diff --git a/mlir/lib/Parser/Parser.cpp b/mlir/lib/Parser/Parser.cpp
--- a/mlir/lib/Parser/Parser.cpp
+++ b/mlir/lib/Parser/Parser.cpp
@@ -701,7 +701,7 @@
   if (useInfo.number < entries.size() && entries[useInfo.number].value) {
     Value result = entries[useInfo.number].value;
     // Check that the type matches the other uses.
-    if (result.getType() == type)
+    if (!type || result.getType() == type)
       return maybeRecordUse(result);
 
     emitError(useInfo.loc, "use of value '")
@@ -713,6 +713,13 @@
     return nullptr;
   }
 
+  if (!type) {
+    emitError(useInfo.loc, "forward reference of value '")
+        .append(useInfo.name, "' requires explicit type specification")
+        .attachNote(getEncodedSourceLocation(entries[useInfo.number].loc));
+    return nullptr;
+  }
+
   // Make sure we have enough slots for this.
   if (entries.size() <= useInfo.number)
     entries.resize(useInfo.number + 1);
diff --git a/mlir/test/lib/Dialect/Test/TestDialect.cpp b/mlir/test/lib/Dialect/Test/TestDialect.cpp
--- a/mlir/test/lib/Dialect/Test/TestDialect.cpp
+++ b/mlir/test/lib/Dialect/Test/TestDialect.cpp
@@ -16,6 +16,7 @@
 #include "mlir/Dialect/Tensor/IR/Tensor.h"
 #include "mlir/IR/BuiltinOps.h"
 #include "mlir/IR/DialectImplementation.h"
+#include "mlir/IR/OpImplementation.h"
 #include "mlir/IR/PatternMatch.h"
 #include "mlir/IR/TypeUtilities.h"
 #include "mlir/Reducer/ReductionPatternInterface.h"
@@ -1095,6 +1096,34 @@
       /*printBlockTerminators=*/false);
 }
 
+//===----------------------------------------------------------------------===//
+// FormatOperandOptionalTypeOp
+//===----------------------------------------------------------------------===//
+
+static bool isDefinedAbove(Value val, Operation *op) {
+  if (val.isa<BlockArgument>())
+    return true;
+
+  return val.getDefiningOp()->getBlock() == op->getBlock() &&
+         val.getDefiningOp()->isBeforeInBlock(op);
+}
+
+static void printOptionalType(OpAsmPrinter &printer,
+                              FormatOperandOptionalTypeOp op, Type type) {
+  if (isDefinedAbove(op.operand(), op))
+    return;
+
+  printer << ":";
+  printer.printType(type);
+}
+
+static ParseResult parseOptionalType(OpAsmParser &parser, Type &type) {
+  if (parser.parseOptionalColon())
+    return success();
+
+  return parser.parseType(type);
+}
+
 #include "TestOpEnums.cpp.inc"
 #include "TestOpInterfaces.cpp.inc"
 #include "TestOpStructs.cpp.inc"
diff --git a/mlir/test/lib/Dialect/Test/TestOps.td b/mlir/test/lib/Dialect/Test/TestOps.td
--- a/mlir/test/lib/Dialect/Test/TestOps.td
+++ b/mlir/test/lib/Dialect/Test/TestOps.td
@@ -1753,6 +1753,9 @@
 def FormatOperandEOp : FormatOperandBase<"e", [{
   $buildable `,` $operand `:` type($buildable) `,` type($operand) attr-dict
 }]>;
+def FormatOperandOptionalTypeOp : FormatOperandBase<"optional_type", [{
+  $buildable `,` $operand custom<OptionalType>(type($operand)) attr-dict
+}]>;
 
 def FormatSuccessorAOp : TEST_Op<"format_successor_a_op", [Terminator]> {
   let successors = (successor VariadicSuccessor<AnySuccessor>:$targets);
diff --git a/mlir/test/mlir-tblgen/op-format.mlir b/mlir/test/mlir-tblgen/op-format.mlir
--- a/mlir/test/mlir-tblgen/op-format.mlir
+++ b/mlir/test/mlir-tblgen/op-format.mlir
@@ -148,6 +148,12 @@
 // CHECK: test.format_operand_e_op %[[I64]], %[[MEMREF]] : i64, memref<1xf64>
 test.format_operand_e_op %i64, %memref : i64, memref<1xf64>
 
+// CHECK: test.format_operand_optional_type_op %[[I64]], %[[MEMREF]]
+test.format_operand_optional_type_op %i64, %memref
+
+// CHECK: test.format_operand_optional_type_op %[[I64]], %[[MEMREF]]
+test.format_operand_optional_type_op %i64, %memref : memref<1xf64>
+
 // CHECK: test.format_variadic_operand %[[I64]], %[[I64]], %[[I64]] : i64, i64, i64
 test.format_variadic_operand %i64, %i64, %i64 : i64, i64, i64