id<1> overloaded operator '*' is ambiguous

[Pennycook]

The following code fails to compile with SimSYCL:

#include <sycl/sycl.hpp>

int foo(sycl::id<1> i, int j)
{
    return i * j;
}

I think this should work, because SYCL 2020 defines an implicit conversion of id<1> to size_t.

The error message is impressively long, but points to an ambiguous overload of *:

test.cpp:5:14: error: use of overloaded operator '*' is ambiguous (with operand types 'sycl::id<1>' and 'int')
    5 |     return i * j;
      |            ~ ^ ~
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:59:5: note: candidate function
   59 |     SIMSYCL_DETAIL_DEFINE_COORDINATE_BINARY_COPY_LHS_OPERATOR(*)
      |     ^
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:51:32: note: expanded from macro 'SIMSYCL_DETAIL_DEFINE_COORDINATE_BINARY_COPY_LHS_OPERATOR'
   51 |     friend constexpr Interface operator op(const Interface &lhs, const size_t &rhs) {                                  \
      |                                ^
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:59:5: note: candidate function
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:46:32: note: expanded from macro 'SIMSYCL_DETAIL_DEFINE_COORDINATE_BINARY_COPY_LHS_OPERATOR'
   46 |     friend constexpr Interface operator op(const Interface &lhs, const Interface &rhs) {                               \
      |                                ^
test.cpp:5:14: note: built-in candidate operator*(unsigned long, int)
    5 |     return i * j;
      |              ^
test.cpp:5:14: note: built-in candidate operator*(unsigned long, float)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, double)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, long double)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, __float128)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long, unsigned __int128)
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:108:5: note: candidate function
  108 |     SIMSYCL_DETAIL_DEFINE_COORDINATE_BINARY_COPY_RHS_OPERATOR(*)
      |     ^
/usr/local/include/sycl/../simsycl/sycl/../detail/coordinate.hh:100:32: note: expanded from macro 'SIMSYCL_DETAIL_DEFINE_COORDINATE_BINARY_COPY_RHS_OPERATOR'
  100 |     friend constexpr Interface operator op(const size_t &lhs, const Interface &rhs) {                                  \
      |                                ^
test.cpp:5:14: note: built-in candidate operator*(float, int)
    5 |     return i * j;
      |              ^
test.cpp:5:14: note: built-in candidate operator*(double, int)
test.cpp:5:14: note: built-in candidate operator*(long double, int)
test.cpp:5:14: note: built-in candidate operator*(__float128, int)
test.cpp:5:14: note: built-in candidate operator*(int, int)
test.cpp:5:14: note: built-in candidate operator*(long, int)
test.cpp:5:14: note: built-in candidate operator*(long long, int)
test.cpp:5:14: note: built-in candidate operator*(__int128, int)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, int)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, int)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, int)
test.cpp:5:14: note: built-in candidate operator*(float, float)
test.cpp:5:14: note: built-in candidate operator*(float, double)
test.cpp:5:14: note: built-in candidate operator*(float, long double)
test.cpp:5:14: note: built-in candidate operator*(float, __float128)
test.cpp:5:14: note: built-in candidate operator*(float, long)
test.cpp:5:14: note: built-in candidate operator*(float, long long)
test.cpp:5:14: note: built-in candidate operator*(float, __int128)
test.cpp:5:14: note: built-in candidate operator*(float, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(float, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(float, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(float, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(double, float)
test.cpp:5:14: note: built-in candidate operator*(double, double)
test.cpp:5:14: note: built-in candidate operator*(double, long double)
test.cpp:5:14: note: built-in candidate operator*(double, __float128)
test.cpp:5:14: note: built-in candidate operator*(double, long)
test.cpp:5:14: note: built-in candidate operator*(double, long long)
test.cpp:5:14: note: built-in candidate operator*(double, __int128)
test.cpp:5:14: note: built-in candidate operator*(double, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(double, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(double, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(double, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(long double, float)
test.cpp:5:14: note: built-in candidate operator*(long double, double)
test.cpp:5:14: note: built-in candidate operator*(long double, long double)
test.cpp:5:14: note: built-in candidate operator*(long double, __float128)
test.cpp:5:14: note: built-in candidate operator*(long double, long)
test.cpp:5:14: note: built-in candidate operator*(long double, long long)
test.cpp:5:14: note: built-in candidate operator*(long double, __int128)
test.cpp:5:14: note: built-in candidate operator*(long double, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(long double, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(long double, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(long double, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(__float128, float)
test.cpp:5:14: note: built-in candidate operator*(__float128, double)
test.cpp:5:14: note: built-in candidate operator*(__float128, long double)
test.cpp:5:14: note: built-in candidate operator*(__float128, __float128)
