[ConstraintSystem] Overhaul ambiguity diagnostics

include/swift/AST/DiagnosticsSema.def

@@ -253,6 +253,10 @@ ERROR(no_candidates_match_result_type,none,
       "no '%0' candidates produce the expected contextual result type %1",
       (StringRef, Type))
 
+ERROR(no_candidates_match_argument_type,none,
+      "no '%0' candidates produce the expected type %1 for parameter #%2",
+      (StringRef, Type, unsigned))
+
 ERROR(cannot_infer_closure_parameter_type,none,
       "unable to infer type of a closure parameter %0 in the current context",
        (StringRef))

lib/Sema/CSFix.cpp

@@ -252,6 +252,33 @@ bool ContextualMismatch::diagnose(const Solution &solution, bool asNote) const {
   return failure.diagnose(asNote);
 }
 
+bool ContextualMismatch::diagnoseForAmbiguity(
+    CommonFixesArray commonFixes) const {
+  auto getTypes =
+      [&](const std::pair<const Solution *, const ConstraintFix *> &entry)
+      -> std::pair<Type, Type> {
+    auto &solution = *entry.first;
+    auto *fix = static_cast<const ContextualMismatch *>(entry.second);
+
+    return {solution.simplifyType(fix->getFromType()),
+            solution.simplifyType(fix->getToType())};
+  };
+
+  auto etalonTypes = getTypes(commonFixes.front());
+  if (llvm::all_of(
+          commonFixes,
+          [&](const std::pair<const Solution *, const ConstraintFix *> &entry) {
+            auto types = getTypes(entry);
+            return etalonTypes.first->isEqual(types.first) &&
+                   etalonTypes.second->isEqual(types.second);
+          })) {
+    const auto &primary = commonFixes.front();
+    return primary.second->diagnose(*primary.first, /*asNote=*/false);
+  }
+
+  return false;
+}
+
 ContextualMismatch *ContextualMismatch::create(ConstraintSystem &cs, Type lhs,
                                                Type rhs,
                                                ConstraintLocator *locator) {
@@ -381,6 +408,57 @@ bool AllowFunctionTypeMismatch::diagnose(const Solution &solution,
   return coalesceAndDiagnose(solution, {}, asNote);
 }
 
+bool AllowFunctionTypeMismatch::diagnoseForAmbiguity(
+    CommonFixesArray commonFixes) const {
+  if (ContextualMismatch::diagnoseForAmbiguity(commonFixes))
+    return true;
+
+  auto *locator = getLocator();
+  // If this is a mismatch between two function types at argument
+  // position, there is a tailored diagnostic for that.
+  if (auto argConv =
+          locator->getLastElementAs<LocatorPathElt::ApplyArgToParam>()) {
+    auto &cs = getConstraintSystem();
+    auto &DE = cs.getASTContext().Diags;
+    auto &solution = *commonFixes[0].first;
+
+    auto info = getStructuralTypeContext(solution, locator);
+    if (!info)
+      return false;
+
+    auto *argLoc = cs.getConstraintLocator(simplifyLocatorToAnchor(locator));
+
+    auto overload = solution.getOverloadChoiceIfAvailable(
+        solution.getCalleeLocator(argLoc));
+    if (!overload)
+      return false;
+
+    auto name = overload->choice.getName().getBaseName();
+    DE.diagnose(getLoc(getAnchor()), diag::no_candidates_match_argument_type,
+                name.userFacingName(), std::get<2>(*info),
+                argConv->getParamIdx());
+
+    for (auto &entry : commonFixes) {
+      auto &solution = *entry.first;
+      auto overload = solution.getOverloadChoiceIfAvailable(
+          solution.getCalleeLocator(argLoc));
+
+      if (!(overload && overload->choice.isDecl()))
+        continue;
+
+      auto *decl = overload->choice.getDecl();
+      if (decl->getLoc().isValid()) {
+        DE.diagnose(decl, diag::found_candidate_type,
+                    solution.simplifyType(overload->openedType));
+      }
+    }
+
+    return true;
+  }
+
+  return false;
+}
+
 AllowFunctionTypeMismatch *
 AllowFunctionTypeMismatch::create(ConstraintSystem &cs, Type lhs, Type rhs,
                                   ConstraintLocator *locator, unsigned index) {
@@ -867,14 +945,19 @@ RemoveAddressOf *RemoveAddressOf::create(ConstraintSystem &cs, Type lhs, Type rh
   return new (cs.getAllocator()) RemoveAddressOf(cs, lhs, rhs, locator);
 }
 
