Index: llvm/lib/TableGen/TGParser.cpp
===================================================================
--- llvm/lib/TableGen/TGParser.cpp
+++ llvm/lib/TableGen/TGParser.cpp
@@ -3509,7 +3509,20 @@
     SubstStack Substs;
     for (unsigned i = 0, e = TArgs.size(); i != e; ++i) {
       if (i < TemplateVals.size()) {
+        // This if block is a hack so we can track down multiclass template
+        // argument values of the wrong type.
+        if (TypedInit *ArgValue = dyn_cast<TypedInit>(TemplateVals[i])) {
+          auto *ArgType = MC->Rec.getValue(TArgs[i])->getType();
+          if (!ArgValue->getCastTo(ArgType))
+            return Error(SubClassLoc,
+                         "Value specified for template argument '" +
+                             TArgs[i]->getAsString() + "' (#" + Twine(i) +
+                             ") is of type " + ArgValue->getType()->getAsString() +
+                             "; expected type " + ArgType->getAsString() + ": " +
+                             ArgValue->getAsString());
+        }
         Substs.emplace_back(TArgs[i], TemplateVals[i]);
+
       } else {
         Init *Default = MC->Rec.getValue(TArgs[i])->getValue();
         if (!Default->isComplete()) {