GHC API reports CPP errors in confusing ways

[MikolajKonarski]

When there is a CPP error, the GHC API provides a normal error message (such as in case of type errors), e.g.,

error: missing binary operator before token "("

but it does not say it's a CPP error. In addition (and unlike for type errors), GHC API throws an exception, e.g.,

phase `C pre-processor' failed (exitcode = 1)

which states the error comes from CPP, but is otherwise uninformative and not tied in any way to the normal error message.

Ideally, all the information would be in the error message and the exception that disrupts the normal control flow would not be thrown.

Here's how to reproduce this: in the following repo (uh, just realised it's a rather large repo)

https://github.com/Mikolaj/ghc/tree/CPPrunGhc

run

ghc --make GhcRun.hs -package ghc-7.6.1

and then

./GhcRun

You should get

Normal error message:
Severity: SevError  SrcSpan: RealSrcSpan (SrcSpanPoint {srcSpanFile = "Ticks.hs", srcSpanLine = 1, srcSpanCol = 0})  MsgDoc:  error: missing binary operator before token "("

Exception:
GhcRun: phase `C pre-processor' failed (exitcode = 1)

[MikolajKonarski]

Attached the source files. Just make sure you run GhcRun? in the same directory that Ticks.hs resides.

[igloo]

Thanks for the report.