[Constraint solver] Improve modeling of fix behavior.

Rather than re-using `DiagnosticBehavior` to describe how a fix should
act, introduce `FixBehavior` to cover the differences between (e.g.)
always-as-awarning and downgrade-to-warning. While here, split the
`isWarning` predicate into two different predicates:

* `canApplySolution`: Whether we can still apply a solution when it
contains this particular fix.
* `affectsSolutionScore`: Whether

These two predicates are currently tied together, because that's the
existing behavior, but we don't necessarily want them to stay that way.

Author: DougGregor

include/swift/Sema/CSFix.h

@@ -25,6 +25,7 @@
 #include "swift/AST/Types.h"
 #include "swift/Basic/Debug.h"
 #include "swift/Sema/ConstraintLocator.h"
+#include "swift/Sema/FixBehavior.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/TrailingObjects.h"
@@ -402,13 +403,12 @@ class ConstraintFix {
   ConstraintLocator *Locator;
 
   /// The behavior limit to apply to the diagnostics emitted.
-  DiagnosticBehavior behaviorLimit;
+  FixBehavior fixBehavior;
 
 public:
   ConstraintFix(ConstraintSystem &cs, FixKind kind, ConstraintLocator *locator,
-                DiagnosticBehavior behaviorLimit =
-                    DiagnosticBehavior::Unspecified)
-      : CS(cs), Kind(kind), Locator(locator), behaviorLimit(behaviorLimit) {}
+                FixBehavior fixBehavior = FixBehavior::Error)
+      : CS(cs), Kind(kind), Locator(locator), fixBehavior(fixBehavior) {}
 
   virtual ~ConstraintFix();
 
@@ -419,14 +419,36 @@ class ConstraintFix {
 
   FixKind getKind() const { return Kind; }
 
-  bool isWarning() const {
-    return behaviorLimit == DiagnosticBehavior::Warning ||
-           behaviorLimit == DiagnosticBehavior::Ignore;
+  /// Whether it is still possible to "apply" a solution containing this kind
+  /// of fix to get a usable AST.
+  bool canApplySolution() const {
+    switch (fixBehavior) {
+    case FixBehavior::AlwaysWarning:
+    case FixBehavior::DowngradeToWarning:
+    case FixBehavior::Suppress:
+      return true;
+
+    case FixBehavior::Error:
+      return false;
+    }
+  }
+
+  /// 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;
+    }
   }
 
   /// The diagnostic behavior limit that will be applied to any emitted
   /// diagnostics.
-  DiagnosticBehavior diagBehaviorLimit() const { return behaviorLimit; }
+  FixBehavior diagfixBehavior() const { return fixBehavior; }
 
   virtual std::string getName() const = 0;
 
@@ -672,16 +694,15 @@ class ContextualMismatch : public ConstraintFix {
 
   ContextualMismatch(ConstraintSystem &cs, Type lhs, Type rhs,
                      ConstraintLocator *locator,
-                     DiagnosticBehavior behaviorLimit)
-      : ConstraintFix(cs, FixKind::ContextualMismatch, locator, behaviorLimit),
+                     FixBehavior fixBehavior)
+      : ConstraintFix(cs, FixKind::ContextualMismatch, locator, fixBehavior),
         LHS(lhs), RHS(rhs) {}
 
 protected:
   ContextualMismatch(ConstraintSystem &cs, FixKind kind, Type lhs, Type rhs,
                      ConstraintLocator *locator,
-                     DiagnosticBehavior behaviorLimit =
-                         DiagnosticBehavior::Unspecified)
-      : ConstraintFix(cs, kind, locator, behaviorLimit), LHS(lhs), RHS(rhs) {}
+                     FixBehavior fixBehavior = FixBehavior::Error)
+      : ConstraintFix(cs, kind, locator, fixBehavior), LHS(lhs), RHS(rhs) {}
 
 public:
   std::string getName() const override { return "fix contextual mismatch"; }
@@ -766,9 +787,9 @@ class MarkExplicitlyEscaping final : public ContextualMismatch {
 class MarkGlobalActorFunction final : public ContextualMismatch {
   MarkGlobalActorFunction(ConstraintSystem &cs, Type lhs, Type rhs,
                           ConstraintLocator *locator,
-                          DiagnosticBehavior behaviorLimit)
+                          FixBehavior fixBehavior)
       : ContextualMismatch(cs, FixKind::MarkGlobalActorFunction, lhs, rhs,
-                           locator, behaviorLimit) {
+                           locator, fixBehavior) {
   }
 
