diff --git a/mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td b/mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td
--- a/mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td
+++ b/mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td
@@ -53,18 +53,25 @@
 class Shape_Op<string mnemonic, list<OpTrait> traits = []> :
     Op<ShapeDialect, mnemonic, traits>;
 
-def Shape_AddOp : Shape_Op<"add", [Commutative, SameOperandsAndResultType]> {
-  let summary = "Addition of sizes";
+def Shape_AddOp : Shape_Op<"add", [Commutative, NoSideEffect]> {
+  let summary = "Addition of sizes and indices";
   let description = [{
-    Adds two valid sizes as follows:
-    * lhs + rhs = unknown if either lhs or rhs unknown;
-    * lhs + rhs = (int)lhs + (int)rhs if known;
+    Adds two sizes or indices. If either operand is an error it will be
+    propagated to the result. The operands can be of type `size` or `index`. If
+    at least one of the operands can hold an error, i.e. if it is of type `size`,
+    then also the result must be of type `size`. If error propagation is not
+    possible because both operands are of type `index` then the result must also
+    be of type `index`.
   }];
 
-  let arguments = (ins Shape_SizeType:$lhs, Shape_SizeType:$rhs);
-  let results = (outs Shape_SizeType:$result);
+  let arguments = (ins Shape_SizeOrIndexType:$lhs, Shape_SizeOrIndexType:$rhs);
+  let results = (outs Shape_SizeOrIndexType:$result);
 
-  let assemblyFormat = "$lhs `,` $rhs attr-dict";
+  let assemblyFormat = [{
+    $lhs `,` $rhs `:` type($lhs) `,` type($rhs) `->` type($result) attr-dict
+  }];
+
+  let verifier = [{ return verifySizeOrIndexOp(*this); }];
 }
 
 def Shape_BroadcastOp : Shape_Op<"broadcast", [Commutative]> {
diff --git a/mlir/test/Dialect/Shape/invalid.mlir b/mlir/test/Dialect/Shape/invalid.mlir
--- a/mlir/test/Dialect/Shape/invalid.mlir
+++ b/mlir/test/Dialect/Shape/invalid.mlir
@@ -154,3 +154,19 @@
   return %result : index
 }
 
+// -----
+
+func @add_error_free(%arg : index) -> !shape.size {
+  // expected-error@+1 {{if none of the operands can hold error values then the result must be of type `index`}}
+  %result = shape.add %arg, %arg : index, index -> !shape.size
+  return %result : !shape.size
+}
+
+// -----
+
+func @add_error_possible(%lhs : !shape.size, %rhs : index) -> index {
+  // expected-error@+1 {{if at least one of the operands can hold error values then the result must be of type `size` to propagate them}}
+  %result = shape.add %lhs, %rhs : !shape.size, index -> index
+  return %result : index
+}
+
diff --git a/mlir/test/Dialect/Shape/ops.mlir b/mlir/test/Dialect/Shape/ops.mlir
--- a/mlir/test/Dialect/Shape/ops.mlir
+++ b/mlir/test/Dialect/Shape/ops.mlir
@@ -120,6 +120,15 @@
   return
 }
 
+func @add(%size_arg : !shape.size, %index_arg : index) {
+  %size_sum = shape.add %size_arg, %size_arg
+      : !shape.size, !shape.size -> !shape.size
+  %index_sum = shape.add %index_arg, %index_arg : index, index -> index
+  %mixed_sum = shape.add %size_arg, %index_arg
+      : !shape.size, index -> !shape.size
+  return
+}
+
 func @const_size() {
   // CHECK: %c1 = shape.const_size 1
   // CHECK: %c2 = shape.const_size 2