[ConstraintSystem] Add a constraint locator path element for tuple types,

which will provide context for tuple element mismatch diagnostics.

Author: hborla

include/swift/Sema/ConstraintLocator.h

@@ -581,6 +581,20 @@ class LocatorPathElt::SynthesizedArgument final : public StoredIntegerElement<2>
   }
 };
 
+class LocatorPathElt::TupleType : public StoredPointerElement<TypeBase> {
+public:
+  TupleType(Type type)
+      : StoredPointerElement(PathElementKind::TupleType, type.getPointer()) {
+    assert(type->getDesugaredType()->is<swift::TupleType>());
+  }
+
+  Type getType() const { return getStoredPointer(); }
+
+  static bool classof(const LocatorPathElt *elt) {
+    return elt->getKind() == PathElementKind::TupleType;
+  }
+};
+
 /// Abstract superclass for any kind of tuple element.
 class LocatorPathElt::AnyTupleElement : public StoredIntegerElement<1> {
 protected:

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -164,6 +164,10 @@ SIMPLE_LOCATOR_PATH_ELT(SubscriptMember)
 /// The missing argument synthesized by the solver.
 CUSTOM_LOCATOR_PATH_ELT(SynthesizedArgument)
 
+/// A tuple type, which provides context for subsequent tuple element
+/// path components.
+CUSTOM_LOCATOR_PATH_ELT(TupleType)
+
 /// Tuple elements.
 ABSTRACT_LOCATOR_PATH_ELT(AnyTupleElement)
   /// A tuple element referenced by position.

lib/Sema/ConstraintLocator.cpp

@@ -63,6 +63,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::WrappedValue:
   case ConstraintLocator::GenericParameter:
   case ConstraintLocator::GenericArgument:
+  case ConstraintLocator::TupleType:
   case ConstraintLocator::NamedTupleElement:
   case ConstraintLocator::TupleElement:
   case ConstraintLocator::ProtocolRequirement:
@@ -356,6 +357,12 @@ void ConstraintLocator::dump(SourceManager *sm, raw_ostream &out) const {
       out << "member reference base";
       break;
 
+    case TupleType: {
+      auto tupleElt = elt.castTo<LocatorPathElt::TupleType>();
+      out << "tuple type '" << tupleElt.getType()->getString(PO) << "'";
+      break;
+    }
+
     case NamedTupleElement: {
       auto tupleElt = elt.castTo<LocatorPathElt::NamedTupleElement>();
       out << "named tuple element #" << llvm::utostr(tupleElt.getIndex());

lib/Sema/ConstraintSystem.cpp

@@ -4273,6 +4273,10 @@ void constraints::simplifyLocator(ASTNode &anchor,
       path = path.slice(1);
       continue;
 
+   case ConstraintLocator::TupleType:
+      path = path.slice(1);
+      continue;
+
     case ConstraintLocator::NamedTupleElement:
     case ConstraintLocator::TupleElement: {
       // Extract tuple element.