 public:
@@ -778,7 +799,7 @@ class MarkGlobalActorFunction final : public ContextualMismatch {
 
   static MarkGlobalActorFunction *create(ConstraintSystem &cs, Type lhs,
                                          Type rhs, ConstraintLocator *locator,
-                                         DiagnosticBehavior behaviorLimit);
+                                         FixBehavior fixBehavior);
 
   static bool classof(ConstraintFix *fix) {
     return fix->getKind() == FixKind::MarkGlobalActorFunction;
@@ -814,9 +835,9 @@ class ForceOptional final : public ContextualMismatch {
 class AddSendableAttribute final : public ContextualMismatch {
   AddSendableAttribute(ConstraintSystem &cs, FunctionType *fromType,
                        FunctionType *toType, ConstraintLocator *locator,
-                       DiagnosticBehavior behaviorLimit)
+                       FixBehavior fixBehavior)
       : ContextualMismatch(cs, FixKind::AddSendableAttribute, fromType, toType,
-                           locator, behaviorLimit) {
+                           locator, fixBehavior) {
     assert(fromType->isSendable() != toType->isSendable());
   }
 
@@ -829,7 +850,7 @@ class AddSendableAttribute final : public ContextualMismatch {
                                       FunctionType *fromType,
                                       FunctionType *toType,
                                       ConstraintLocator *locator,
-                                      DiagnosticBehavior behaviorLimit);
+                                      FixBehavior fixBehavior);
 
   static bool classof(ConstraintFix *fix) {
     return fix->getKind() == FixKind::AddSendableAttribute;
@@ -1392,11 +1413,14 @@ class AllowTypeOrInstanceMember final : public AllowInvalidMemberRef {
 };
 
 class AllowInvalidPartialApplication final : public ConstraintFix {
+  bool isWarning;
+
   AllowInvalidPartialApplication(bool isWarning, ConstraintSystem &cs,
                                  ConstraintLocator *locator)
       : ConstraintFix(cs, FixKind::AllowInvalidPartialApplication, locator,
-                      isWarning ? DiagnosticBehavior::Warning
-                                : DiagnosticBehavior::Unspecified) {}
+                      isWarning ? FixBehavior::AlwaysWarning
+                                : FixBehavior::Error),
+        isWarning(isWarning) {}
 
 public:
   std::string getName() const override {
@@ -2130,10 +2154,9 @@ class AllowArgumentMismatch : public ContextualMismatch {
 
   AllowArgumentMismatch(ConstraintSystem &cs, FixKind kind, Type argType,
                         Type paramType, ConstraintLocator *locator,
-                        DiagnosticBehavior behaviorLimit =
-                            DiagnosticBehavior::Unspecified)
+                        FixBehavior fixBehavior = FixBehavior::Error)
       : ContextualMismatch(
-            cs, kind, argType, paramType, locator, behaviorLimit) {}
+            cs, kind, argType, paramType, locator, fixBehavior) {}
 
 public:
   std::string getName() const override {
@@ -2281,9 +2304,9 @@ class TreatEphemeralAsNonEphemeral final : public AllowArgumentMismatch {
   TreatEphemeralAsNonEphemeral(ConstraintSystem &cs, ConstraintLocator *locator,
                                Type srcType, Type dstType,
                                ConversionRestrictionKind conversionKind,
-                               DiagnosticBehavior behaviorLimit)
+                               FixBehavior fixBehavior)
       : AllowArgumentMismatch(cs, FixKind::TreatEphemeralAsNonEphemeral,
-                              srcType, dstType, locator, behaviorLimit),
+                              srcType, dstType, locator, fixBehavior),
         ConversionKind(conversionKind) {}
 
 public:
@@ -2447,7 +2470,7 @@ class AllowCoercionToForceCast final : public ContextualMismatch {
   AllowCoercionToForceCast(ConstraintSystem &cs, Type fromType, Type toType,
                            ConstraintLocator *locator)
       : ContextualMismatch(cs, FixKind::AllowCoercionToForceCast, fromType,
-                           toType, locator, DiagnosticBehavior::Warning) {}
+                           toType, locator, FixBehavior::AlwaysWarning) {}
 
 public:
   std::string getName() const override {
@@ -2561,7 +2584,7 @@ class SpecifyLabelToAssociateTrailingClosure final : public ConstraintFix {
   SpecifyLabelToAssociateTrailingClosure(ConstraintSystem &cs,
                                          ConstraintLocator *locator)
       : ConstraintFix(cs, FixKind::SpecifyLabelToAssociateTrailingClosure,
-                      locator, DiagnosticBehavior::Warning) {}
+                      locator, FixBehavior::AlwaysWarning) {}
 
