diff --git a/mlir/include/mlir/IR/AffineMap.h b/mlir/include/mlir/IR/AffineMap.h
--- a/mlir/include/mlir/IR/AffineMap.h
+++ b/mlir/include/mlir/IR/AffineMap.h
@@ -166,9 +166,10 @@
   /// when the caller knows it is safe to do so.
   unsigned getDimPosition(unsigned idx) const;
 
-  /// Extracts the permuted position where given input index resides.
-  /// Fails when called on a non-permutation.
-  unsigned getPermutedPosition(unsigned input) const;
+  /// Extracts the permuted position where the given input index resides.
+  /// Returns `llvm::None` if the input index is projected. Asserts on
+  /// non-projected permutation maps.
+  Optional<unsigned> getPermutedPosition(unsigned input) const;
 
   /// Return true if any affine expression involves AffineDimExpr `position`.
   bool isFunctionOfDim(unsigned position) const {
diff --git a/mlir/lib/Dialect/SparseTensor/IR/SparseTensorDialect.cpp b/mlir/lib/Dialect/SparseTensor/IR/SparseTensorDialect.cpp
--- a/mlir/lib/Dialect/SparseTensor/IR/SparseTensorDialect.cpp
+++ b/mlir/lib/Dialect/SparseTensor/IR/SparseTensorDialect.cpp
@@ -300,7 +300,9 @@
     auto order = enc.getDimOrdering();
     if (order) {
       assert(order.isPermutation());
-      return order.getPermutedPosition(d);
+      auto maybePos = order.getPermutedPosition(d);
+      assert(maybePos.has_value());
+      return *maybePos;
     }
   }
   return d;
diff --git a/mlir/lib/IR/AffineMap.cpp b/mlir/lib/IR/AffineMap.cpp
--- a/mlir/lib/IR/AffineMap.cpp
+++ b/mlir/lib/IR/AffineMap.cpp
@@ -328,12 +328,12 @@
   return getResult(idx).cast<AffineDimExpr>().getPosition();
 }
 
-unsigned AffineMap::getPermutedPosition(unsigned input) const {
-  assert(isPermutation() && "invalid permutation request");
+Optional<unsigned> AffineMap::getPermutedPosition(unsigned input) const {
+  assert(isProjectedPermutation() && "invalid projected permutation request");
   for (unsigned i = 0, numResults = getNumResults(); i < numResults; i++)
     if (getDimPosition(i) == input)
       return i;
-  llvm_unreachable("incorrect permutation request");
+  return llvm::None;
 }
 
 /// Folds the results of the application of an affine map on the provided