Matrix product always falls back to native implementation

I am trying to implement HPX backend for matrix_product.
I am not certainly clear with this section of code (below).
The following always evalutes to false right ?
Because both qualified lookup and unqualified lookup of matrix_product return void, so the second part of the expression always evaluates to false and hence whole std::enable_if_t is false.

template <class Exec, class A_t, class B_t, class C_t>
struct is_custom_matrix_product_avail<
  Exec, A_t, B_t, C_t,
  std::enable_if_t<
    std::is_void_v<
      decltype(
	      matrix_product(
          std::declval<Exec>(),
          std::declval<A_t>(),
		      std::declval<B_t>(),
		      std::declval<C_t>()))
      >
    && !std::is_same_v< // see #218
      decltype(
        std::experimental::linalg::matrix_product(
          std::declval<Exec>(),
          std::declval<A_t>(),
		      std::declval<B_t>(),
		      std::declval<C_t>())),
      decltype(
        matrix_product(
          std::declval<Exec>(),
          std::declval<A_t>(),
		      std::declval<B_t>(),
		      std::declval<C_t>()))
      >
    && !linalg::impl::is_inline_exec_v<Exec>
    >
  >
  : std::true_type{};

Which makes this : https://github.com/kokkos/stdBLAS/blob/main/include/experimental/__p1673_bits/blas3_matrix_product.hpp#L757 always false and hence falls backs to native sequential implementation.

Read include/experimental/__p1673_bits/blas3_matrix_product.hpp around line 757 and trace is_custom_matrix_product_avail for an HPX execution type. Confirm how qualified and unqualified matrix_product lookup behaves, then determine why the custom path is rejected. Done means the HPX backend can be selected instead of always falling back to the native sequential implementation.