diff --git a/mlir/include/mlir/Dialect/Shape/IR/ShapeBase.td b/mlir/include/mlir/Dialect/Shape/IR/ShapeBase.td
--- a/mlir/include/mlir/Dialect/Shape/IR/ShapeBase.td
+++ b/mlir/include/mlir/Dialect/Shape/IR/ShapeBase.td
@@ -104,11 +104,20 @@
   }];
 }
 
+def Shape_ExtentTensorType :
+    1DTensorOf<[Index]>,
+    BuildableType<"::mlir::RankedTensorType::get({ShapedType::kDynamicSize}, "
+                  "$_builder.getType<::mlir::IndexType>())"> {
+  let typeDescription = [{
+    The extent tensor is a tensor of rank one with arbitrarily many index
+    elements. Like `!shape.shape`, it is used to represent shapes with the
+    difference that it is guaranteed to be error-free.
+  }];
+}
+
 def Shape_ShapeOrSizeType : AnyTypeOf<[Shape_SizeType, Shape_ShapeType],
   "shape or size">;
 
-def Shape_ExtentTensorType : 1DTensorOf<[Index]>;
-
 def Shape_ShapeOrExtentTensorType : AnyTypeOf<[Shape_ShapeType,
                                                Shape_ExtentTensorType],
                                               "shape or extent tensor">;
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
@@ -390,8 +390,15 @@
 def Shape_ShapeOfOp : Shape_Op<"shape_of", [NoSideEffect]> {
   let summary = "Returns shape of a value or shaped type operand";
 
+  let description = [{
+    The operation takes a value-shape tuple or a shaped operand as an argument
+    and returns a description of the shape. Because this operation cannot yield
+    error shapes the result is an extent tensor `tensor<?xindex>`, not a shape
+    type.
+  }];
+
   let arguments = (ins AnyTypeOf<[AnyShaped, Shape_ValueShapeType]>:$arg);
-  let results = (outs Shape_ShapeType:$result);
+  let results = (outs Shape_ExtentTensorType:$result);
 
   let assemblyFormat = "$arg `:` type($arg) attr-dict";
 
diff --git a/mlir/lib/Conversion/ShapeToStandard/ShapeToStandard.cpp b/mlir/lib/Conversion/ShapeToStandard/ShapeToStandard.cpp
--- a/mlir/lib/Conversion/ShapeToStandard/ShapeToStandard.cpp
+++ b/mlir/lib/Conversion/ShapeToStandard/ShapeToStandard.cpp
@@ -86,9 +86,11 @@
       }
     }
 
-    // Materialize shape as ranked tensor.
-    rewriter.replaceOpWithNewOp<TensorFromElementsOp>(op.getOperation(),
-                                                      dimValues);
+    // Materialize extent tensor.
+    Value staticExtentTensor =
+        rewriter.create<TensorFromElementsOp>(loc, dimValues);
+    rewriter.replaceOpWithNewOp<TensorCastOp>(op, staticExtentTensor,
+                                              op.getType());
     return success();
   }
 };
diff --git a/mlir/lib/Dialect/Shape/IR/Shape.cpp b/mlir/lib/Dialect/Shape/IR/Shape.cpp
--- a/mlir/lib/Dialect/Shape/IR/Shape.cpp
+++ b/mlir/lib/Dialect/Shape/IR/Shape.cpp
@@ -24,7 +24,7 @@
 }
 
 namespace {
-RankedTensorType getExtentTensorType(OpBuilder &builder) {
+RankedTensorType inline getExtentTensorType(OpBuilder &builder) {
   return RankedTensorType::get({ShapedType::kDynamicSize},
                                builder.getIndexType());
 }
diff --git a/mlir/test/Conversion/ShapeToStandard/shape-to-standard.mlir b/mlir/test/Conversion/ShapeToStandard/shape-to-standard.mlir
--- a/mlir/test/Conversion/ShapeToStandard/shape-to-standard.mlir
+++ b/mlir/test/Conversion/ShapeToStandard/shape-to-standard.mlir
@@ -95,7 +95,8 @@
   // CHECK-DAG: %[[C1:.*]] = constant 1 : index
   // CHECK-DAG: %[[C2:.*]] = constant 2 : index
   // CHECK-DAG: %[[C3:.*]] = constant 3 : index
-  // CHECK-DAG: %[[SHAPE:.*]] = tensor_from_elements(%[[C1]], %[[C2]], %[[C3]]) : tensor<3xindex>
+  // CHECK-DAG: %[[SHAPE_UNCASTED:.*]] = tensor_from_elements(%[[C1]], %[[C2]], %[[C3]]) : tensor<3xindex>
+  // CHECK-DAG: %[[SHAPE:.*]] = tensor_cast %[[SHAPE_UNCASTED]] : tensor<3xindex> to tensor<?xindex>
   %shape = shape.shape_of %arg : tensor<1x2x3xf32>
   return
 }
@@ -110,7 +111,8 @@
   // CHECK-DAG: %[[C5:.*]] = constant 5 : index
   // CHECK-DAG: %[[C2:.*]] = constant 2 : index
   // CHECK-DAG: %[[DYN_DIM:.*]] = dim %[[ARG]], %[[C2]] : tensor<1x5x?xf32>
-  // CHECK-DAG: %[[SHAPE:.*]] = tensor_from_elements(%[[C1]], %[[C5]], %[[DYN_DIM]]) : tensor<3xindex>
+  // CHECK-DAG: %[[SHAPE_UNCASTED:.*]] = tensor_from_elements(%[[C1]], %[[C5]], %[[DYN_DIM]]) : tensor<3xindex>
+  // CHECK-DAG: %[[SHAPE:.*]] = tensor_cast %[[SHAPE_UNCASTED]] : tensor<3xindex> to tensor<?xindex>
   %shape = shape.shape_of %arg : tensor<1x5x?xf32>
   return
 }
@@ -139,7 +141,7 @@
   // CHECK: %[[RESULT:.*]] = dim %[[ARG]], %[[IDX]] : tensor<2x3xf32>
   // CHECK: return %[[RESULT]] : index
   %shape = shape.shape_of %arg : tensor<2x3xf32>
-  %result = shape.get_extent %shape, %idx : !shape.shape
+  %result = shape.get_extent %shape, %idx : tensor<?xindex>
   return %result : !shape.size
 }
 
