[ConstraintLocator] Add a locator path element that stores an opened pack element

generic signature.

Author: hborla

include/swift/Sema/ConstraintLocator.h

@@ -687,6 +687,21 @@ class LocatorPathElt::PackElement final : public StoredIntegerElement<1> {
   }
 };
 
+class LocatorPathElt::OpenedPackElement final
+    : public StoredPointerElement<GenericEnvironment> {
+public:
+  OpenedPackElement(GenericEnvironment *env)
+      : StoredPointerElement(PathElementKind::OpenedPackElement, env) {}
+
+  GenericEnvironment *getGenericEnvironment() const {
+    return getStoredPointer();
+  }
+
+  static bool classof(const LocatorPathElt *elt) {
+    return elt->getKind() == PathElementKind::OpenedPackElement;
+  }
+};
+
 class LocatorPathElt::KeyPathComponent final : public StoredIntegerElement<1> {
 public:
   KeyPathComponent(unsigned index)

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -195,6 +195,9 @@ CUSTOM_LOCATOR_PATH_ELT(PackType)
 /// An element of a pack type - the T in <T, U, V, ...>
 CUSTOM_LOCATOR_PATH_ELT(PackElement)
 
+/// Stores the generic environment for an opened pack element.
+CUSTOM_LOCATOR_PATH_ELT(OpenedPackElement)
+
 /// The shape of a parameter pack.
 SIMPLE_LOCATOR_PATH_ELT(PackShape)
 

lib/Sema/ConstraintLocator.cpp

@@ -97,6 +97,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::SyntacticElement:
   case ConstraintLocator::PackType:
   case ConstraintLocator::PackElement:
+  case ConstraintLocator::OpenedPackElement:
   case ConstraintLocator::PackShape:
   case ConstraintLocator::PackExpansionPattern:
   case ConstraintLocator::PatternBindingElement:
@@ -448,6 +449,11 @@ void LocatorPathElt::dump(raw_ostream &out) const {
     break;
   }
 
+  case ConstraintLocator::OpenedPackElement: {
+    out << "opened pack element";
+    break;
+  }
+
   case ConstraintLocator::PackShape: {
     out << "pack shape";
     break;

lib/Sema/ConstraintSystem.cpp

@@ -5338,6 +5338,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
       break;
 
     case ConstraintLocator::PackElement:
+    case ConstraintLocator::OpenedPackElement:
     case ConstraintLocator::PackShape:
       break;