Address @adam-smnk's comments

Author: rolfmorel

mlir/include/mlir/Dialect/Linalg/IR/LinalgStructuredOps.td

@@ -769,6 +769,7 @@ def ContractOp : LinalgStructuredBase_Op<"contract", [
 
   let extraClassDeclaration = structuredOpsBaseDecls # [{
     // Declare/implement functions necessary for LinalgStructuredInterface.
+
     /// Infer iterator types for each dim in the domain of IndexingMaps.
     SmallVector<utils::IteratorType> getIteratorTypesArray();
 

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

@@ -3746,22 +3746,29 @@ LogicalResult ContractOp::verify() {
       return failure(); // NOTE: checking lambda will emit error.
 
   bool hasContractingDim = false;
-  for (auto &&[inOccCount, outOccCount] : zip(inOccurrences, outOccurrences)) {
+  for (size_t dimIndex = 0; dimIndex < (size_t)iterationSpaceDims; dimIndex++) {
+    size_t inOccCount = inOccurrences[dimIndex];
+    size_t outOccCount = outOccurrences[dimIndex];
+
     hasContractingDim |= inOccCount == 2 && outOccCount == 0;
 
     if (inOccCount == 0)
-      return emitError("iteration space dim not used by either input");
-
-    // NB: A dim which occurs for only one input operand and not for the output.
-    //     In terms of einsum semantics, such dims have a sensible meaning -
-    //     namely an additional reduction per such dim - though this can also
-    //     always be expressed through an additional op. Additionally, at time
-    //     of writing, vector.contract's verifier accepts these dims but many of
-    //     its lowerings do not handle these kinds of dims. Hence...
+      return emitError() << "iteration space dim at index " << dimIndex
+                         << " not used by either input";
+
+    // NB: We disallow a dim which occurs for only one input operand and not
+    //     for the output. In terms of einsum semantics such dims have a
+    //     sensible meaning - namely an additional reduction per each such dim.
+    //     By contrast, the ContractionOpInterface does not know about this
+    //     iter type - cf. inferContractionDims' supported dim kinds. Similarly,
+    //     while vector.contract's verifier accepts dims of this kind many of
+    //     its lowerings give up on encountering these dims.
     // TODO: Remove following once we have comprehensive support for input-only
     //       reduction dims, at both the linalg- and vector-dialect levels.
     if (inOccCount == 1 && outOccCount != 1)
-      return emitError("iter type of dim is not one of M, N, K or batch");
+      return emitError()
+             << "iteration space dim at index " << dimIndex
+             << " is neither a contracting dim nor of parallel iteration type";
   }
 
   if (!hasContractingDim)

mlir/test/Dialect/Linalg/invalid.mlir

@@ -617,7 +617,7 @@ func.func @mismatch_type_affine_map_and_operand_contraction(%lhs: f32, %rhs: ten
 // -----
 
 func.func @unused_iteration_space_dim_contraction(%lhs: tensor<4x1xf32>, %rhs: tensor<1x64xf32>, %init: tensor<4x64xf32>) {
-  // expected-error @+1 {{iteration space dim not used by either input}}
+  // expected-error @+1 {{iteration space dim at index 3 not used by either input}}
   linalg.contract indexing_maps = [
                     affine_map<(d0, d1, d2, d3) -> (d0, d2)>,
                     affine_map<(d0, d1, d2, d3) -> (d2, d1)>,
@@ -631,7 +631,7 @@ func.func @unused_iteration_space_dim_contraction(%lhs: tensor<4x1xf32>, %rhs: t
 // -----
 
 func.func @unused_iteration_space_dim_contraction(%lhs: tensor<8x4x1xf32>, %rhs: tensor<1x64xf32>, %init: tensor<4x64xf32>) {
-  // expected-error @+1 {{iter type of dim is not one of M, N, K or batch}}
+  // expected-error @+1 {{iteration space dim at index 3 is neither a contracting dim nor of parallel iteration type}}
   linalg.contract indexing_maps = [
                     affine_map<(d0, d1, d2, d3) -> (d0, d2, d3)>,
                     affine_map<(d0, d1, d2, d3) -> (d2, d1)>,