diff --git a/mlir/include/mlir/Transforms/DialectConversion.h b/mlir/include/mlir/Transforms/DialectConversion.h
--- a/mlir/include/mlir/Transforms/DialectConversion.h
+++ b/mlir/include/mlir/Transforms/DialectConversion.h
@@ -103,8 +103,8 @@
   ///       conversion function to perform the conversion.
   /// Note: When attempting to convert a type, e.g. via 'convertType', the
   ///       mostly recently added conversions will be invoked first.
-  template <typename FnT,
-            typename T = typename llvm::function_traits<FnT>::template arg_t<0>>
+  template <typename FnT, typename T = typename llvm::function_traits<
+                              std::decay_t<FnT>>::template arg_t<0>>
   void addConversion(FnT &&callback) {
     registerConversion(wrapCallback<T>(std::forward<FnT>(callback)));
   }
@@ -124,8 +124,8 @@
   ///
   /// This method registers a materialization that will be called when
   /// converting an illegal block argument type, to a legal type.
-  template <typename FnT,
-            typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
+  template <typename FnT, typename T = typename llvm::function_traits<
+                              std::decay_t<FnT>>::template arg_t<1>>
   void addArgumentMaterialization(FnT &&callback) {
     argumentMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
@@ -133,16 +133,16 @@
   /// This method registers a materialization that will be called when
   /// converting a legal type to an illegal source type. This is used when
   /// conversions to an illegal type must persist beyond the main conversion.
-  template <typename FnT,
-            typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
+  template <typename FnT, typename T = typename llvm::function_traits<
+                              std::decay_t<FnT>>::template arg_t<1>>
   void addSourceMaterialization(FnT &&callback) {
     sourceMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
   }
   /// This method registers a materialization that will be called when
   /// converting type from an illegal, or source, type to a legal type.
-  template <typename FnT,
-            typename T = typename llvm::function_traits<FnT>::template arg_t<1>>
+  template <typename FnT, typename T = typename llvm::function_traits<
+                              std::decay_t<FnT>>::template arg_t<1>>
   void addTargetMaterialization(FnT &&callback) {
     targetMaterializations.emplace_back(
         wrapMaterialization<T>(std::forward<FnT>(callback)));
diff --git a/mlir/test/lib/Dialect/Test/TestPatterns.cpp b/mlir/test/lib/Dialect/Test/TestPatterns.cpp
--- a/mlir/test/lib/Dialect/Test/TestPatterns.cpp
+++ b/mlir/test/lib/Dialect/Test/TestPatterns.cpp
@@ -492,8 +492,17 @@
   TestTypeConverter() {
     addConversion(convertType);
     addArgumentMaterialization(materializeCast);
-    addArgumentMaterialization(materializeOneToOneCast);
     addSourceMaterialization(materializeCast);
+
+    /// Materialize the cast for one-to-one conversion from i64 to f64.
+    const auto materializeOneToOneCast =
+        [](OpBuilder &builder, IntegerType resultType, ValueRange inputs,
+           Location loc) -> Optional<Value> {
+      if (resultType.getWidth() == 42 && inputs.size() == 1)
+        return builder.create<TestCastOp>(loc, resultType, inputs).getResult();
+      return llvm::None;
+    };
+    addArgumentMaterialization(materializeOneToOneCast);
   }
 
   static LogicalResult convertType(Type t, SmallVectorImpl<Type> &results) {
@@ -532,16 +541,6 @@
       return inputs[0];
     return builder.create<TestCastOp>(loc, resultType, inputs).getResult();
   }
-
-  /// Materialize the cast for one-to-one conversion from i64 to f64.
-  static Optional<Value> materializeOneToOneCast(OpBuilder &builder,
-                                                 IntegerType resultType,
-                                                 ValueRange inputs,
-                                                 Location loc) {
-    if (resultType.getWidth() == 42 && inputs.size() == 1)
-      return builder.create<TestCastOp>(loc, resultType, inputs).getResult();
-    return llvm::None;
-  }
 };
 
 struct TestLegalizePatternDriver