diff --git a/mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td b/mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td
--- a/mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td
+++ b/mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td
@@ -820,6 +820,11 @@
       llvm.mlir.cast %v : llvm.float to f32
       llvm.mlir.cast %v : !llvm<"<2 x float>"> to vector<2xf32>
   }];
+  let extraClassDeclaration = [{
+    /// Return true if `a` and `b` are valid operand and result types for the
+    /// operation.
+    static bool areCastCompatible(Type a, Type b);
+  }];
   let assemblyFormat = "$in attr-dict `:` type($in) `to` type($res)";
   let verifier = "return ::verify(*this);";
 }
diff --git a/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp b/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
--- a/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
+++ b/mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp
@@ -170,8 +170,9 @@
                                Location loc) -> Optional<Value> {
     if (inputs.size() != 1)
       return llvm::None;
-    // FIXME: These should check LLVM::DialectCastOp can actually be constructed
-    // from the input and result.
+    if (!LLVM::DialectCastOp::areCastCompatible(inputs[0].getType(),
+                                                resultType))
+      return llvm::None;
     return builder.create<LLVM::DialectCastOp>(loc, resultType, inputs[0])
         .getResult();
   });
@@ -180,8 +181,9 @@
                                Location loc) -> Optional<Value> {
     if (inputs.size() != 1)
       return llvm::None;
-    // FIXME: These should check LLVM::DialectCastOp can actually be constructed
-    // from the input and result.
+    if (!LLVM::DialectCastOp::areCastCompatible(inputs[0].getType(),
+                                                resultType))
+      return llvm::None;
     return builder.create<LLVM::DialectCastOp>(loc, resultType, inputs[0])
         .getResult();
   });
diff --git a/mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp b/mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp
--- a/mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp
+++ b/mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp
@@ -981,26 +981,25 @@
 // Verifier for LLVM::DialectCastOp.
 //===----------------------------------------------------------------------===//
 
-static LogicalResult verify(DialectCastOp op) {
-  auto verifyMLIRCastType = [&op](Type type) -> LogicalResult {
+bool mlir::LLVM::DialectCastOp::areCastCompatible(Type a, Type b) {
+  auto verifyMLIRCastType = [&](Type type) -> bool {
     if (auto llvmType = type.dyn_cast<LLVM::LLVMType>()) {
       if (llvmType.isVectorTy())
         llvmType = llvmType.getVectorElementType();
       if (llvmType.isIntegerTy() || llvmType.isBFloatTy() ||
           llvmType.isHalfTy() || llvmType.isFloatTy() ||
           llvmType.isDoubleTy()) {
-        return success();
+        return true;
       }
-      return op.emitOpError("type must be non-index integer types, float "
-                            "types, or vector of mentioned types.");
+      return false;
     }
     if (auto vectorType = type.dyn_cast<VectorType>()) {
       if (vectorType.getShape().size() > 1)
-        return op.emitOpError("only 1-d vector is allowed");
+        return false;
       type = vectorType.getElementType();
     }
     if (type.isSignlessIntOrFloat())
-      return success();
+      return true;
     // Note that memrefs are not supported. We currently don't have a use case
     // for it, but even if we do, there are challenges:
     // * if we allow memrefs to cast from/to memref descriptors, then the
@@ -1010,11 +1009,16 @@
     // alternatively want metadata that only present in the descriptor.
     //
     // TODO: re-evaluate the memref cast design when it's needed.
-    return op.emitOpError("type must be non-index integer types, float types, "
-                          "or vector of mentioned types.");
+    return false;
   };
-  return failure(failed(verifyMLIRCastType(op.in().getType())) ||
-                 failed(verifyMLIRCastType(op.getType())));
+  return verifyMLIRCastType(a) && verifyMLIRCastType(b);
+}
+
+static LogicalResult verify(DialectCastOp op) {
+  if (!DialectCastOp::areCastCompatible(op.in().getType(), op.getType()))
+    return op.emitOpError("types must be non-index integer types, float types, "
+                          "or 1-d vector of mentioned types.");
+  return success();
 }
 
 // Parses one of the keywords provided in the list `keywords` and returns the
diff --git a/mlir/test/Conversion/StandardToLLVM/invalid.mlir b/mlir/test/Conversion/StandardToLLVM/invalid.mlir
--- a/mlir/test/Conversion/StandardToLLVM/invalid.mlir
+++ b/mlir/test/Conversion/StandardToLLVM/invalid.mlir
@@ -1,7 +1,7 @@
 // RUN: mlir-opt %s -convert-std-to-llvm -verify-diagnostics -split-input-file
 
 func @mlir_cast_to_llvm(%0 : index) -> !llvm.i64 {
-  // expected-error@+1 {{'llvm.mlir.cast' op type must be non-index integer types, float types, or vector of mentioned types}}
+  // expected-error@+1 {{'llvm.mlir.cast' op types must be non-index integer types, float types, or 1-d vector of mentioned types}}
   %1 = llvm.mlir.cast %0 : index to !llvm.i64
   return %1 : !llvm.i64
 }
@@ -9,7 +9,7 @@
 // -----
 
 func @mlir_cast_from_llvm(%0 : !llvm.i64) -> index {
-  // expected-error@+1 {{'llvm.mlir.cast' op type must be non-index integer types, float types, or vector of mentioned types}}
+  // expected-error@+1 {{'llvm.mlir.cast' op types must be non-index integer types, float types, or 1-d vector of mentioned types}}
   %1 = llvm.mlir.cast %0 : !llvm.i64 to index
   return %1 : index
 }
@@ -25,7 +25,7 @@
 // -----
 
 func @mlir_cast_to_llvm_vec(%0 : vector<1x1xf32>) -> !llvm.vec<1 x float> {
-  // expected-error@+1 {{'llvm.mlir.cast' op only 1-d vector is allowed}}
+  // expected-error@+1 {{'llvm.mlir.cast' op types must be non-index integer types, float types, or 1-d vector of mentioned types}}
   %1 = llvm.mlir.cast %0 : vector<1x1xf32> to !llvm.vec<1 x float>
   return %1 : !llvm.vec<1 x float>
 }