diff --git a/mlir/include/mlir/IR/StandardTypes.h b/mlir/include/mlir/IR/StandardTypes.h
--- a/mlir/include/mlir/IR/StandardTypes.h
+++ b/mlir/include/mlir/IR/StandardTypes.h
@@ -330,7 +330,7 @@
     // element type within that dialect.
     return type.isa<ComplexType>() || type.isa<FloatType>() ||
            type.isa<IntegerType>() || type.isa<OpaqueType>() ||
-           type.isa<VectorType>() ||
+           type.isa<VectorType>() || type.isa<IndexType>() ||
            (type.getKind() > Type::Kind::LAST_STANDARD_TYPE);
   }
 
@@ -342,8 +342,8 @@
 };
 
 /// Ranked tensor types represent multi-dimensional arrays that have a shape
-/// with a fixed number of dimensions. Each shape element can be a non-negative
-/// integer or unknown (represented by -1).
+/// with a fixed number of dimensions. Each shape element can be a positive
+/// integer or unknown (represented -1).
 class RankedTensorType
     : public Type::TypeBase<RankedTensorType, TensorType,
                             detail::RankedTensorTypeStorage> {
@@ -416,8 +416,8 @@
 
 /// MemRef types represent a region of memory that have a shape with a fixed
 /// number of dimensions. Each shape element can be a non-negative integer or
-/// unknown (represented by -1). MemRef types also have an affine map
-/// composition, represented as an array AffineMap pointers.
+/// unknown (represented by any negative integer). MemRef types also have an
+/// affine map composition, represented as an array AffineMap pointers.
 class MemRefType : public Type::TypeBase<MemRefType, BaseMemRefType,
                                          detail::MemRefTypeStorage> {
 public:
diff --git a/mlir/test/IR/invalid.mlir b/mlir/test/IR/invalid.mlir
--- a/mlir/test/IR/invalid.mlir
+++ b/mlir/test/IR/invalid.mlir
@@ -23,10 +23,6 @@
 
 func @indexmemref(memref<? x index>) -> () // expected-error {{invalid memref element type}}
 
-// -----
-
-func @indextensor(tensor<4 x index>) -> () // expected-error {{invalid tensor element type}}
-
 // -----
 // Test no map in memref type.
 func @memrefs(memref<2x4xi8, >) // expected-error {{expected list element}}