Nothing stopped a type variable being instantiated at dyn, because dyn
is an ordinary case of Types.t and substituted like any other type. The
copy was then made and walked into the dyn answers that are not all
there, and the refusal arrived from inside the generic's own source:
(or-else (Some d) e) over two dyns was reported against <prelude>:385,
a line the caller did not write and cannot act on.
Refused at the binding instead, where the call site is. The message
does not only say no: two models answer "one body, many types" here
and they are not rivals -- this one copies per written type at compile
time, defgeneric/defmethod dispatch at run time on a value that
carries its own -- so a dyn argument is asking the second question of
the first machinery, and the sentence names the other spelling.
Only the unbounded half is new. A variable carrying a {:where} clause
was already refused, because pred_holds says no to dyn for all four
predicates, and that refusal is left in front of this one on purpose:
it names the predicate the signature wrote down, which is the more
specific of the two answers.
Whether dyn should eventually flow through a generic is the author's
call and is recorded as open. Refusing now is the direction that can
be walked back: allowing it later adds programs, and nothing written
under this rule stops compiling.
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