Fail self-recursive deferred given impl #22595
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At typer, fail if implementing a deferred given picks the implementing member, which can happen in the companion to the type class.
The recursion is not detected by
CheckLoopingImplicits, where a warning might have sufficed, because the RHS gets the symbol of the deferred member and not its implementation.It behooves typer to refuse to infer the member in this case.
The feature is intended to reduce boilerplate where the implicit value is found in lexical scope, so that not too much magic is demanded.
Fixes #22589