Sema: Fix crash with ambiguous overload inside a trailing closure

Any ambiguity inside of a call with a trailing closure
call was going down the code path that would tell you to
add an argument label... but the ambiguity might not be
with the call that has the trailing closure itself, and
instead something inside.

Author: slavapestov

lib/Sema/CSDiag.cpp

@@ -325,13 +325,17 @@ static DeclName getOverloadChoiceName(ArrayRef<OverloadChoice> choices) {
 
 /// Returns true if any diagnostics were emitted.
 static bool
-tryDiagnoseTrailingClosureAmbiguity(TypeChecker &tc, const Expr *expr,
+tryDiagnoseTrailingClosureAmbiguity(TypeChecker &tc,
+                                    const Expr *expr,
+                                    const Expr *anchor,
                                     ArrayRef<OverloadChoice> choices) {
   auto *callExpr = dyn_cast<CallExpr>(expr);
   if (!callExpr)
     return false;
   if (!callExpr->hasTrailingClosure())
     return false;
+  if (callExpr->getFn() != anchor)
+    return false;
 
   llvm::SmallMapVector<Identifier, const ValueDecl *, 8> choicesByLabel;
   for (const OverloadChoice &choice : choices) {
@@ -465,7 +469,7 @@ static bool diagnoseAmbiguity(ConstraintSystem &cs,
                                   : diag::ambiguous_decl_ref,
                 name);
 
-    if (tryDiagnoseTrailingClosureAmbiguity(tc, expr, overload.choices))
+    if (tryDiagnoseTrailingClosureAmbiguity(tc, expr, anchor, overload.choices))
       return true;
 
     // Emit candidates.  Use a SmallPtrSet to make sure only emit a particular

test/Constraints/diagnostics.swift

@@ -931,3 +931,12 @@ let _ = (r29850459() ? r29850459_a : r29850459_b) + 42.0 // expected-error {{bin
 // expected-note@-1 {{overloads for '+' exist with these partially matching parameter lists: (Double, Double), (Int, Int), (Int, UnsafeMutablePointer<Pointee>), (Int, UnsafePointer<Pointee>)}}
 let _ = ((r29850459_flag || r29850459()) ? r29850459_a : r29850459_b) + 42.0 // expected-error {{binary operator '+' cannot be applied to operands of type 'Int' and 'Double'}}
 // expected-note@-1 {{overloads for '+' exist with these partially matching parameter lists: (Double, Double), (Int, Int), (Int, UnsafeMutablePointer<Pointee>), (Int, UnsafePointer<Pointee>)}}
+
+// Ambiguous overload inside a trailing closure
+
+func ambiguousCall() -> Int {} // expected-note {{found this candidate}}
+func ambiguousCall() -> Float {} // expected-note {{found this candidate}}
+
+func takesClosure(fn: () -> ()) {}
+
+takesClosure() { ambiguousCall() } // expected-error {{ambiguous use of 'ambiguousCall()'}}