Build failure -- missing dependency? Help!

[Simon Peyton Jones]

I'm getting this (with 'sh validate -legacy').  Oddly

  *   It does not happen on HEAD
  *   It does happen on wip/T19495, a tiny patch with one innocuous change to GHC.Tc.Gen.HsType
I can't see how my patch could possible cause "missing files" in ghc-bignum!
I'm guessing that there is a missing dependency that someone doesn't show up in master, but does in my branch, randomly.
There's something funny about ghc-bignum; it doesn't seem to be a regular library
Can anyone help?
Thanks
Simon

"inplace/bin/ghc-stage1" -hisuf hi -osuf  o -hcsuf hc -static  -O -H64m -Wall -fllvm-fill-undef-with-garbage    -Werror    -this-unit-id base-4.16.0.0 -hide-all-packages -package-env - -i -ilibraries/base/. -ilibraries/base/dist-install/build -Ilibraries/base/dist-install/build -ilibraries/base/dist-install/build/./autogen -Ilibraries/base/dist-install/build/./autogen -Ilibraries/base/include -Ilibraries/base/dist-install/build/include    -optP-include -optPlibraries/base/dist-install/build/./autogen/cabal_macros.h -package-id ghc-bignum-1.0 -package-id ghc-prim-0.8.0 -package-id rts -this-unit-id base -Wcompat -Wnoncanonical-monad-instances -XHaskell2010 -O -dcore-lint -ticky -Wwarn  -no-user-package-db -rtsopts  -Wno-trustworthy-safe -Wno-deprecated-flags     -Wnoncanonical-monad-instances  -outputdir libraries/base/dist-install/build  -dynamic-too -c libraries/base/./GHC/Exception/Type.hs-boot -o libraries/base/dist-install/build/GHC/Exception/Type.o-boot -dyno libraries/base/dist-install/build/GHC/Exception/Type.dyn_o-boot

Failed to load interface for 'GHC.Num.Integer'

There are files missing in the 'ghc-bignum' package,

try running 'ghc-pkg check'.

Use -v (or `:set -v` in ghci) to see a list of the files searched for.

make[1]: *** [libraries/base/ghc.mk:4: libraries/base/dist-install/build/GHC/Exception/Type.o-boot] Error 1

make[1]: *** Waiting for unfinished jobs....
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20210314/cec830ab/attachment.html>