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Ömer Sinan Ağacan authored
Currently, GHC's warning generation code is assuming that a name (`RdrName`) can be imported at most once. This is a correct assumption, because 1) it's OK to import same names as long as we don't use any of them 2) when we use one of them, GHC generates an error because it doesn't disambiguate it automatically. But apparently the story is different with typeclass methods. If I import two methods with same names, it's OK to use them in typeclass instance declarations, because the context specifies which one to use. For example, this is OK (where modules A and B define typeclasses A and B, both with a function has), import A import B data Blah = Blah instance A Blah where has = Blah instance B Blah where has = Blah But GHC's warning generator is not taking this into account, and so if I change import list of this program to: import A (A (has)) import B (B (has)) GHC is printing these warnings: Main.hs:5:1: Warning: The import of ‘A.has’ from module ‘A’ is redundant Main.hs:6:1: Warning: The import of ‘B.has’ from module ‘B’ is redundant Why? Because warning generation code is _silently_ ignoring multiple symbols with same names. With this patch, GHC takes this into account. If there's only one name, then this patch reduces to the previous version, that is, it works exactly the same as current GHC (thanks goes to @quchen for realizing this). Reviewed By: austin Differential Revision: https://phabricator.haskell.org/D1257 GHC Trac Issues: #10890
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