diff --git a/mlir/include/mlir/IR/PatternMatch.h b/mlir/include/mlir/IR/PatternMatch.h
--- a/mlir/include/mlir/IR/PatternMatch.h
+++ b/mlir/include/mlir/IR/PatternMatch.h
@@ -980,6 +980,7 @@
     static_assert(always_false<T>,
                   "`std::string` arguments require a string copy, use "
                   "`StringRef` for string-like arguments instead");
+    return {};
   }
   static void processAsResult(PatternRewriter &rewriter, PDLResultList &results,
                               StringRef value) {