From: simonpj <unknown>
Date: Fri, 25 Feb 2005 14:15:53 +0000 (+0000)
Subject: [project @ 2005-02-25 14:15:53 by simonpj]
X-Git-Tag: Initial_conversion_from_CVS_complete~1017
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=9ced2b5962e84d84694cde112e1befe52f970d7b

[project @ 2005-02-25 14:15:53 by simonpj]
-----------------------------------------
	Improve ambiguity reporting in TcSimplify
	-----------------------------------------

	Merge to STABLE

The test for ambiguity in tcSimplifyTop caused us to say thing like
	No instance for (Show c)
which is a bit confusing -- and in fact exposed a separate buglet in
the fix-reporting code.

This commit deals with the origial problem.  tcfail133 tests it.
---

diff --git a/ghc/compiler/typecheck/TcSimplify.lhs b/ghc/compiler/typecheck/TcSimplify.lhs
index 8b1fed0..e7cfee9 100644
--- a/ghc/compiler/typecheck/TcSimplify.lhs
+++ b/ghc/compiler/typecheck/TcSimplify.lhs
@@ -1967,16 +1967,10 @@ tc_simplify_top is_interactive wanteds
 		-- Collect together all the bad guys
 	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 (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.  
-	-- Otherwise, addNoInstanceErrs does the right thing
-	-- 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.
+    	(ambigs, no_insts) = partition isTyVarDict non_ips
+	-- If the dict has no type constructors involved, it must be ambiguous,
+	-- except I suppose that another error with fundeps maybe should have
+	-- constrained those type variables
     in
 
 	-- Report definite errors