Improve error message if you co_await a non-awaitable #702
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Previous error message was ``` error C2672: 'get_awaiter': no matching overloaded function found ``` New error message is ``` error C2338: Not an awaitable type ``` If you `co_await` something that has both a member `operator co_await` as well as a free `operator co_await`, the error message used to be ``` error C2668: 'winrt::impl::get_awaiter': ambiguous call to overloaded function ``` and it is now ``` error C2338: Ambiguous operator co_await (as both member and free function) ``` The C++ standard has rules for resolving the ambiguity, but they are difficult to express in template metaprogramming, so we don't try. Hasn't been a problem yet. Added some ambiguous awaiters to the unit tests to verify that we resolve to the correct one. Remove vestigial support for free awaiters, which is left over from an earlier iteration of the coroutine TS. (There is an interesting asymmetry in awaiter_finder: We always search for `await_ready` on an lvalue, whereas the `operator co_await` uses the original reference class of the object being awaited. This is a consequence of the language specification, which converts the awaiter to an lvalue *after* applying `operator co_await` but *before* calling `await_ready`.)
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Previous error message was
New error message is
If you
co_awaitsomething that has both a memberoperator co_awaitas well as a freeoperator co_await, the error message used to beand it is now
The C++ standard has rules for resolving the ambiguity, but they are difficult to express in template metaprogramming, so we don't try and simply reject them. Hasn't been a problem yet. Added some ambiguous awaiters to the unit tests to verify that we resolve to the correct one in the cases we support.
Remove vestigial support for free awaiters, which is left over from an earlier iteration of the coroutine TS.
(There is an interesting asymmetry in
awaiter_finder: We always search forawait_readyon an lvalue, whereas theoperator co_awaituses the original reference category of the object being awaited. This is a consequence of the language specification, which converts the awaiter to an lvalue after applyingoperator co_awaitbut before callingawait_ready.)