[Constraint solver] Restrict key path application to subscript[keyPath:].

Extend argument label matching to handle key path application choices,
which are accessed via subscript syntax with the argument labels “keyPath:”.
This gets us to the point where it’s possible to filter down to a single
subscript if there is no type-based overloading going on.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -110,9 +110,12 @@ bool constraints::areConservativelyCompatibleArgumentLabels(
       baseType = baseType->getRValueType();
     break;
 
+  case OverloadChoiceKind::KeyPathApplication:
+    // Key path applications are written as if subscript[keyPath:].
+    return !hasTrailingClosure && labels.size() == 1 && labels[0].is("keyPath");
+
   case OverloadChoiceKind::BaseType:
   case OverloadChoiceKind::DynamicMemberLookup:
-  case OverloadChoiceKind::KeyPathApplication:
   case OverloadChoiceKind::TupleIndex:
     return true;
   }

test/Constraints/overload_filtering.swift

@@ -22,7 +22,9 @@ struct X {
 }
 
 func testSubscript(x: X, i: Int) {
-  // CHECK: disabled disjunction term {{.*}}X.subscript(_:)
+  // CHECK: disabled disjunction term {{.*}} bound to key path application
+  // CHECK-NEXT: disabled disjunction term {{.*}}X.subscript(_:)
   // CHECK-NEXT: disabled disjunction term {{.*}}X.subscript(_:_:_:)
+  // CHECK-NEXT: introducing single enabled disjunction term {{.*}} bound to decl overload_filtering.(file).X.subscript(_:_:)
   _ = x[i, i]
 }