[ConstraintSystem] Add a new locator for key path subscript parameter

Author: xedin

include/swift/Sema/ConstraintLocatorPathElts.def

@@ -131,6 +131,9 @@ SIMPLE_LOCATOR_PATH_ELT(KeyPathType)
 /// The value of a key path.
 SIMPLE_LOCATOR_PATH_ELT(KeyPathValue)
 
+/// The index parameter of a special `subscript(keyPath: KeyPath<...>)`.
+SIMPLE_LOCATOR_PATH_ELT(KeyPathSubscriptIndex)
+
 /// An implicit @lvalue-to-inout conversion; only valid for operator
 /// arguments.
 SIMPLE_LOCATOR_PATH_ELT(LValueConversion)

lib/Sema/ConstraintLocator.cpp

@@ -111,6 +111,7 @@ unsigned LocatorPathElt::getNewSummaryFlags() const {
   case ConstraintLocator::PackExpansionType:
   case ConstraintLocator::ThrownErrorType:
   case ConstraintLocator::FallbackType:
+  case ConstraintLocator::KeyPathSubscriptIndex:
     return 0;
 
   case ConstraintLocator::FunctionArgument:
@@ -528,6 +529,10 @@ void LocatorPathElt::dump(raw_ostream &out) const {
   case ConstraintLocator::FallbackType: {
     out << "fallback type";
     break;
+
+  case ConstraintLocator::KeyPathSubscriptIndex:
+    out << "key path subscript index parameter";
+    break;
   }
   }
 }

lib/Sema/ConstraintSystem.cpp

@@ -6097,6 +6097,7 @@ void constraints::simplifyLocator(ASTNode &anchor,
     case ConstraintLocator::OptionalPayload:
     case ConstraintLocator::ImplicitlyUnwrappedDisjunctionChoice:
     case ConstraintLocator::FallbackType:
+    case ConstraintLocator::KeyPathSubscriptIndex:
       break;
     }