[Trailing closures] Attempt both forward and backward scans.

To better preserve source compatibility, teach the constraint
solver to try both the new forward scanning rule as well as the
backward scanning rule when matching a single, unlabeled trailing
closure. In the extreme case, where the unlabeled trailing closure
matches different parameters with the different rules, and yet both
produce a potential match, introduce a disjunction to explore both
possibilities.

Prefer solutions that involve forward scans to those that involve
backward scans, so we only use the backward scan as a fallback.

Author: DougGregor

include/swift/AST/Attr.h

@@ -55,7 +55,6 @@ class GenericFunctionType;
 class LazyConformanceLoader;
 class LazyMemberLoader;
 class PatternBindingInitializer;
-enum class TrailingClosureMatching: uint8_t;
 class TrailingWhereClause;
 
 /// TypeAttributes - These are attributes that may be applied to types.

lib/AST/Expr.cpp

@@ -1271,15 +1271,29 @@ SourceRange TupleExpr::getSourceRange() const {
       return { SourceLoc(), SourceLoc() };
     } else {
       // Scan backwards for a valid source loc.
+      bool hasSingleTrailingClosure = hasTrailingClosure();
       for (Expr *expr : llvm::reverse(getElements())) {
         // Default arguments are located at the start of their parent tuple, so
         // skip over them.
         if (isa<DefaultArgumentExpr>(expr))
           continue;
-        end = expr->getEndLoc();
-        if (end.isValid()) {
-          break;
+
+        SourceLoc newEnd = expr->getEndLoc();
+        if (newEnd.isInvalid())
+          continue;
+
+        // There is a quirk with the backward scan logic for trailing
+        // closures that can cause arguments to be flipped. If there is a
+        // single trailing closure, only stop when the "end" point we hit comes
+        // after the close parenthesis (if there is one).
+        if (end.isInvalid() ||
+            end.getOpaquePointerValue() < newEnd.getOpaquePointerValue()) {
+          end = newEnd;
         }
+
+        if (!hasSingleTrailingClosure || RParenLoc.isInvalid() ||
+            RParenLoc.getOpaquePointerValue() < end.getOpaquePointerValue())
+          break;
       }
     }
   } else {

lib/Sema/CSApply.cpp

@@ -5754,19 +5754,34 @@ Expr *ExprRewriter::coerceCallArguments(
   AnyFunctionType::relabelParams(args, argLabels);
 
   MatchCallArgumentListener listener;
-  SmallVector<ParamBinding, 4> parameterBindings;
   auto unlabeledTrailingClosureIndex =
       arg->getUnlabeledTrailingClosureIndexOfPackedArgument();
-  bool failed = constraints::matchCallArguments(args, params,
-                                                paramInfo,
-                                                unlabeledTrailingClosureIndex,
-                                                /*allowFixes=*/false, listener,
-                                                parameterBindings);
-
-  assert((matchCanFail || !failed) && "Call arguments did not match up?");
-  (void)failed;
+
+  // Determine the trailing closure matching rule that was applied. This
+  // is only relevant for explicit calls and subscripts.
+  auto trailingClosureMatching = TrailingClosureMatching::Forward;
+  {
+    SmallVector<LocatorPathElt, 4> path;
+    auto anchor = locator.getLocatorParts(path);
+    if (!path.empty() && path.back().is<LocatorPathElt::ApplyArgument>() &&
+        (anchor.isExpr(ExprKind::Call) || anchor.isExpr(ExprKind::Subscript))) {
+      auto locatorPtr = cs.getConstraintLocator(locator);
+      assert(solution.trailingClosureMatchingChoices.count(locatorPtr) == 1);
+      trailingClosureMatching = solution.trailingClosureMatchingChoices.find(
+          locatorPtr)->second;
+    }
+  }
+
+  auto callArgumentMatch = constraints::matchCallArguments(
+      args, params, paramInfo, unlabeledTrailingClosureIndex,
+      /*allowFixes=*/false, listener, trailingClosureMatching);
+
+  assert((matchCanFail || callArgumentMatch) &&
+         "Call arguments did not match up?");
   (void)matchCanFail;
 
+  auto parameterBindings = std::move(callArgumentMatch->parameterBindings);
+
   // We should either have parentheses or a tuple.
   auto *argTuple = dyn_cast<TupleExpr>(arg);
   auto *argParen = dyn_cast<ParenExpr>(arg);

lib/Sema/CSRanking.cpp

@@ -49,6 +49,10 @@ void ConstraintSystem::increaseScore(ScoreKind kind, unsigned value) {
       log << "use of an unavailable declaration";
       break;
 
+    case SK_BackwardTrailingClosure:
+      llvm::errs() << "backward scan when matching a trailing closure";
+      break;
+
     case SK_Fix:
       log << "attempting to fix the source";
       break;