[ConstraintSystem] Add a new constraint which connects base with result of the member chain

The first type represents a result of an unresolved member chain,
and the second type is its base type. This constraint acts almost
like `Equal` but also enforces following semantics:

- It's possible to infer a base from a result type by looking through
  this constraint, but it's only solved when both types are bound.

- If base is a protocol metatype, this constraint becomes a conformance
  check instead of an equality.

Author: xedin

include/swift/Sema/Constraint.h

@@ -177,6 +177,16 @@ enum class ConstraintKind : char {
   /// - Handled specially by binding inference, specifically contributes
   ///   to the bindings only if there are no contextual types available.
   DefaultClosureType,
+  /// The first type represents a result of an unresolved member chain,
+  /// and the second type is its base type. This constraint acts almost
+  /// like `Equal` but also enforces following semantics:
+  ///
+  /// - It's possible to infer a base from a result type by looking through
+  ///   this constraint, but it's only solved when both types are bound.
+  ///
+  /// - If base is a protocol metatype, this constraint becomes a conformance
+  ///   check instead of an equality.
+  UnresolvedMemberChainBase,
 };
 
 /// Classification of the different kinds of constraints.
@@ -570,6 +580,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
     case ConstraintKind::OneWayEqual:
     case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
+    case ConstraintKind::UnresolvedMemberChainBase:
       return ConstraintClassification::Relational;
 
     case ConstraintKind::ValueMember:

include/swift/Sema/ConstraintSystem.h

@@ -4568,6 +4568,12 @@ class ConstraintSystem {
                                         TypeMatchOptions flags,
                                         ConstraintLocatorBuilder locator);
 
+  /// Simplify an equality constraint between result and base types of
+  /// a unresolved member chain.
+  SolutionKind simplifyUnresolvedMemberChainBaseConstraint(
+      Type first, Type second, TypeMatchOptions flags,
+      ConstraintLocatorBuilder locator);
+
   /// Simplify a conversion constraint by applying the given
   /// reduction rule, which is known to apply at the outermost level.
   SolutionKind simplifyRestrictedConstraintImpl(

lib/Sema/CSBindings.cpp

@@ -1158,7 +1158,8 @@ void PotentialBindings::infer(Constraint *constraint) {
   case ConstraintKind::Conversion:
   case ConstraintKind::ArgumentConversion:
   case ConstraintKind::OperatorArgumentConversion:
-  case ConstraintKind::OptionalObject: {
+  case ConstraintKind::OptionalObject:
+  case ConstraintKind::UnresolvedMemberChainBase: {
     auto binding = inferFromRelational(constraint);
     if (!binding)
       break;

lib/Sema/CSSimplify.cpp

@@ -1541,6 +1541,7 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     llvm_unreachable("Not a conversion");
   }
 
@@ -1678,6 +1679,7 @@ static bool matchFunctionRepresentations(FunctionType::ExtInfo einfo1,
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     return true;
   }
 
@@ -2032,6 +2034,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     llvm_unreachable("Not a relational constraint");
   }
 
@@ -4896,6 +4899,7 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     case ConstraintKind::OneWayEqual:
     case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
+    case ConstraintKind::UnresolvedMemberChainBase:
       llvm_unreachable("Not a relational constraint");
     }
   }
@@ -7955,6 +7959,27 @@ ConstraintSystem::simplifyOneWayConstraint(
       secondSimplified, first, ConstraintKind::BindParam, flags, locator);
 }
 
+ConstraintSystem::SolutionKind
+ConstraintSystem::simplifyUnresolvedMemberChainBaseConstraint(
+    Type first, Type second, TypeMatchOptions flags,
+    ConstraintLocatorBuilder locator) {
+  auto resultTy = getFixedTypeRecursive(first, flags, /*wantRValue=*/true);
+  auto baseTy = getFixedTypeRecursive(second, flags, /*wantRValue=*/true);
+
+  if (baseTy->isTypeVariableOrMember() || resultTy->isTypeVariableOrMember()) {
+    if (flags.contains(TMF_GenerateConstraints)) {
+      addUnsolvedConstraint(
+          Constraint::create(*this, ConstraintKind::UnresolvedMemberChainBase,
+                             first, second, getConstraintLocator(locator)));
+      return SolutionKind::Solved;
+    }
+
+    return SolutionKind::Unsolved;
+  }
+
+  return matchTypes(baseTy, resultTy, ConstraintKind::Equal, flags, locator);
+}
+
 static Type getOpenedResultBuilderTypeFor(ConstraintSystem &cs,
                                             ConstraintLocatorBuilder locator) {
   auto lastElt = locator.last();
@@ -10761,6 +10786,10 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
   case ConstraintKind::OneWayBindParam:
     return simplifyOneWayConstraint(kind, first, second, subflags, locator);
 
+  case ConstraintKind::UnresolvedMemberChainBase:
+    return simplifyUnresolvedMemberChainBaseConstraint(first, second, subflags,
+                                                       locator);
+
   case ConstraintKind::ValueMember:
   case ConstraintKind::UnresolvedValueMember:
   case ConstraintKind::ValueWitness:
@@ -11280,6 +11309,11 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
                                     constraint.getFirstType(),
                                     constraint.getSecondType(),
                                     /*flags*/ None, constraint.getLocator());
+
+  case ConstraintKind::UnresolvedMemberChainBase:
+    return simplifyUnresolvedMemberChainBaseConstraint(
+        constraint.getFirstType(), constraint.getSecondType(),
+        /*flags=*/None, constraint.getLocator());
   }
 
   llvm_unreachable("Unhandled ConstraintKind in switch.");

lib/Sema/Constraint.cpp

@@ -67,6 +67,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second,
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
+  case ConstraintKind::UnresolvedMemberChainBase:
     assert(!First.isNull());
     assert(!Second.isNull());
     break;
@@ -142,6 +143,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second, Type Third,
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     llvm_unreachable("Wrong constructor");
 
   case ConstraintKind::KeyPath:
@@ -269,6 +271,7 @@ Constraint *Constraint::clone(ConstraintSystem &cs) const {
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     return create(cs, getKind(), getFirstType(), getSecondType(), getLocator());
 
   case ConstraintKind::ApplicableFunction:
@@ -364,6 +367,9 @@ void Constraint::print(llvm::raw_ostream &Out, SourceManager *sm) const {
   case ConstraintKind::DefaultClosureType:
     Out << " closure can default to ";
     break;
+  case ConstraintKind::UnresolvedMemberChainBase:
+    Out << " unresolved member chain base ";
+    break;
   case ConstraintKind::KeyPath:
       Out << " key path from ";
       Out << getSecondType()->getString(PO);
@@ -590,6 +596,7 @@ gatherReferencedTypeVars(Constraint *constraint,
   case ConstraintKind::OneWayEqual:
   case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
+  case ConstraintKind::UnresolvedMemberChainBase:
     constraint->getFirstType()->getTypeVariables(typeVars);
     constraint->getSecondType()->getTypeVariables(typeVars);
     break;