We set the `declaring_impl_decls` state to modify the evaluation of
LookupImplWitness instructions formed inside an `impl as` decl's
constraint. However while checking that constraint, we may form other
specifics through calls to generic classes, etc. Specifics are canonical
and are keyed by their arguments and the generic ID. This means every
time we see the same arguments for a specific, we must produce the same
instructions. The `declaring_impl_decls` state can break that, causing
instructions inside the specific to be formed in a way they wouldn't at
another time.
To fix this, remove the effect of `declaring_impl_decls` while resolving
a specific. This keeps the specific's evaluation consistent so that the
key truly represents the canonical specific for its arguments.
This doesn't break anything. Existing LookupImplWitnesses (such as in a
class decl) are not guaranteed to point to an `impl as` when they are
modified by a specific. It is designators in the `impl as` constraint
that are guaranteed as such. Designators in an `extend require impls`
statement in a named constraint will be pointed to the `impl as` through
another mechanism in a followup, which will make the
`impl_as_named_constraint.carbon` test pass again.
Dana Jansens
71af207401