Throwing LinearOperatorException when storage types cannot be promoted

src/operations.jl

@@ -136,8 +136,15 @@ function *(op1::AbstractLinearOperator, op2::AbstractLinearOperator)
     throw(LinearOperatorException("shape mismatch"))
   end
   S = promote_type(storage_type(op1), storage_type(op2))
-  isconcretetype(S) ||
-    throw(LinearOperatorException("storage types cannot be promoted to a concrete type"))
+  if !isconcretetype(S)
+    throw(
+      LinearOperatorException(
+        "storage types $(storage_type(op1)) and $(storage_type(op2)) " *
+        "cannot be promoted to a concrete type. " *
+        "Ensure both operators use compatible storage types (e.g., both GPU or both CPU).",
+      ),
+    )
+  end
   #tmp vector for products
   vtmp = fill!(S(undef, m2), zero(T))
   utmp = fill!(S(undef, n1), zero(T))