Make it a warning (not error) to hide an import that isn't exported

We often see build failures like

    Module `Prelude' does not export `catch'

elicited by an import statement

  import Prelude hiding( catch )

John Lato and others suggest making this a warning, not an error. Simon and I agree and we propose to do this for GHC 7.6, thus:

• Warn, rather than error, when a hiding clause mentions something that the module does not export.
• Suppress with warning with -fno-warn-dodgy-imports, an existing warning-suppression flag.
• Document the change, in the -fwarn-docugy-imports flag, and the release notes.

We propose to make this change without a language extension. Doing so is technically wrong, since H2010 says it's an error to mention something in hiding that isn't exported, but it seems too heavyweight to make a language extension just for this; and in any case it would have to be on by default to be any use in practice.

Paolo, can you do all this?

• The relevant function is RnNames.filterImports.
• I suggest you make lookup_ie return TcRn [(LIE Name,AvailInfo)] rather than the MaybeErr ... that it currently returns.
• The want_hiding flag is True if this is a hiding clause; in that case bad_ie should warn (unless the warning is suppressed) rather than error.

Trac metadata

Trac field	Value
Version	7.4.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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