[Sema] In simplifyType, if substitution fails retry with IUOs stripped. #7196
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We can end up with IUOs bound as a result of overload resolution. When this happens and the resulting type is used as a generic, we can end up with dependent member types that fail to simplify properly as a result of failing to substitute due to the IUO getting in the way. In these cases it should be completely safe to strip the IUO off and retry substitution with the base type. This isn't really an ideal fix by any means, and we need to revisit how we handle bindings to make it more uniform, but for now I think this is a reasonable path to move forward. Fixes a source compatibility regression. rdar://problem/29960575 (cherry picked from ecedb33 with edits to handle conflicts between branches)
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We can end up with IUOs bound as a result of overload resolution. When
this happens and the resulting type is used as a generic, we can end up
with dependent member types that fail to simplify properly as a result
of failing to substitute due to the IUO getting in the way. In these
cases it should be completely safe to strip the IUO off and retry
substitution with the base type.
This isn't really an ideal fix by any means, and we need to revisit how
we handle bindings to make it more uniform, but for now I think this is
a reasonable path to move forward.
Fixes a source compatibility regression.
rdar://problem/29960575
(cherry picked from ecedb33 with edits
to handle conflicts between branches)