 public:
   std::string getName() const override {
@@ -2731,7 +2754,7 @@ class SpecifyBaseTypeForOptionalUnresolvedMember final : public ConstraintFix {
                                              DeclNameRef memberName,
                                              ConstraintLocator *locator)
       : ConstraintFix(cs, FixKind::SpecifyBaseTypeForOptionalUnresolvedMember,
-                      locator, DiagnosticBehavior::Warning),
+                      locator, FixBehavior::AlwaysWarning),
         MemberName(memberName) {}
   DeclNameRef MemberName;
 
@@ -2762,7 +2785,7 @@ class CheckedCastContextualMismatchWarning : public ContextualMismatch {
                                        CheckedCastKind kind,
                                        ConstraintLocator *locator)
       : ContextualMismatch(cs, fixKind, fromType, toType, locator,
-                           DiagnosticBehavior::Warning),
+                           FixBehavior::AlwaysWarning),
         CastKind(kind) {}
   CheckedCastKind CastKind;
 };
@@ -2873,7 +2896,7 @@ class AllowTupleLabelMismatch final : public ContextualMismatch {
   AllowTupleLabelMismatch(ConstraintSystem &cs, Type fromType, Type toType,
                           ConstraintLocator *locator)
       : ContextualMismatch(cs, FixKind::AllowTupleLabelMismatch, fromType,
-                           toType, locator, DiagnosticBehavior::Warning) {}
+                           toType, locator, FixBehavior::AlwaysWarning) {}
 
 public:
   std::string getName() const override { return "allow tuple label mismatch"; }

include/swift/Sema/FixBehavior.h

@@ -0,0 +1,41 @@
+//===--- FixBehavior.h - Constraint Fix Behavior --------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides information about how a constraint fix should behavior.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SEMA_FIXBEHAVIOR_H
+#define SWIFT_SEMA_FIXBEHAVIOR_H
+
+namespace swift {
+namespace constraints {
+
+/// Describes the behavior of the diagnostic corresponding to a given fix.
+enum class FixBehavior {
+  /// The diagnostic is an error, and should prevent constraint application.
+  Error,
+  /// The diagnostic is always a warning, which should not prevent constraint
+  /// application.
+  AlwaysWarning,
+  /// The diagnostic should be downgraded to a warning, and not prevent
+  /// constraint application.
+  DowngradeToWarning,
+  /// The diagnostic should be suppressed, and not prevent constraint
+  /// application.
+  Suppress
+};
+
+}
+}
+
+#endif // SWIFT_SEMA_FIXBEHAVIOR_H

lib/Sema/CSApply.cpp

@@ -8480,7 +8480,7 @@ bool ConstraintSystem::applySolutionFixes(const Solution &solution) {
 
         auto diagnosed =
             primaryFix->coalesceAndDiagnose(solution, secondaryFixes);
-        if (primaryFix->isWarning()) {
+        if (primaryFix->canApplySolution()) {
           assert(diagnosed && "warnings should always be diagnosed");
           (void)diagnosed;
         } else {
@@ -9113,7 +9113,7 @@ Optional<SolutionApplicationTarget> ConstraintSystem::applySolution(
     // 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->isWarning();
+          return fix->canApplySolution();
         })) {
       // If we already diagnosed any errors via fixes, that's it.
       if (diagnosedErrorsViaFixes)

lib/Sema/CSDiagnostics.cpp

@@ -103,6 +103,22 @@ template <typename... ArgTypes>
 InFlightDiagnostic
 FailureDiagnostic::emitDiagnosticAt(ArgTypes &&... Args) const {
   auto &DE = getASTContext().Diags;
+  DiagnosticBehavior behaviorLimit;
+  switch (fixBehavior) {
+  case FixBehavior::Error:
+  case FixBehavior::AlwaysWarning:
+    behaviorLimit = DiagnosticBehavior::Unspecified;
+    break;
+
+  case FixBehavior::DowngradeToWarning:
+    behaviorLimit = DiagnosticBehavior::Warning;
+    break;
+
+  case FixBehavior::Suppress:
+    behaviorLimit = DiagnosticBehavior::Ignore;
+    break;
+  }
+
   return std::move(DE.diagnose(std::forward<ArgTypes>(Args)...)
                      .limitBehavior(behaviorLimit));
 }