diff --git a/mlir/test/Dialect/Shape/canonicalize.mlir b/mlir/test/Dialect/Shape/canonicalize.mlir
--- a/mlir/test/Dialect/Shape/canonicalize.mlir
+++ b/mlir/test/Dialect/Shape/canonicalize.mlir
@@ -1,10 +1,10 @@
-// RUN: mlir-opt -split-input-file -allow-unregistered-dialect -canonicalize <%s | FileCheck %s
+// RUN: mlir-opt -split-input-file -allow-unregistered-dialect -canonicalize %s | FileCheck %s
 
 // CHECK-LABEL: func @f
-func @f(%arg0: tensor<2x3x4xf32>) -> !shape.shape {
-  // CHECK: shape.const_shape [2, 3, 4] : !shape.shape
-  %0 = "shape.shape_of"(%arg0) : (tensor<2x3x4xf32>) -> !shape.shape
-  return %0 : !shape.shape
+func @f(%arg0: tensor<2x3x4xf32>) -> tensor<?xindex> {
+  // CHECK: shape.const_shape [2, 3, 4] : tensor<?xindex>
+  %0 = shape.shape_of %arg0 : tensor<2x3x4xf32>
+  return %0 : tensor<?xindex>
 }
 
 // -----
@@ -510,7 +510,7 @@
   // CHECK-DAG: %[[RESULT:.*]] = shape.const_size 3
   // CHECK-DAG: return %[[RESULT]] : !shape.size
   %shape = shape.shape_of %arg : tensor<1x2x?xf32>
-  %rank = shape.rank %shape : !shape.shape
+  %rank = shape.rank %shape : tensor<?xindex>
   return %rank : !shape.size
 }
 
@@ -524,7 +524,7 @@
   // CHECK-DAG: %[[SIZE:.*]] = shape.rank %[[SHAPE]]
   // CHECK-DAG: return %[[SIZE]] : !shape.size
   %shape = shape.shape_of %arg : tensor<*xf32>
-  %rank = shape.rank %shape : !shape.shape
+  %rank = shape.rank %shape : tensor<?xindex>
   return %rank : !shape.size
 }
 
@@ -560,7 +560,7 @@
   // CHECK-NEXT: return
   %0 = shape.const_shape [] : !shape.shape
   %1 = shape.shape_of %arg0 : tensor<?xf32>
-  %2 = shape.cstr_broadcastable %0, %1 : !shape.shape, !shape.shape
+  %2 = shape.cstr_broadcastable %0, %1 : !shape.shape, tensor<?xindex>
   "consume.witness"(%2) : (!shape.witness) -> ()
   return
 }
@@ -577,7 +577,7 @@
   // CHECK-NEXT: return
   %0 = shape.shape_of %arg0 : tensor<?xf32>
   %1 = shape.shape_of %arg1 : tensor<?xf32>
-  %2 = shape.cstr_broadcastable %0, %1 : !shape.shape, !shape.shape
+  %2 = shape.cstr_broadcastable %0, %1 : tensor<?xindex>, tensor<?xindex>
   "consume.witness"(%2) : (!shape.witness) -> ()
   return
 }
@@ -592,7 +592,7 @@
   // CHECK-NEXT: return
   %0 = shape.shape_of %arg1 : tensor<index>
   %1 = shape.shape_of %arg0 : tensor<*xf32>
-  %2 = shape.cstr_broadcastable %0, %1 : !shape.shape, !shape.shape
+  %2 = shape.cstr_broadcastable %0, %1 : tensor<?xindex>, tensor<?xindex>
   "consume.witness"(%2) : (!shape.witness) -> ()
   return
 }
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
@@ -79,9 +79,9 @@
   return
 }
 
-func @test_shape_of(%arg0: tensor<?xf32>) -> !shape.shape {
+func @test_shape_of(%arg0: tensor<?xf32>) -> tensor<?xindex> {
   %0 = shape.shape_of %arg0 : tensor<?xf32>
-  return %0 : !shape.shape
+  return %0 : tensor<?xindex>
 }
 
 func @test_constraints() {