__fp16 is ambiguous error in C++

[Jim Wilson]

On Thu, Jun 24, 2021 at 7:26 PM ALO via Gcc <gcc@gcc.gnu.org> wrote:

> foo.c: In function '__fp16 foo(__fp16, __fp16)':
> foo.c:6:23: error: call of overloaded 'exp(__fp16&)' is ambiguous
> 6 | return a + std::exp(b);
> | ^
>

No, there isn't a solution for this.  You might want to try an ARM port
clang/gcc to see what they do, but it probably isn't much better than the
RISC-V port.  Looks like the same gcc result to me with a quick check.  And
note that only the non-upstream V extension branch for RISC-V has the
__fp16 support because the vector extension depends on it.  It is hard to
argue for changes when the official RISC-V GCC port has no __fp16 support.

Kito started a related thread in March, and there was tentative agreement
to add _Float16 support to the GCC C++ front end.
https://gcc.gnu.org/pipermail/gcc/2021-March/234971.html
That may or may not help you.

I think it will be difficult to do anything useful here until the C and C++
standards figure out how they want half-float support to work.  If we do
something before then, it will probably end up incompatible with the
official solution and we will end up stuck with a mess.

Jim