Opened 13 months ago

Last modified 10 months ago

#15594 patch bug

--abi-hash with Backpack incorrectly loads modules from dependent packages

Reported by: ezyang Owned by:
Priority: normal Milestone: 8.6.1
Component: Compiler Version: 8.4.3
Keywords: backpack Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s): Phab:D5123
Wiki Page:

Description (last modified by ezyang)

Repro at:

If you use Backpack with data families, you might fail during --abi-hash with:

exited with an error:
Failed to load interface for ‘Stuff’
no unit id matching ‘backpack-trans-’ was

Change History (4)

comment:1 Changed 13 months ago by ezyang

Differential Rev(s): Phab:D5123
Status: newpatch

comment:2 Changed 12 months ago by ezyang

Description: modified (diff)

comment:3 Changed 12 months ago by ezyang

Description: modified (diff)

comment:4 Changed 10 months ago by Edward Z. Yang <ezyang@…>

In 13ff0b7c/ghc:

Fix #15594 (--abi-hash with Backpack sometimes fails)

For holes, its necessary to "see through" the instantiation
of the hole to get accurate family instance dependencies.
For example, if B imports <A>, and <A> is instantiated with
F, we must grab and include all of the dep_finsts from
F to have an accurate transitive dep_finsts list.

However, we MUST NOT do this for regular modules.
First, for efficiency reasons, doing this
bloats the the dep_finsts list, because we *already* had
those modules in the list (it wasn't a hole module, after
all). But there's a second, more important correctness
consideration: we perform module renaming when running
--abi-hash.  In this case, GHC's contract to the user is that
it will NOT go and read out interfaces of any dependencies
(; the point of
--abi-hash is just to get a hash of the on-disk interfaces
for this *specific* package.  If we go off and tug on the
interface for /everything/ in dep_finsts, we're gonna have a
bad time.  (It's safe to do do this for hole modules, though,
because the hmap for --abi-hash is always trivial, so the
interface we request is local.  Though, maybe we ought
not to do it in this case either...)

Signed-off-by: Edward Z. Yang <>

Test Plan: validate

Reviewers: alexbiehl, goldfire, bgamari

Subscribers: ppk, shlevy, rwbarton, carter

GHC Trac Issues: #15594

Differential Revision:
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