[project @ 2005-01-06 10:04:30 by simonpj]

Wibble to ambiguity reporting

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index c1d0673..97aca4d 100644 (file)
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1933,15 +1933,15 @@ tc_simplify_top is_interactive wanteds
        bad_guys           = non_stds ++ concat std_bads
        (non_ips, bad_ips) = partition isClassDict bad_guys
        (ambigs, no_insts) = partition is_ambig non_ips
-       is_ambig d         = not (isEmptyVarSet (tyVarsOfInst d))
+       is_ambig d         = not (tyVarsOfInst d `subVarSet` fixed_tvs)
+       fixed_tvs          = oclose (fdPredsOfInsts irreds) emptyVarSet
        -- If the dict has free type variables, it's almost certainly ambiguous,
-       -- and that's the first thing to fix
+       -- and that's the first thing to fix.  
        -- Otherwise, addNoInstanceErrs does the right thing
-       -- [ Previously, there was a different no_inst definition:
-       --      no_inst d = not (isTyVarDict d) || tyVarsOfInst d `subVarSet` fixed_tvs
-       --      fixed_tvs = oclose (fdPredsOfInsts tidy_dicts) emptyVarSet
-       --   But that seems over-elaborate to me; it only bites for class decls with
-       --   fundeps like this:         class C a b | -> b where ...]
+       -- I say "almost certain" because we might have
+       --       class C a b | a -> B where ...
+       -- plus an Inst (C Int x).  Then the 'x' isn't ambiguous; it's just that
+       -- there's no instance decl for (C Int ...).  Hence the oclose.
     in
 
        -- Report definite errors