[project @ 2004-11-09 12:45:04 by simonpj]

[ghc-hetmet.git] / ghc / compiler / typecheck / TcTyClsDecls.lhs

diff --git a/ghc/compiler/typecheck/TcTyClsDecls.lhs b/ghc/compiler/typecheck/TcTyClsDecls.lhs
index 9516686..3e72f0e 100644 (file)
--- a/ghc/compiler/typecheck/TcTyClsDecls.lhs
+++ b/ghc/compiler/typecheck/TcTyClsDecls.lhs
@@ -447,6 +447,9 @@ tcConDecl unbox_strict DataType tycon tc_tvs        -- Ordinary data types
     ; let 
        is_vanilla = null ex_tvs && null (unLoc ex_ctxt) 
                -- Vanilla iff no ex_tvs and no context
+               -- Must check the context too because of
+               -- implicit params; e.g.
+               --  data T = (?x::Int) => MkT Int
 
        tc_datacon is_infix field_lbls btys
          = do { let { bangs = map getBangStrictness btys }
@@ -461,7 +464,10 @@ tcConDecl unbox_strict DataType tycon tc_tvs       -- Ordinary data types
     ; case details of
        PrefixCon btys     -> tc_datacon False [] btys
        InfixCon bty1 bty2 -> tc_datacon True [] [bty1,bty2]
-       RecCon fields      -> do { checkTc is_vanilla (exRecConErr name)
+       RecCon fields      -> do { checkTc (null ex_tvs) (exRecConErr name)
+               -- It's ok to have an implicit-parameter context
+               -- for the data constructor, provided it binds
+               -- no type variables
                                 ; let { (field_names, btys) = unzip fields }
                                 ; tc_datacon False field_names btys } }