[ConstraintSystem] Add a locator element to represent a generic type

This locator is going to be used as a "parent" element for
"generic argument" elements.

include/swift/Sema/ConstraintLocator.h

@@ -643,6 +643,20 @@ public:
   }
 };
 
+class LocatorPathElt::GenericType : public StoredPointerElement<TypeBase> {
+public:
+  GenericType(Type type)
+      : StoredPointerElement(PathElementKind::GenericType, type.getPointer()) {
+    assert(type->getDesugaredType()->is<BoundGenericType>());
+  }
+
+  Type getType() const { return getStoredPointer(); }
+
+  static bool classof(const LocatorPathElt *elt) {
+    return elt->getKind() == PathElementKind::GenericType;
+  }
+};
+
 /// Abstract superclass for any kind of tuple element.
 class LocatorPathElt::AnyTupleElement : public StoredIntegerElement<1> {
 protected:

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -181,6 +181,10 @@ CUSTOM_LOCATOR_PATH_ELT(SynthesizedArgument)
 /// path components.
 CUSTOM_LOCATOR_PATH_ELT(TupleType)
 
+/// A generic type, which provides context for subsequent generic
+/// argument path components.
+CUSTOM_LOCATOR_PATH_ELT(GenericType)
+
 /// Tuple elements.
 ABSTRACT_LOCATOR_PATH_ELT(AnyTupleElement)
   /// A tuple element referenced by position.

lib/Sema/ConstraintLocator.cpp

@@ -67,6 +67,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::GenericParameter:
   case ConstraintLocator::GenericArgument:
   case ConstraintLocator::TupleType:
+  case ConstraintLocator::GenericType:
   case ConstraintLocator::NamedTupleElement:
   case ConstraintLocator::TupleElement:
   case ConstraintLocator::ProtocolRequirement:
@@ -242,6 +243,12 @@ void LocatorPathElt::dump(raw_ostream &out) const {
     break;
   }
 
+  case ConstraintLocator::GenericType: {
+    auto genericTyElt = elt.castTo<LocatorPathElt::GenericType>();
+    out << "generic type '" << genericTyElt.getType()->getString(PO) << "'";
+    break;
+  }
+
   case ConstraintLocator::NamedTupleElement: {
     auto tupleElt = elt.castTo<LocatorPathElt::NamedTupleElement>();
     out << "named tuple element #" << llvm::utostr(tupleElt.getIndex());

lib/Sema/ConstraintSystem.cpp

@@ -5372,6 +5372,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
       continue;
 
    case ConstraintLocator::TupleType:
+   case ConstraintLocator::GenericType:
       path = path.slice(1);
       continue;