[ConstraintSystem] Add a new transitive conformance constraint

Conformance constraints could be transferred through conversions,
but that would also require checking implicit conversions
such as optional and pointer promotions for conformance is the
type itself doesn't conform, for that let's add a special constraint
`TransitivelyConformsTo`.

Author: xedin

include/swift/Sema/Constraint.h

@@ -190,6 +190,11 @@ enum class ConstraintKind : char {
   /// The first type is a property wrapper with a wrapped-value type
   /// equal to the second type.
   PropertyWrapper,
+  /// The first type (or its optional or pointer version) must conform to a
+  /// second type (protocol type). This is not a direct requirement but one
+  /// inferred from a conversion, so the check is more relax comparing to
+  /// `ConformsTo`.
+  TransitivelyConformsTo,
 };
 
 /// Classification of the different kinds of constraints.
@@ -579,6 +584,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
     case ConstraintKind::OperatorArgumentConversion:
     case ConstraintKind::ConformsTo:
     case ConstraintKind::LiteralConformsTo:
+    case ConstraintKind::TransitivelyConformsTo:
     case ConstraintKind::CheckedCast:
     case ConstraintKind::SelfObjectOfProtocol:
     case ConstraintKind::ApplicableFunction:

include/swift/Sema/ConstraintSystem.h

@@ -4513,6 +4513,12 @@ class ConstraintSystem {
                                             ConstraintLocatorBuilder locator,
                                             TypeMatchOptions flags);
 
+  /// Similar to \c simplifyConformsToConstraint but also checks for
+  /// optional and pointer derived a given type.
+  SolutionKind simplifyTransitivelyConformsTo(Type type, Type protocol,
+                                              ConstraintLocatorBuilder locator,
+                                              TypeMatchOptions flags);
+
   /// Attempt to simplify a checked-cast constraint.
   SolutionKind simplifyCheckedCastConstraint(Type fromType, Type toType,
                                              TypeMatchOptions flags,

lib/Sema/CSBindings.cpp

@@ -1312,6 +1312,12 @@ void PotentialBindings::infer(Constraint *constraint) {
     // Constraints from which we can't do anything.
     break;
 
+  // For now let's avoid inferring protocol requirements from
+  // this constraint, but in the future we could do that to
+  // to filter bindings.
+  case ConstraintKind::TransitivelyConformsTo:
+    break;
+
   case ConstraintKind::DynamicTypeOf: {
     // Direct binding of the left-hand side could result
     // in `DynamicTypeOf` failure if right-hand side is

lib/Sema/CSSimplify.cpp

@@ -1542,6 +1542,7 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
   case ConstraintKind::BindOverload:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::Defaultable:
   case ConstraintKind::Disjunction:
   case ConstraintKind::DynamicTypeOf:
@@ -1682,6 +1683,7 @@ static bool matchFunctionRepresentations(FunctionType::ExtInfo einfo1,
   case ConstraintKind::BindOverload:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::Defaultable:
   case ConstraintKind::Disjunction:
   case ConstraintKind::DynamicTypeOf:
@@ -2072,6 +2074,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   case ConstraintKind::BindOverload:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::ConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::Defaultable:
   case ConstraintKind::Disjunction:
   case ConstraintKind::DynamicTypeOf:
@@ -4983,6 +4986,7 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     case ConstraintKind::BridgingConversion:
     case ConstraintKind::CheckedCast:
     case ConstraintKind::ConformsTo:
+    case ConstraintKind::TransitivelyConformsTo:
     case ConstraintKind::Defaultable:
     case ConstraintKind::Disjunction:
     case ConstraintKind::DynamicTypeOf:
@@ -6186,6 +6190,12 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
   return SolutionKind::Error;
 }
 
+ConstraintSystem::SolutionKind ConstraintSystem::simplifyTransitivelyConformsTo(
+    Type type, Type protocolTy, ConstraintLocatorBuilder locator,
+    TypeMatchOptions flags) {
+  return SolutionKind::Error;
+}
+
 /// Determine the kind of checked cast to perform from the given type to
 /// the given type.
 ///
@@ -11259,6 +11269,10 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
     return simplifyConformsToConstraint(first, second, kind, locator,
                                         subflags);
 
