"buildable: False" should fail the current configuration

[bos]

(Imported from Trac #231, reported by @dcoutts on 2008-02-05)

Currently resolving configurations does not take account of the buildable field. It should act rather like build-depends: non-existant, though with a different error message.

If possible the error message should relate to the guard condition expression (the conjunction if there are several nested guards).

At the moment it is not considered at all and we don't notice that the package cannot be built until the build step, rather than the configure step.

One problem is that it's not clear if buildable: False is always fatal. People sometimes do that for test executables that are not supposed to be installed. So what should the behaviour be? The lib or only exe failing to configure must be an error.

[bos]

(Imported comment by @dcoutts on 2008-02-05)

Another possibility is to deprecate buildable for this use case and add fail instead which would provide an error message to indicate why a particular configuration is not possible.

The semantics would be slightly different to buildable: False. It would make configuring the whole package fail, not just disable building that executable or library.

[bos]

(Imported comment by @dcoutts on 2008-08-15)

This is doable as part of a larger overhaul of configuration search.

[hvr]

Another possibility is to deprecate buildable for this use case and add fail instead which would provide an error message to indicate why a particular configuration is not possible.

I guess fail would have a similiar effect to build-depends: base<0?

[Ericson2314]

Now that #1725 is fixed (with #3039), can we revisit this? I might take a stab at undoing 04dff51

If I understand correctly, that other commit works for flags but may not work in other situations such as:

name:                Foo
version:             0.1.0.0

library
  if impl(ghcjs)
    buildable: false
  default-language:    Haskell2010

name:                Foo
version:             0.1.0.1

library
  default-language:    Haskell2010

name:                Bar
version:             0.2.0.0

library
  default-language:    Haskell2010
  Build-Depends:       Foo >= 0.1

CC @luite