+RemoveReturn::RemoveReturn(ConstraintSystem &cs, Type resultTy,
+                           ConstraintLocator *locator)
+    : ContextualMismatch(cs, FixKind::RemoveReturn, resultTy,
+                         cs.getASTContext().TheEmptyTupleType, locator) {}
+
 bool RemoveReturn::diagnose(const Solution &solution, bool asNote) const {
   ExtraneousReturnFailure failure(solution, getLocator());
   return failure.diagnose(asNote);
 }
 
-RemoveReturn *RemoveReturn::create(ConstraintSystem &cs,
+RemoveReturn *RemoveReturn::create(ConstraintSystem &cs, Type resultTy,
                                    ConstraintLocator *locator) {
-  return new (cs.getAllocator()) RemoveReturn(cs, locator);
+  return new (cs.getAllocator()) RemoveReturn(cs, resultTy, locator);
 }
 
 bool CollectionElementContextualMismatch::diagnose(const Solution &solution,
@@ -1079,6 +1162,43 @@ bool IgnoreAssignmentDestinationType::diagnose(const Solution &solution,
   return failure.diagnose(asNote);
 }
 
+bool IgnoreAssignmentDestinationType::diagnoseForAmbiguity(
+    CommonFixesArray commonFixes) const {
+  auto &cs = getConstraintSystem();
+
+  // If all of the types are the same let's try to diagnose
+  // this as if there is no ambiguity.
+  if (ContextualMismatch::diagnoseForAmbiguity(commonFixes))
+    return true;
+
+  auto *commonLocator = getLocator();
+  auto *assignment = castToExpr<AssignExpr>(commonLocator->getAnchor());
+
+  auto &solution = *commonFixes.front().first;
+  auto *calleeLocator = solution.getCalleeLocator(
+      solution.getConstraintLocator(assignment->getSrc()));
+  auto overload = solution.getOverloadChoiceIfAvailable(calleeLocator);
+  if (!overload)
+    return false;
+
+  auto memberName = overload->choice.getName().getBaseName();
+  auto destType = solution.getType(assignment->getDest());
+
+  auto &DE = cs.getASTContext().Diags;
+  // TODO(diagnostics): It might be good to add a tailored diagnostic
+  // for cases like this instead of using "contextual" one.
+  DE.diagnose(assignment->getSrc()->getLoc(),
+              diag::no_candidates_match_result_type,
+              memberName.userFacingName(),
+              solution.simplifyType(destType)->getRValueType());
+
+  for (auto &entry : commonFixes) {
+    entry.second->diagnose(*entry.first, /*asNote=*/true);
+  }
+
+  return true;
+}
+
 IgnoreAssignmentDestinationType *
 IgnoreAssignmentDestinationType::create(ConstraintSystem &cs, Type sourceTy,
                                         Type destTy,

lib/Sema/CSFix.h

@@ -369,6 +369,10 @@ class TreatRValueAsLValue final : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static TreatRValueAsLValue *create(ConstraintSystem &cs,
                                      ConstraintLocator *locator);
 };
@@ -522,6 +526,8 @@ class ContextualMismatch : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
+
   static ContextualMismatch *create(ConstraintSystem &cs, Type lhs, Type rhs,
                                     ConstraintLocator *locator);
 };
@@ -798,6 +804,10 @@ class UsePropertyWrapper final : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static UsePropertyWrapper *create(ConstraintSystem &cs, VarDecl *wrapped,
                                     bool usingStorageWrapper, Type base,
                                     Type wrapper, ConstraintLocator *locator);
@@ -825,6 +835,10 @@ class UseWrappedValue final : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static UseWrappedValue *create(ConstraintSystem &cs, VarDecl *propertyWrapper,
                                  Type base, Type wrapper,
                                  ConstraintLocator *locator);
@@ -1103,6 +1117,8 @@ class AllowFunctionTypeMismatch final : public ContextualMismatch {
                            bool asNote = false) const override;
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
 };
 
 
@@ -1399,16 +1415,16 @@ class AllowInvalidRefInKeyPath final : public ConstraintFix {
                                           ConstraintLocator *locator);
 };
 
