[CSDiagnostics] A tailored diagnostic when passing key path as an argument to non-key path parameter

Produce a tailored diagnostic that omits a fully unresolved key path
type (`KeyPath<_, _>`) when key path without an explicit root type is
passed as an argument to non-keypath parameter type (i.e. `Int`).

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -376,6 +376,10 @@ ERROR(cannot_convert_argument_value,none,
       "cannot convert value of type %0 to expected argument type %1",
       (Type,Type))
 
+ERROR(cannot_convert_unresolved_key_path_argument_value,none,
+      "cannot convert value of key path type to expected argument type %0",
+      (Type))
+
 ERROR(cannot_convert_argument_value_for_swift_func,none,
       "cannot convert value of type %0 to expected argument type %1 "
       "because %kind2 was not imported from C header",

lib/Sema/CSDiagnostics.cpp

@@ -7295,6 +7295,18 @@ bool ArgumentMismatchFailure::diagnoseAsError() {
 
   auto argType = getFromType();
 
+  // Unresolved key path argument requires a tailored diagnostic
+  // that doesn't mention a fallback type - `KeyPath<_, _>`.
+  if (argType->isKeyPath() && !isKnownKeyPathType(paramType)) {
+    auto keyPathTy = argType->castTo<BoundGenericType>();
+    auto rootTy = keyPathTy->getGenericArgs()[0];
+    if (rootTy->is<UnresolvedType>()) {
+      emitDiagnostic(diag::cannot_convert_unresolved_key_path_argument_value,
+                     paramType);
+      return true;
+    }
+  }
+
   if (paramType->isAnyObject()) {
     emitDiagnostic(diag::cannot_convert_argument_value_anyobject, argType,
                    paramType);

test/expr/unary/keypath/keypath.swift

@@ -914,7 +914,9 @@ 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) {} 
   provideValueButNotRoot(\.x) // expected-error {{cannot infer key path type from context; consider explicitly specifying a root type}}

validation-test/Sema/SwiftUI/rdar84580119.swift

@@ -16,6 +16,7 @@ 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'}}
   }
 }