Make use of boot TyThings during typechecking.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T14396f.hs b/testsuite/tests/typecheck/should_compile/T14396f.hs
new file mode 100644
index 0000000000..e6230fd11b
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T14396f.hs
@@ -0,0 +1,3 @@
+{-# LANGUAGE TypeFamilies #-}
+module T14396f where
+type family F a