[ConstraintSystem] Key path literals with completion tokens should be marked as invalid

Capability couldn't be determined for expressions like that which
means that inference should be delayed until root becomes available.

Resolves: #69936

Author: xedin

lib/Sema/CSBindings.cpp

@@ -641,14 +641,15 @@ void BindingSet::finalize(
           updatedBindings.insert(
               {keyPathTy, AllowedBindingKind::Exact, keyPathLoc});
         } else if (CS.shouldAttemptFixes()) {
+          auto fixedRootTy = CS.getFixedType(rootTy);
           // If key path is structurally correct and has a resolved root
           // type, let's promote the fallback type into a binding because
           // root would have been inferred from explicit type already and
           // it's benefitial for diagnostics to assign a non-placeholder
           // type to key path literal to propagate root/value to the context.
           if (!keyPath->hasSingleInvalidComponent() &&
               (keyPath->getParsedRoot() ||
-               !CS.getFixedType(rootTy)->isTypeVariableOrMember())) {
+               (fixedRootTy && !fixedRootTy->isTypeVariableOrMember()))) {
             auto fallback = llvm::find_if(Defaults, [](const auto &entry) {
               return entry.second->getKind() == ConstraintKind::FallbackType;
             });

lib/Sema/ConstraintSystem.cpp

@@ -7469,8 +7469,7 @@ ConstraintSystem::inferKeyPathLiteralCapability(KeyPathExpr *keyPath) {
       break;
 
     case KeyPathExpr::Component::Kind::CodeCompletion: {
-      capability = KeyPathCapability::ReadOnly;
-      break;
+      return fail();
     }
     case KeyPathExpr::Component::Kind::Property:
     case KeyPathExpr::Component::Kind::Subscript:

test/IDE/complete_issue-69936.swift

@@ -0,0 +1,13 @@
+// RUN: %batch-code-completion
+
+struct Test {
+  var currentHeight: Int
+
+  func subscribeToKeyboardEvents() {
+    objectAndKeyPath(object: self, keyPath: \.#^COMPLETE^#)
+  }
+}
+
+func objectAndKeyPath<Root, Input>(object: Root, keyPath: WritableKeyPath<Root, Input>) {}
+
+// COMPLETE: Decl[InstanceVar]/CurrNominal:      currentHeight[#Int#]; name=currentHeight