[ConstraintSystem] Fail key path capability inference if root is a placeholder

If root type of a key path has been determined to be a hole there
is no reason to delay the inference decision which should be a
failure because none of the components would be inferrable from
a placeholder root.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -7648,6 +7648,13 @@ ConstraintSystem::inferKeyPathLiteralCapability(KeyPathExpr *keyPath) {
   if (keyPath->hasSingleInvalidComponent())
     return fail();
 
+  // If root is determined to be a hole it means that none of the components
+  // are resolvable and key path is not viable.
+  auto rootTy =
+      getFixedTypeRecursive(getKeyPathRootType(keyPath), /*wantRValue=*/false);
+  if (rootTy->isPlaceholder())
+    return fail();
+
   auto mutability = KeyPathMutability::Writable;
   for (unsigned i : indices(keyPath->getComponents())) {
     auto &component = keyPath->getComponents()[i];

test/expr/unary/keypath/keypath.swift

@@ -914,13 +914,13 @@ func testKeyPathHole() {
 
   func f(_ i: Int) {}
   f(\.x) // expected-error {{cannot infer key path type from context; consider explicitly specifying a root type}} {{6-6=<#Root#>}}
-  // expected-error@-1 {{cannot convert value of key path type to expected argument type 'Int'}}
   f(\.x.y) // expected-error {{cannot infer key path type from context; consider explicitly specifying a root type}} {{6-6=<#Root#>}}
-  // expected-error@-1 {{cannot convert value of key path type to expected argument type 'Int'}}
 
-  func provideValueButNotRoot<T>(_ fn: (T) -> String) {} 
+func provideValueButNotRoot<T>(_ fn: (T) -> String) {} // expected-note 2 {{in call to function 'provideValueButNotRoot'}}
   provideValueButNotRoot(\.x) // expected-error {{cannot infer key path type from context; consider explicitly specifying a root type}}
+  // expected-error@-1 {{generic parameter 'T' could not be inferred}}
   provideValueButNotRoot(\.x.y) // expected-error {{cannot infer key path type from context; consider explicitly specifying a root type}}
+// expected-error@-1 {{generic parameter 'T' could not be inferred}}
   provideValueButNotRoot(\String.foo) // expected-error {{value of type 'String' has no member 'foo'}}
 
   func provideKPValueButNotRoot<T>(_ kp: KeyPath<T, String>) {} 
@@ -1237,3 +1237,9 @@ func test_keypath_coercion_to_function() {
 func test_keypath_application_with_composition(v: String, kp: any KeyPath<String, Int> & PP) {
   _ = v[keyPath: kp] // Ok
 }
+
+func test_leading_dot_key_path_without_context() {
+  func test(_: AnyKeyPath?) {}
+  test(\.utf8)
+  // expected-error@-1 {{cannot infer key path type from context; consider explicitly specifying a root type}}
+}

validation-test/Sema/SwiftUI/rdar84580119.swift

@@ -16,7 +16,6 @@ extension EnvironmentValues {
     set { self[\.MyHorizontalAlignmentEnvironmentKey.self] = newValue }
     // expected-error@-1 {{generic parameter 'K' could not be inferred}}
     // expected-error@-2 {{cannot infer key path type from context; consider explicitly specifying a root type}}
-    // expected-error@-3 {{cannot convert value of key path type to expected argument type 'K.Type'}}
   }
 }