+  case ConstraintKind::TransitivelyConformsTo:
+    return simplifyTransitivelyConformsTo(first, second, locator,
+                                          subflags);
+
   case ConstraintKind::CheckedCast:
     return simplifyCheckedCastConstraint(first, second, subflags, locator);
 
@@ -11727,6 +11741,13 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
              constraint.getLocator(),
              None);
 
+  case ConstraintKind::TransitivelyConformsTo:
+    return simplifyTransitivelyConformsTo(
+             constraint.getFirstType(),
+             constraint.getSecondType(),
+             constraint.getLocator(),
+             None);
+
   case ConstraintKind::CheckedCast: {
     auto result = simplifyCheckedCastConstraint(constraint.getFirstType(),
                                                 constraint.getSecondType(),

lib/Sema/CSSolver.cpp

@@ -2232,6 +2232,7 @@ void DisjunctionChoice::propagateConversionInfo(ConstraintSystem &cs) const {
         case ConstraintKind::Defaultable:
         case ConstraintKind::ConformsTo:
         case ConstraintKind::LiteralConformsTo:
+        case ConstraintKind::TransitivelyConformsTo:
           return false;
 
         default:

lib/Sema/Constraint.cpp

@@ -56,6 +56,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second,
   case ConstraintKind::OperatorArgumentConversion:
   case ConstraintKind::ConformsTo:
   case ConstraintKind::LiteralConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::SelfObjectOfProtocol:
   case ConstraintKind::DynamicTypeOf:
@@ -124,6 +125,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second, Type Third,
   case ConstraintKind::OperatorArgumentConversion:
   case ConstraintKind::ConformsTo:
   case ConstraintKind::LiteralConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::SelfObjectOfProtocol:
   case ConstraintKind::DynamicTypeOf:
@@ -241,7 +243,8 @@ Constraint::Constraint(ConstraintKind kind, ConstraintFix *fix, Type first,
 ProtocolDecl *Constraint::getProtocol() const {
   assert((Kind == ConstraintKind::ConformsTo ||
           Kind == ConstraintKind::LiteralConformsTo ||
-          Kind == ConstraintKind::SelfObjectOfProtocol)
+          Kind == ConstraintKind::SelfObjectOfProtocol ||
+          Kind == ConstraintKind::TransitivelyConformsTo)
           && "Not a conformance constraint");
   return Types.Second->castTo<ProtocolType>()->getDecl();
 }
@@ -259,6 +262,7 @@ Constraint *Constraint::clone(ConstraintSystem &cs) const {
   case ConstraintKind::OperatorArgumentConversion:
   case ConstraintKind::ConformsTo:
   case ConstraintKind::LiteralConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::CheckedCast:
   case ConstraintKind::DynamicTypeOf:
   case ConstraintKind::EscapableFunctionOf:
@@ -357,6 +361,7 @@ void Constraint::print(llvm::raw_ostream &Out, SourceManager *sm) const {
       Out << " operator arg conv "; break;
   case ConstraintKind::ConformsTo: Out << " conforms to "; break;
   case ConstraintKind::LiteralConformsTo: Out << " literal conforms to "; break;
+  case ConstraintKind::TransitivelyConformsTo: Out << " transitive conformance to "; break;
   case ConstraintKind::CheckedCast: Out << " checked cast to "; break;
   case ConstraintKind::SelfObjectOfProtocol: Out << " Self type of "; break;
   case ConstraintKind::ApplicableFunction: Out << " applicable fn "; break;
@@ -599,6 +604,7 @@ gatherReferencedTypeVars(Constraint *constraint,
   case ConstraintKind::Defaultable:
   case ConstraintKind::ConformsTo:
   case ConstraintKind::LiteralConformsTo:
+  case ConstraintKind::TransitivelyConformsTo:
   case ConstraintKind::SelfObjectOfProtocol:
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
@@ -659,6 +665,7 @@ Constraint *Constraint::create(ConstraintSystem &cs, ConstraintKind kind,
 
   // Conformance constraints expect an existential on the right-hand side.
   assert((kind != ConstraintKind::ConformsTo &&
+          kind != ConstraintKind::TransitivelyConformsTo &&
           kind != ConstraintKind::SelfObjectOfProtocol) ||
          second->isExistentialType());