[Constraint solver] Treat downgraded errors as "disfavored overloads".

This allows us to still maintain them in the score kind, but not treat
them as being as severe as an error requiring a fix.

Author: DougGregor

include/swift/Sema/CSFix.h

@@ -46,6 +46,7 @@ class ConstraintSystem;
 class ConstraintLocator;
 class ConstraintLocatorBuilder;
 enum class ConversionRestrictionKind;
+enum ScoreKind: unsigned int;
 class Solution;
 struct MemberLookupResult;
 
@@ -433,18 +434,9 @@ class ConstraintFix {
     }
   }
 
-  /// Whether this kind of fix affects the solution score.
-  bool affectsSolutionScore() const {
-    switch (fixBehavior) {
-    case FixBehavior::AlwaysWarning:
-    case FixBehavior::DowngradeToWarning:
-    case FixBehavior::Suppress:
-      return false;
-
-    case FixBehavior::Error:
-      return true;
-    }
-  }
+  /// Whether this kind of fix affects the solution score, and which score
+  /// it affects.
+  Optional<ScoreKind> affectsSolutionScore() const;
 
   /// The diagnostic behavior limit that will be applied to any emitted
   /// diagnostics.

include/swift/Sema/ConstraintSystem.h

@@ -775,7 +775,7 @@ class FunctionArgApplyInfo {
 
 /// Describes an aspect of a solution that affects its overall score, i.e., a
 /// user-defined conversions.
-enum ScoreKind {
+enum ScoreKind: unsigned int {
   // These values are used as indices into a Score value.
 
   /// A fix needs to be applied to the source.

lib/Sema/CSApply.cpp

@@ -9105,16 +9105,23 @@ Optional<SolutionApplicationTarget> ConstraintSystem::applySolution(
     Solution &solution, SolutionApplicationTarget target) {
   // If any fixes needed to be applied to arrive at this solution, resolve
   // them to specific expressions.
+  unsigned numResolvableFixes = 0;
   if (!solution.Fixes.empty()) {
     if (shouldSuppressDiagnostics())
       return None;
 
     bool diagnosedErrorsViaFixes = applySolutionFixes(solution);
+    bool canApplySolution = true;
+    for (const auto fix : solution.Fixes) {
+      if (!fix->canApplySolution())
+        canApplySolution = false;
+      if (fix->affectsSolutionScore() == SK_Fix && fix->canApplySolution())
+        ++numResolvableFixes;
+    }
+
     // If all of the available fixes would result in a warning,
     // we can go ahead and apply this solution to AST.
-    if (!llvm::all_of(solution.Fixes, [](const ConstraintFix *fix) {
-          return fix->canApplySolution();
-        })) {
+    if (!canApplySolution) {
       // If we already diagnosed any errors via fixes, that's it.
       if (diagnosedErrorsViaFixes)
         return None;
@@ -9131,7 +9138,7 @@ Optional<SolutionApplicationTarget> ConstraintSystem::applySolution(
   // produce a fallback diagnostic to highlight the problem.
   {
     const auto &score = solution.getFixedScore();
-    if (score.Data[SK_Fix] > 0 || score.Data[SK_Hole] > 0) {
+    if (score.Data[SK_Fix] > numResolvableFixes || score.Data[SK_Hole] > 0) {
       maybeProduceFallbackDiagnostic(target);
       return None;
     }

lib/Sema/CSFix.cpp

@@ -37,6 +37,20 @@ using namespace constraints;
 
 ConstraintFix::~ConstraintFix() {}
 
+Optional<ScoreKind> ConstraintFix::affectsSolutionScore() const {
+  switch (fixBehavior) {
+  case FixBehavior::AlwaysWarning:
+    return None;
+
+  case FixBehavior::Error:
+    return SK_Fix;
+
+  case FixBehavior::DowngradeToWarning:
+  case FixBehavior::Suppress:
+    return SK_DisfavoredOverload;
+  }
+}
+
 ASTNode ConstraintFix::getAnchor() const { return getLocator()->getAnchor(); }
 
 void ConstraintFix::print(llvm::raw_ostream &Out) const {

lib/Sema/CSSimplify.cpp

@@ -12706,8 +12706,8 @@ bool ConstraintSystem::recordFix(ConstraintFix *fix, unsigned impact) {
   // Record the fix.
 
   // If this should affect the solution score, do so.
-  if (fix->affectsSolutionScore())
-    increaseScore(SK_Fix, impact);
+  if (auto scoreKind = fix->affectsSolutionScore())
+    increaseScore(*scoreKind, impact);
 
   // If we've made the current solution worse than the best solution we've seen
   // already, stop now.