test.cpp:5:14: note: built-in candidate operator*(__float128, long)
test.cpp:5:14: note: built-in candidate operator*(__float128, long long)
test.cpp:5:14: note: built-in candidate operator*(__float128, __int128)
test.cpp:5:14: note: built-in candidate operator*(__float128, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(__float128, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(__float128, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(__float128, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(int, float)
test.cpp:5:14: note: built-in candidate operator*(int, double)
test.cpp:5:14: note: built-in candidate operator*(int, long double)
test.cpp:5:14: note: built-in candidate operator*(int, __float128)
test.cpp:5:14: note: built-in candidate operator*(int, long)
test.cpp:5:14: note: built-in candidate operator*(int, long long)
test.cpp:5:14: note: built-in candidate operator*(int, __int128)
test.cpp:5:14: note: built-in candidate operator*(int, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(int, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(int, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(int, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(long, float)
test.cpp:5:14: note: built-in candidate operator*(long, double)
test.cpp:5:14: note: built-in candidate operator*(long, long double)
test.cpp:5:14: note: built-in candidate operator*(long, __float128)
test.cpp:5:14: note: built-in candidate operator*(long, long)
test.cpp:5:14: note: built-in candidate operator*(long, long long)
test.cpp:5:14: note: built-in candidate operator*(long, __int128)
test.cpp:5:14: note: built-in candidate operator*(long, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(long, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(long, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(long, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(long long, float)
test.cpp:5:14: note: built-in candidate operator*(long long, double)
test.cpp:5:14: note: built-in candidate operator*(long long, long double)
test.cpp:5:14: note: built-in candidate operator*(long long, __float128)
test.cpp:5:14: note: built-in candidate operator*(long long, long)
test.cpp:5:14: note: built-in candidate operator*(long long, long long)
test.cpp:5:14: note: built-in candidate operator*(long long, __int128)
test.cpp:5:14: note: built-in candidate operator*(long long, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(long long, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(long long, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(long long, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(__int128, float)
test.cpp:5:14: note: built-in candidate operator*(__int128, double)
test.cpp:5:14: note: built-in candidate operator*(__int128, long double)
test.cpp:5:14: note: built-in candidate operator*(__int128, __float128)
test.cpp:5:14: note: built-in candidate operator*(__int128, long)
test.cpp:5:14: note: built-in candidate operator*(__int128, long long)
test.cpp:5:14: note: built-in candidate operator*(__int128, __int128)
test.cpp:5:14: note: built-in candidate operator*(__int128, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(__int128, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(__int128, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(__int128, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, float)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, double)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, long double)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, __float128)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, long)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned int, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, float)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, double)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, long double)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, __float128)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned long long, unsigned __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, float)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, double)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, long double)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, __float128)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, long)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, __int128)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, unsigned int)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, unsigned long)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, unsigned long long)
test.cpp:5:14: note: built-in candidate operator*(unsigned __int128, unsigned __int128)
1 error generated.

[fknorr]

I believe this is correctly diagnosed, because there also exists an implicit conversion constructor from size_t to id<1>. The compiler could either convert the LHS id<1> to size_t and call built-in operator*(size_t, int) or convert the RHS int to size_t and call friend operator*(id<1>, size_t). To my understanding both these paths have equal weight because they require one implicit argument conversion each. Making j a size_t should solve the problem.

[Pennycook]

This code compiles with DPC++. Maybe we need a Khronos issue to determine which behavior is desired?

[fknorr]

DPC++ has an explicit workaround for this ambiguity. I'm pretty sure the spec doesn't disallow this, but I also could not find any wording that requires it. I agree this needs clarification, the disambiguating overloads seem like a good idea to me.

[Pennycook]

I've opened KhronosGroup/SYCL-Docs#711 to discuss this.