Fix @_implements type resolution diagnostics

If a type in an `@_implements` attribute failed to resolve, Sema would assume it was because the type existed but wasn’t a protocol, even if there was another reason for the problem (such as the type not existing). Explicitly resolve the TypeRepr again through a path that will produce diagnostics.

Author: beccadax

lib/Sema/TypeCheckAttr.cpp

@@ -4281,8 +4281,25 @@ void AttributeChecker::visitImplementsAttr(ImplementsAttr *attr) {
   ProtocolDecl *PD = attr->getProtocol(DC);
 
   if (!PD) {
-    diagnose(attr->getLocation(), diag::implements_attr_non_protocol_type)
-      .highlight(attr->getProtocolTypeRepr()->getSourceRange());
+    // `ImplementsAttr::getProtocol()` returns `nullptr` both when a type fails
+    // to resolve, and when it resolves to something other than a protocol.
+    // Due to layering concerns, it doesn't resolve in a way that emits
+    // diagnostics.
+    //
+    // Distinguish between these situations by trying to resolve the type again.
+    // If it doesn't resolve, TypeResolution will have diagnosed the problem; if
+    // it does, it must have resolved to a non-protocol, so emit a diagnostic to
+    // that effect.
+    TypeResolutionOptions options(TypeResolverContext::None);
+    auto resolvedType = TypeResolution::resolveContextualType(
+        attr->getProtocolTypeRepr(), DC, options,
+        // Unbound generics and placeholders are not allowed within this attr.
+        /*unboundTyOpener*/ nullptr, /*placeholderHandler*/ nullptr,
+        /*packElementOpener*/ nullptr);
+
+    if (resolvedType && !resolvedType->hasError())
+      diagnose(attr->getLocation(), diag::implements_attr_non_protocol_type)
+        .highlight(attr->getProtocolTypeRepr()->getSourceRange());
     return;
   }
 

test/attr/attr_implements_bad_types.swift

@@ -21,5 +21,9 @@ struct S0 : NeedsF0 {  // expected-error {{type 'S0' does not conform to protoco
   @_implements(Equatable, ==(_:_:)) // expected-error {{containing type 'S0' does not conform to protocol 'Equatable'}}
   static func gg2(x:S0, y:S0) -> Bool {
   }
+
+  @_implements(NonexistentType, ff3()) // expected-error {{cannot find type 'NonexistentType' in scope}}
+  func gg3() {
+  }
 }