[CSSolver] Attempt disabled disjunction choices only if they have a fix

This draws a distinction between disabled choices with and without
fixes - former is disabled because it's unrelated, latter because
it only makes sense to attempt it during "salvage" mode while trying
to produce diagnostics.

Author: xedin

lib/Sema/CSSolver.cpp

@@ -1305,8 +1305,10 @@ ConstraintSystem::filterDisjunction(
   for (unsigned constraintIdx : indices(disjunction->getNestedConstraints())) {
     auto constraint = disjunction->getNestedConstraints()[constraintIdx];
 
-    // Skip already-disabled constraints.
-    if (constraint->isDisabled())
+    // Skip already-disabled constraints. Let's treat disabled
+    // choices which have a fix as "enabled" ones here, so we can
+    // potentially infer some type information from them.
+    if (constraint->isDisabled() && !constraint->getFix())
       continue;
 
     if (pred(constraint)) {

lib/Sema/CSStep.cpp

@@ -430,7 +430,9 @@ StepResult DisjunctionStep::resume(bool prevFailed) {
 bool DisjunctionStep::shouldSkip(const DisjunctionChoice &choice) const {
   auto &ctx = CS.getASTContext();
 
-  if (choice.isDisabled()) {
+  bool attemptFixes = CS.shouldAttemptFixes();
+  // Enable "fixed" overload choices in "diagnostic" mode.
+  if (!(attemptFixes && choice.hasFix()) && choice.isDisabled()) {
     if (isDebugMode()) {
       auto &log = getDebugLogger();
       log << "(skipping ";
@@ -442,7 +444,7 @@ bool DisjunctionStep::shouldSkip(const DisjunctionChoice &choice) const {
   }
 
   // Skip unavailable overloads unless solver is in the "diagnostic" mode.
-  if (!CS.shouldAttemptFixes() && choice.isUnavailable())
+  if (!attemptFixes && choice.isUnavailable())
     return true;
 
   if (ctx.LangOpts.DisableConstraintSolverPerformanceHacks)