[Macros] Separate determining whether a custom attribute is a macro and

type checking macro attribute arguments into two different requests.

lib/AST/NameLookup.cpp

@@ -3041,14 +3041,40 @@ GenericParamListRequest::evaluate(Evaluator &evaluator, GenericContext *value) c
       parsedGenericParams->getRAngleLoc());
 }
 
+void swift::findMacroForCustomAttr(CustomAttr *attr, DeclContext *dc,
+                                   llvm::TinyPtrVector<ValueDecl *> &macros) {
+  auto *identTypeRepr = dyn_cast_or_null<IdentTypeRepr>(attr->getTypeRepr());
+  if (!identTypeRepr)
+    return;
+
+  // 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
+  // the way.
+  auto moduleScopeDC = dc->getModuleScopeContext();
+  ASTContext &ctx = moduleScopeDC->getASTContext();
+  UnqualifiedLookupDescriptor descriptor(
+      identTypeRepr->getNameRef(), moduleScopeDC
+  );
+  auto lookup = evaluateOrDefault(
+      ctx.evaluator, UnqualifiedLookupRequest{descriptor}, {});
+  for (const auto &result : lookup.allResults()) {
+    // Only keep attached macros, which can be spelled as custom attributes.
+    if (auto macro = dyn_cast<MacroDecl>(result.getValueDecl()))
+      if (isAttachedMacro(macro->getMacroRoles()))
+        macros.push_back(macro);
+  }
+}
+
 MacroOrNominalTypeDecl
 CustomAttrDeclRequest::evaluate(Evaluator &evaluator,
                                 CustomAttr *attr, DeclContext *dc) const {
   // Look for names at module scope, so we don't trigger name lookup for
   // nested scopes. At this point, we're looking to see whether there are
   // any suitable macros.
-  if (auto macro = findMacroForCustomAttr(attr, dc))
-    return macro.getValue();
+  llvm::TinyPtrVector<ValueDecl *> macros;
+  findMacroForCustomAttr(attr, dc, macros);
+  if (!macros.empty())
+    return nullptr;
 
   // Find the types referenced by the custom attribute.
   auto &ctx = dc->getASTContext();