[Diagnostics] Diagnose re-labeling failures in ambiguity conditions

If all solutions point to the same overload choice that needs
re-labeling it's safe to diagnose it as if there was no ambiguity
because the call site is static.

(cherry picked from commit f3ff87b)

Author: xedin

include/swift/Sema/CSFix.h

@@ -528,6 +528,8 @@ class RelabelArguments final
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
+
   static RelabelArguments *create(ConstraintSystem &cs,
                                   llvm::ArrayRef<Identifier> correctLabels,
                                   ConstraintLocator *locator);

lib/Sema/CSFix.cpp

@@ -242,6 +242,38 @@ bool RelabelArguments::diagnose(const Solution &solution, bool asNote) const {
   return failure.diagnose(asNote);
 }
 
+bool RelabelArguments::diagnoseForAmbiguity(
+    CommonFixesArray commonFixes) const {
+  SmallPtrSet<ValueDecl *, 4> overloadChoices;
+
+  // First, let's find overload choice associated with each
+  // re-labeling fix.
+  for (const auto &fix : commonFixes) {
+    auto &solution = *fix.first;
+
+    auto calleeLocator = solution.getCalleeLocator(getLocator());
+    if (!calleeLocator)
+      return false;
+
+    auto overloadChoice = solution.getOverloadChoiceIfAvailable(calleeLocator);
+    if (!overloadChoice)
+      return false;
+
+    auto *decl = overloadChoice->choice.getDeclOrNull();
+    if (!decl)
+      return false;
+
+    (void)overloadChoices.insert(decl);
+  }
+
+  // If all of the fixes point to the same overload choice then it's
+  // exactly the same issue since the call site is static.
+  if (overloadChoices.size() == 1)
+    return diagnose(*commonFixes.front().first);
+
+  return false;
+}
+
 RelabelArguments *
 RelabelArguments::create(ConstraintSystem &cs,
                          llvm::ArrayRef<Identifier> correctLabels,

test/Constraints/closures.swift

@@ -1170,7 +1170,6 @@ func test(arr: [[Int]]) {
   }
 
   arr.map { ($0 as? [Int]).map { A($0) } } // expected-error {{missing argument label 'arg:' in call}} {{36-36=arg: }}
-  // expected-warning@-1 {{conditional cast from '[Int]' to '[Int]' always succeeds}}
 }
 
 func closureWithCaseArchetype<T>(_: T.Type) {

test/Constraints/generics.swift

@@ -902,7 +902,8 @@ func rdar78781552() {
     // expected-error@-1 {{generic struct 'Test' requires that '(((Int) throws -> Bool) throws -> [Int])?' conform to 'RandomAccessCollection'}}
     // expected-error@-2 {{generic parameter 'Content' could not be inferred}} expected-note@-2 {{explicitly specify the generic arguments to fix this issue}}
     // expected-error@-3 {{cannot convert value of type '(((Int) throws -> Bool) throws -> [Int])?' to expected argument type '[(((Int) throws -> Bool) throws -> [Int])?]'}}
-    // expected-error@-4 {{missing argument for parameter 'filter' in call}}
+    // expected-error@-4 {{missing argument label 'data:' in call}}
+    // expected-error@-5 {{missing argument for parameter 'filter' in call}}
   }
 }
 

test/expr/unary/keypath/keypath.swift

@@ -507,6 +507,7 @@ func testLabeledSubscript() {
   // TODO: These ought to work without errors.
   let _ = \AA.[keyPath: k]
   // expected-error@-1 {{cannot convert value of type 'KeyPath<AA, Int>' to expected argument type 'Int'}}
+  // expected-error@-2 {{extraneous argument label 'keyPath:' in call}}
 
   let _ = \AA.[keyPath: \AA.[labeled: 0]] // expected-error {{extraneous argument label 'keyPath:' in call}}
   // expected-error@-1 {{cannot convert value of type 'KeyPath<AA, Int>' to expected argument type 'Int'}}