[Macros] Distinguish between attributes that do not reference macros and

invalid macro attributes in `findMacroForCustomAttr`.

Author: hborla

include/swift/AST/MacroDeclaration.h

@@ -71,7 +71,12 @@ class CustomAttr;
 
 /// Perform lookup to determine whether the given custom attribute refers to
 /// a macro declaration, and return that macro declaration.
-MacroDecl *findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc);
+///
+/// \Returns \c None if the custom attribute name does not match any macro
+/// declarations, \c nullptr if the macro reference has errors in the argument
+/// list, or a resolved macro declaration for a valid macro attribute.
+Optional<MacroDecl *>
+findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc);
 
 class MacroIntroducedDeclName {
 public:

lib/AST/NameLookup.cpp

@@ -3048,7 +3048,7 @@ CustomAttrDeclRequest::evaluate(Evaluator &evaluator,
   // nested scopes. At this point, we're looking to see whether there are
   // any suitable macros.
   if (auto macro = findMacroForCustomAttr(attr, dc))
-    return macro;
+    return macro.getValue();
 
   // Find the types referenced by the custom attribute.
   auto &ctx = dc->getASTContext();

lib/Sema/TypeCheckMacros.cpp

@@ -1083,11 +1083,11 @@ bool swift::expandAttributes(CustomAttr *attr, MacroDecl *macro, Decl *member) {
   return addedAttributes;
 }
 
-MacroDecl *
+Optional<MacroDecl *>
 swift::findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc) {
   auto *identTypeRepr = dyn_cast_or_null<IdentTypeRepr>(attr->getTypeRepr());
   if (!identTypeRepr)
-    return nullptr;
+    return None;
 
   // Look for macros at module scope. They can only occur at module scope, and
   // we need to be sure not to trigger name lookup into type contexts along
@@ -1108,7 +1108,7 @@ swift::findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc) {
   }
 
   if (macros.empty())
-    return nullptr;
+    return None;
 
   // Extract macro arguments from the attribute, or create an empty list.
   ArgumentList *attrArgs;
@@ -1133,5 +1133,7 @@ swift::findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc) {
     if (auto *macro = dyn_cast<MacroDecl>(fn->getDecl()))
       return macro;
 
-  return dyn_cast<MacroDecl>(macros.front());
+  // If we couldn't resolve a macro decl, the attribute is invalid.
+  attr->setInvalid();
+  return nullptr;
 }