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Summary:
Suppose that you are typechecking A.hs, which transitively imports,
via B.hs, A.hs-boot. When we poke on B.hs and discover that it
has a reference to a type from A, what TyThing should we wire
it up with? Clearly, if we have already typechecked A, we
should use the most up-to-date TyThing: the one we freshly
generated when we typechecked A. But what if we haven't typechecked
it yet?
For the longest time, GHC adopted the policy that this was
*an error condition*; that you MUST NEVER poke on B.hs's reference
to a thing defined in A.hs until A.hs has gotten around to checking
this. However, actually ensuring this is the case has proven
to be a bug farm. The problem was especially poignant with
type family consistency checks, which eagerly happen before
any typechecking takes place.
This patch takes a different strategy: if we ever try to access
an entity from A which doesn't exist, we just fall back on the
definition of A from the hs-boot file. This means that you may
end up with a mix of A.hs and A.hs-boot TyThings during the
course of typechecking.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: validate
Reviewers: simonpj, bgamari, austin, goldfire
Subscribers: thomie, rwbarton
GHC Trac Issues: #14396
Differential Revision: https://phabricator.haskell.org/D4154
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