`wasi32` cross target GHC failure

[dhess]

Hi! Immense thanks for making the wasi32 cross-target available in haskell.nix!

We've been building our application using the ghc-meta-wasm flake for about a year, and are now trying to get a working build with the haskell.nix wasi32 target, but we're encountering what looks like an internal GHC bug when compiling generics-sop:

Running phase: unpackPhase
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking source archive /nix/store/2smbbljgf6max5nk6zvjml826dmmbf5p-generics-sop-0.5.1.4.tar.gz
source root is generics-sop-0.5.1.4
setting SOURCE_DATE_EPOCH to timestamp 1000000000 of file "generics-sop-0.5.1.4/test/HTransExample.hs"
Running phase: patchPhase
@nix { "action": "setPhase", "phase": "patchPhase" }
'/tmp/nix-build-generics-sop-lib-generics-sop-static-wasm32-unknown-wasi-0.5.1.4.drv-0/.attr-0h9ydb7s8pfhl5kfs3rmmwnmpi49yyiw7jgr5dpw9rmvv8sy0f2b' -> 'generics-sop.cabal'
Running phase: updateAutotoolsGnuConfigScriptsPhase
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
Running phase: updateAutotoolsGnuConfigScriptsPhase
@nix { "action": "setPhase", "phase": "updateAutotoolsGnuConfigScriptsPhase" }
Running phase: configurePhase
@nix { "action": "setPhase", "phase": "configurePhase" }
Configure flags:
--prefix=/nix/store/95sha6bw7h8h7lczgvdrmh678s4x44dv-generics-sop-lib-generics-sop-static-wasm32-unknown-wasi-0.5.1.4 --ghc-option=-fwrite-ide-info --ghc-option=-hiedir/nix/store/6b2kszwfqywz55n9ncz0j358gg7yjn9m-generics-sop-lib-generics-sop-static-wasm32-unknown-wasi-0.5.1.4-hie lib:generics-sop --package-db=clear --package-db=/tmp/nix-build-generics-sop-lib-generics-sop-static-wasm32-unknown-wasi-0.5.1.4.drv-0/tmp.CFCdoAk>
Configuring library for generics-sop-0.5.1.4...
Running phase: buildPhase
@nix { "action": "setPhase", "phase": "buildPhase" }
Preprocessing library for generics-sop-0.5.1.4...
Building library for generics-sop-0.5.1.4...
[ 1 of 14] Compiling Generics.SOP.BasicFunctors ( src/Generics/SOP/BasicFunctors.hs, dist/build/Generics/SOP/BasicFunctors.o, dist/build/Generics/SOP/BasicFunctors.dyn_o )
[ 2 of 14] Compiling Generics.SOP.Classes ( src/Generics/SOP/Classes.hs, dist/build/Generics/SOP/Classes.o, dist/build/Generics/SOP/Classes.dyn_o )
[ 3 of 14] Compiling Generics.SOP.Constraint ( src/Generics/SOP/Constraint.hs, dist/build/Generics/SOP/Constraint.o, dist/build/Generics/SOP/Constraint.dyn_o )
[ 4 of 14] Compiling Generics.SOP.Dict ( src/Generics/SOP/Dict.hs, dist/build/Generics/SOP/Dict.o, dist/build/Generics/SOP/Dict.dyn_o )
[ 5 of 14] Compiling Generics.SOP.NP  ( src/Generics/SOP/NP.hs, dist/build/Generics/SOP/NP.o, dist/build/Generics/SOP/NP.dyn_o )
[ 6 of 14] Compiling Generics.SOP.Metadata ( src/Generics/SOP/Metadata.hs, dist/build/Generics/SOP/Metadata.o, dist/build/Generics/SOP/Metadata.dyn_o )
[ 7 of 14] Compiling Generics.SOP.NS  ( src/Generics/SOP/NS.hs, dist/build/Generics/SOP/NS.o, dist/build/Generics/SOP/NS.dyn_o )
[ 8 of 14] Compiling Generics.SOP.Sing ( src/Generics/SOP/Sing.hs, dist/build/Generics/SOP/Sing.o, dist/build/Generics/SOP/Sing.dyn_o )
[ 9 of 14] Compiling Generics.SOP.Type.Metadata ( src/Generics/SOP/Type/Metadata.hs, dist/build/Generics/SOP/Type/Metadata.o, dist/build/Generics/SOP/Type/Metadata.dyn_o )
[10 of 14] Compiling Generics.SOP.GGP ( src/Generics/SOP/GGP.hs, dist/build/Generics/SOP/GGP.o, dist/build/Generics/SOP/GGP.dyn_o )
[11 of 14] Compiling Generics.SOP.Universe ( src/Generics/SOP/Universe.hs, dist/build/Generics/SOP/Universe.o, dist/build/Generics/SOP/Universe.dyn_o )
[12 of 14] Compiling Generics.SOP.TH  ( src/Generics/SOP/TH.hs, dist/build/Generics/SOP/TH.o, dist/build/Generics/SOP/TH.dyn_o )
src/Generics/SOP/TH.hs:15:1: warning: [GHC-66111] [-Wunused-imports]
    The import of ‘Data.List’ is redundant
      except perhaps to import instances from ‘Data.List’
    To import instances alone, use: import Data.List()
   |
15 | import Data.List (foldl')
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

src/Generics/SOP/TH.hs:23:1: warning: [GHC-66111] [-Wunused-imports]
    The import of ‘Language.Haskell.TH.Datatype.TyVarBndr’ is redundant
      except perhaps to import instances from ‘Language.Haskell.TH.Datatype.TyVarBndr’
    To import instances alone, use: import Language.Haskell.TH.Datatype.TyVarBndr()
   |
23 | import Language.Haskell.TH.Datatype.TyVarBndr
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Generics/SOP/TH.hs:319:34: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘head’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Use pattern matching, 'Data.List.uncons' or 'Data.Maybe.listToMaybe' instead. Consider refactoring to use "Data.List.NonEmpty"."
    |
319 |                         $(mdCon (head cs))
    |                                  ^^^^

src/Generics/SOP/TH.hs:393:38: warning: [GHC-63394] [-Wx-partial]
    In the use of ‘head’
    (imported from Prelude, but defined in GHC.Internal.List):
    "This is a partial function, it throws an error on empty lists. Use pattern matching, 'Data.List.uncons' or 'Data.Maybe.listToMaybe' instead. Consider refactoring to use "Data.List.NonEmpty"."
    |
393 |                             $(mdCon (head cs))
    |                                      ^^^^

[13 of 14] Compiling Generics.SOP.Instances ( src/Generics/SOP/Instances.hs, dist/build/Generics/SOP/Instances.o, dist/build/Generics/SOP/Instances.dyn_o )
<no location info>: error:
    compiler/GHC/Runtime/Interpreter/Wasm.hs:(36,7)-(39,38): Non-exhaustive patterns in Just ghci_unit_id

This error does not occur with the ghc-meta-wasm version of GHC 9.12.2 (specifically, 9.12.2.20250828 from the ghc-meta-wasm flake).

Build details:

• Build system (eg. x86_64-linux or aarch64-darwin): both x86_64-linux and aarch64-darwin
• GHC version: ghc9122
• Haskell.nix version (or commit) used: f104621