[m-dev.] for review: fix a bug in the handling of type_desc

[Thomas Conway]

Hi

Zoltan and Fergus, can you verify that this is correct?
The test cases that were aborting on arrays of univs in the debug
grades were doing so because in asm_fast.gc, the type specific
unification pred was being called for type_desc, but in debug
grade, generic unify was being called. The code in the latter
was treating type_desc as c_pointer which is wrong.

-- 
 Thomas Conway              Mercurian )O+  
 <conway at cs.mu.oz.au>       Every sword has two edges.

Fix a bug in the representation of type_desc which meant that
comparison and equality on them didn't work in some cases.

library/std_util.m:
	Use MR_TYPECTOR_REP_TYPEINFO rather than MR_TYPECTOR_REP_C_POINTER
	to denote how type_desc should be handled in generic unifications
	and comparisons.

Index: std_util.m
===================================================================
RCS file: /home/staff/zs/imp/mercury/library/std_util.m,v
retrieving revision 1.217
diff -u -r1.217 std_util.m
--- std_util.m	2001/01/09 23:30:18	1.217
+++ std_util.m	2001/01/16 23:47:48
@@ -1238,7 +1238,7 @@
 #else
 
 MR_DEFINE_BUILTIN_TYPE_CTOR_INFO(std_util, type_desc, 0,
-	MR_TYPECTOR_REP_C_POINTER);
+	MR_TYPECTOR_REP_TYPEINFO);
 
 MR_define_extern_entry(mercury____Unify___std_util__type_desc_0_0);
 MR_define_extern_entry(mercury____Compare___std_util__type_desc_0_0);
@@ -1301,7 +1301,7 @@
 :- pragma foreign_code("MC++", "
 
 MR_DEFINE_BUILTIN_TYPE_CTOR_INFO(std_util, type_desc, 0, 
-        MR_TYPECTOR_REP_C_POINTER)
+        MR_TYPECTOR_REP_TYPEINFO)
 
 static int MR_compare_type_info(MR_TypeInfo x, MR_TypeInfo y) {
 	mercury::runtime::Errors::SORRY(""foreign code for this function"");
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