Hi Simon,
Note that this is occurring while compiling `ghc-boot-th-next`. This package vendors the TemplateHaskell interfaces from ghc-internal and is built by the boot compiler. You will find a bunch of CPP in the TemplateHaskell modules like `GHC.Internal.TH.Syntax` that switches out `ghc-internal` imports for `base` imports when bootstrapping so we can compile with the boot compiler. For more context see `Note [Bootstrapping Template Haskell]`.
I think to avoid this you will need to be careful to only apply these flags when the BOOTSTRAP_TH flag is set.
In particular, I think what's happening is that we try to find the GHC.KnownKey module and then start pulling in all of ghc-internal.
Cheers,
Teo
On Wed, 25 Mar 2026, at 12:56 PM, Brandon Allbery via ghc-devs wrote:
I'd suspect it's your boot compiler producing that error, and that what changed might have been the file being compiled, causing a recompile with the wrong compiler?
Friends
After 10 days of working on !15719 I have suddenly started getting
/home/simonpj/code/HEAD-19/libraries/ghc-boot-th-next/../ghc-internal/src/GHC/Internal/Classes.hs:10:17: error: [GHC-04924]
Unknown flag in {-# OPTIONS_GHC #-} pragma: -fdefines-known-key-names
|
10 | {-# OPTIONS_GHC -fdefines-known-key-names #-}
| ^^^^^^^^^^^^^^^^^^^^^^^^^
I have no idea why this was fine until 10am this morning, and has suddenly started happening now.
How does GHC decide what runtime flags it allows? `-fdefines-known-key-names` is in GHC.Driver.Session.fFlagsDeps.
I'm boggled that it worked until 10am and then abruptly stopped.
Any advice?
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