-class RemoveReturn final : public ConstraintFix {
-  RemoveReturn(ConstraintSystem &cs, ConstraintLocator *locator)
-      : ConstraintFix(cs, FixKind::RemoveReturn, locator) {}
+class RemoveReturn final : public ContextualMismatch {
+  RemoveReturn(ConstraintSystem &cs, Type resultTy, ConstraintLocator *locator);
 
 public:
   std::string getName() const override { return "remove or omit return type"; }
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
-  static RemoveReturn *create(ConstraintSystem &cs, ConstraintLocator *locator);
+  static RemoveReturn *create(ConstraintSystem &cs, Type resultTy,
+                              ConstraintLocator *locator);
 };
 
 class CollectionElementContextualMismatch final : public ContextualMismatch {
@@ -1527,10 +1543,6 @@ class IgnoreContextualType : public ContextualMismatch {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
-  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
-    return diagnose(*commonFixes.front().first);
-  }
-
   static IgnoreContextualType *create(ConstraintSystem &cs, Type resultTy,
                                       Type specifiedTy,
                                       ConstraintLocator *locator);
@@ -1548,6 +1560,8 @@ class IgnoreAssignmentDestinationType final : public ContextualMismatch {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override;
+
   static IgnoreAssignmentDestinationType *create(ConstraintSystem &cs,
                                                  Type sourceTy, Type destTy,
                                                  ConstraintLocator *locator);

lib/Sema/CSSimplify.cpp

@@ -1449,6 +1449,20 @@ assessRequirementFailureImpact(ConstraintSystem &cs, Type requirementType,
   if (!anchor)
     return impact;
 
+  // If this is a conditional requirement failure associated with a
+  // call, let's increase impact of the fix to show that such failure
+  // makes member/function unreachable in current context or with
+  // given arguments.
+  if (auto last = locator.last()) {
+    if (last->isConditionalRequirement()) {
+      if (auto *expr = getAsExpr(anchor)) {
+        auto *parent = cs.getParentExpr(expr);
+        if (parent && isa<ApplyExpr>(parent))
+          return 5;
+      }
+    }
+  }
+
   // If this requirement is associated with a member reference and it
   // was possible to check it before overload choice is bound, that means
   // types came from the context (most likely Self, or associated type(s))
@@ -1475,17 +1489,21 @@ assessRequirementFailureImpact(ConstraintSystem &cs, Type requirementType,
       return 10;
   }
 
-  // Increase the impact of a conformance fix for a standard library type,
-  // as it's unlikely to be a good suggestion. Also do the same for the builtin
-  // compiler types Any and AnyObject, which cannot conform to protocols.
+  // Increase the impact of a conformance fix for a standard library
+  // or foundation type, as it's unlikely to be a good suggestion.
+  //
+  // Also do the same for the builtin compiler types Any and AnyObject,
+  // which cannot conform to protocols.
+  //
   // FIXME: We ought not to have the is<TypeVariableType>() condition here, but
   // removing it currently regresses the diagnostic for the test case for
   // rdar://60727310. Once we better handle the separation of conformance fixes
   // from argument mismatches in cases like SR-12438, we should be able to
   // remove it from the condition.
   auto resolvedTy = cs.simplifyType(requirementType);
   if ((requirementType->is<TypeVariableType>() && resolvedTy->isStdlibType()) ||
-      resolvedTy->isAny() || resolvedTy->isAnyObject()) {
+      resolvedTy->isAny() || resolvedTy->isAnyObject() ||
+      getKnownFoundationEntity(resolvedTy->getString())) {
     if (auto last = locator.last()) {
       if (auto requirement = last->getAs<LocatorPathElt::AnyRequirement>()) {
         auto kind = requirement->getRequirementKind();
@@ -3863,11 +3881,16 @@ bool ConstraintSystem::repairFailures(
     if (lhs->isTypeVariableOrMember() || rhs->isTypeVariableOrMember())
       break;
 
+    // If there is already a fix for contextual failure, let's not
+    // record a duplicate one.
+    if (hasFixFor(getConstraintLocator(locator)))
+      return true;
+
     auto purpose = getContextualTypePurpose(anchor);
     if (rhs->isVoid() &&
         (purpose == CTP_ReturnStmt || purpose == CTP_ReturnSingleExpr)) {
       conversionsOrFixes.push_back(
-          RemoveReturn::create(*this, getConstraintLocator(locator)));
+          RemoveReturn::create(*this, lhs, getConstraintLocator(locator)));
       return true;
     }