Replace CSFix::canApplySolution() with isFatal().

Author: DougGregor

include/swift/Sema/CSFix.h

@@ -428,17 +428,17 @@ class ConstraintFix {
 
   FixKind getKind() const { return Kind; }
 
-  /// Whether it is still possible to "apply" a solution containing this kind
-  /// of fix to get a usable AST.
-  bool canApplySolution() const {
+  /// Whether this fix fatal for the constraint solver, meaning that it cannot
+  /// produce a usable type-checked AST.
+  bool isFatal() const {
     switch (fixBehavior) {
     case FixBehavior::AlwaysWarning:
     case FixBehavior::DowngradeToWarning:
     case FixBehavior::Suppress:
-      return true;
+      return false;
 
     case FixBehavior::Error:
-      return false;
+      return true;
     }
   }
 

lib/Sema/CSApply.cpp

@@ -8467,7 +8467,7 @@ bool ConstraintSystem::applySolutionFixes(const Solution &solution) {
 
         auto diagnosed =
             primaryFix->coalesceAndDiagnose(solution, secondaryFixes);
-        if (primaryFix->canApplySolution()) {
+        if (!primaryFix->isFatal()) {
           assert(diagnosed && "warnings should always be diagnosed");
           (void)diagnosed;
         } else {
@@ -9127,9 +9127,9 @@ Optional<SolutionApplicationTarget> ConstraintSystem::applySolution(
     bool diagnosedErrorsViaFixes = applySolutionFixes(solution);
     bool canApplySolution = true;
     for (const auto fix : solution.Fixes) {
-      if (!fix->canApplySolution())
+      if (fix->isFatal())
         canApplySolution = false;
-      if (fix->affectsSolutionScore() == SK_Fix && fix->canApplySolution())
+      if (fix->affectsSolutionScore() == SK_Fix && !fix->isFatal())
         ++numResolvableFixes;
     }
 

lib/Sema/ConstraintSystem.cpp

@@ -4616,7 +4616,7 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
   // let's diagnose this as regular ambiguity.
   if (llvm::all_of(solutions, [](const Solution &solution) {
         return llvm::all_of(solution.Fixes, [](const ConstraintFix *fix) {
-          return fix->canApplySolution();
+          return !fix->isFatal();
         });
       })) {
     return diagnoseAmbiguity(solutions);