[m-rev.] fix missed mode error for free-free unification

[Fergus Henderson]

Estimated hours taken: 2
Branches: main

Fix a bug where we were missing mode errors, and in some cases generating
incorrect code as a result.

compiler/modecheck_unify.m:
	Disallow the case of free-free unifications where both variables
	are locked.

tests/invalid/Mmakefile:
tests/invalid/freefree.err_exp
	Enable the freefree test case, since we now pass it.

BUGS:
	Delete the bug report for this bug.

Workspace: /home/ceres/fjh/mercury
Index: BUGS
===================================================================
RCS file: /home/mercury1/repository/mercury/BUGS,v
retrieving revision 1.17
diff -u -d -r1.17 BUGS
--- BUGS	25 Jun 2003 06:57:33 -0000	1.17
+++ BUGS	3 Aug 2003 10:48:54 -0000
@@ -120,36 +120,6 @@
 but I thought I'd point it out to you before I forgot it.
 
 -----------------------------------------------------------------------------
-Subject: missed mode error
-Date: Tue, 28 May 1996 02:27:34 +1000 (EST)
-
-Another one for the bug report file:
-
-The goal `some [X, Y] X \= Y' should be a mode error,
-but the current mode checker doesn't report an error.
-Instead, the compiler goes on to generate code which gives
-the wrong answer.  For example, the following program prints out `no'.
-The same problem also occurs with `some [X, Y] (X = Y -> fail ; true)'.
-
-:- module bug.
-:- interface.
-:- import_module io.
-
-:- pred main(io__state::di, io__state::uo) is det.
-
-:- implementation.
-
-main --> 
-	( { p } -> io__write_string("yes\n") ; io__write_string("no\n") ).
-
-:- pred p is semidet.
-p :-
-	some [X, Y] X \= Y.
-
-The bug occurs only when the variables being unified inside a negated
-context are not live, i.e. when it is the last occurrence of those variables.
-
------------------------------------------------------------------------------
 Subject: bug with PC values on Alpha
 Date: Wed, 12 Jun 1996 15:45:59 +1000
 
Index: compiler/modecheck_unify.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/modecheck_unify.m,v
retrieving revision 1.60
diff -u -d -r1.60 modecheck_unify.m
--- compiler/modecheck_unify.m	25 Jul 2003 02:27:22 -0000	1.60
+++ compiler/modecheck_unify.m	3 Aug 2003 10:25:15 -0000
@@ -88,7 +88,21 @@
 			BothLive = dead
 		),
 		abstractly_unify_inst(BothLive, InstOfX, InstOfY,
-			real_unify, ModuleInfo0, UnifyInst, Det1, ModuleInfo1)
+			real_unify, ModuleInfo0, UnifyInst, Det1, ModuleInfo1),
+		% Don't allow free-free unifications if both variables are
+		% locked.  (Normally the checks for binding locked variables
+		% are done in modecheck_set_var_inst, which is called below,
+		% but that won't catch this case, because the inst of the
+		% variable will remain `free'.  XXX are there other cases
+		% like this one?)
+		\+ (
+			UnifyInst = free,
+			mode_info_var_is_locked(ModeInfo0, X, _XLockedReason),
+			mode_info_var_is_locked(ModeInfo0, Y, _YLockedReason),
+			% a unification of the form `X = X' doesn't bind X,
+			% and thus should be allowed even if X is locked
+			X \= Y
+		)
 	->
 		Inst = UnifyInst,
 		Det = Det1,
Index: tests/invalid/Mmakefile
===================================================================
RCS file: /home/mercury1/repository/tests/invalid/Mmakefile,v
retrieving revision 1.139
diff -u -d -r1.139 Mmakefile
--- tests/invalid/Mmakefile	25 Jul 2003 06:30:08 -0000	1.139
+++ tests/invalid/Mmakefile	3 Aug 2003 10:17:30 -0000
@@ -69,6 +69,7 @@
 	foreign_type_visibility \
 	func_errors \
 	funcs_as_preds \
+	freefree \
 	ho_default_func_1 \
 	ho_default_func_3 \
 	ho_type_mode_bug \
@@ -175,7 +176,6 @@
 #		when doing dynamic linking, or when the library
 #		was built with `--split-c-files')
 #	parent.undeclared_child (just not yet implemented)
-#	freefree 	(need bromage's aliasing stuff)
 #	typeclass_test_8 (minor formatting error in the output --
 #			the type class name should be in quotes)
 #	typeclass_mode_{2,3,4} (compiler calls error/1)

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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