COMPLETE pragmas for enhanced pattern exhaustiveness checking

This patch adds a new pragma so that users can specify `COMPLETE` sets of
`ConLike`s in order to sate the pattern match checker.

A function which matches on all the patterns in a complete grouping
will not cause the exhaustiveness checker to emit warnings.

```
pattern P :: ()
pattern P = ()

{-# COMPLETE P #-}

foo P = ()
```

This example would previously have caused the checker to warn that
all cases were not matched even though matching on `P` is sufficient to
make `foo` covering. With the addition of the pragma, the compiler
will recognise that matching on `P` alone is enough and not emit
any warnings.

Reviewers: goldfire, gkaracha, alanz, austin, bgamari

Reviewed By: alanz

Subscribers: lelf, nomeata, gkaracha, thomie

Differential Revision: https://phabricator.haskell.org/D2669

GHC Trac Issues: #8779

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/rename/RnBinds.hs b/compiler/rename/RnBinds.hs
index c232e76ea0..f6a22f5df2 100644
--- a/compiler/rename/RnBinds.hs
+++ b/compiler/rename/RnBinds.hs
@@ -950,6 +950,13 @@ renameSig ctxt sig@(SCCFunSig st v s)
   = do  { new_v <- lookupSigOccRn ctxt sig v
         ; return (SCCFunSig st new_v s, emptyFVs) }
 
+-- COMPLETE Sigs can refer to imported IDs which is why we use
+-- lookupLocatedOccRn rather than lookupSigOccRn
+renameSig _ctxt (CompleteMatchSig s (L l bf) mty)
+  = do new_bf <- traverse lookupLocatedOccRn bf
+       new_mty  <- traverse lookupLocatedOccRn mty
+       return (CompleteMatchSig s (L l new_bf) new_mty, emptyFVs)
+
 ppr_sig_bndrs :: [Located RdrName] -> SDoc
 ppr_sig_bndrs bs = quotes (pprWithCommas ppr bs)
 
@@ -991,6 +998,9 @@ okHsSig ctxt (L _ sig)
      (SCCFunSig {}, HsBootCtxt {}) -> False
      (SCCFunSig {}, _)             -> True
 
+     (CompleteMatchSig {}, TopSigCtxt {} ) -> True
+     (CompleteMatchSig {}, _)              -> False
+
 -------------------
 findDupSigs :: [LSig RdrName] -> [[(Located RdrName, Sig RdrName)]]
 -- Check for duplicates on RdrName version,