init(a: Int? = 42, b: String = "") {}
+    func callAsFunction(_: () -> Void) -> S { S() }
+  }
+
+  _ = S { _ = 42 }
+  _ = S(a: 42) { _ = 42 }
+  _ = S(b: "") { _ = 42 }
+}