[ConstraintSystem] Rank solutions based on overload choices only after fix diagnostics have failed

If there are multiple solutions with fixes, let's not try to rank
them based on overload choices because doing so would filter out
viable candidates.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -2719,6 +2719,11 @@ SolutionResult ConstraintSystem::salvage() {
     // Solve the system.
     solveImpl(viable);
 
+    // Before removing any "fixed" solutions, let's check
+    // if ambiguity is caused by fixes and diagnose if possible.
+    if (diagnoseAmbiguityWithFixes(viable))
+      return SolutionResult::forAmbiguous(viable);
+
     // Check whether we have a best solution; this can happen if we found
     // a series of fixes that worked.
     if (auto best = findBestSolution(viable, /*minimize=*/true)) {
@@ -2728,11 +2733,6 @@ SolutionResult ConstraintSystem::salvage() {
       return SolutionResult::forSolved(std::move(viable[0]));
     }
 
-    // Before removing any "fixed" solutions, let's check
-    // if ambiguity is caused by fixes and diagnose if possible.
-    if (diagnoseAmbiguityWithFixes(viable))
-      return SolutionResult::forAmbiguous(viable);
-
     // FIXME: If we were able to actually fix things along the way,
     // we may have to hunt for the best solution. For now, we don't care.
 
@@ -3071,7 +3071,7 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
 
   if (diagnoseConflictingGenericArguments(*this, solutionDiff, solutions))
     return true;
-  
+
   if (auto bestScore = solverState->BestScore) {
     solutions.erase(llvm::remove_if(solutions,
                                     [&](const Solution &solution) {
@@ -3087,6 +3087,9 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
       return false;
   }
 
+  if (solutions.size() < 2)
+    return false;
+
   if (diagnoseAmbiguityWithEphemeralPointers(*this, solutions))
     return true;