[m-rev.] diff: fix context disagreement bug

[Zoltan Somogyi]

compiler/prog_io.m:
	Fix an old bug: don't insist that two parts of the same field access
	goal have the same context.

	Always get the context for field access goals from the ^ character.

tests/hard_coded/field_access.m:
	Modify this old test case by giving different parts of a field access
	different contexts. The old version of the compiler mistakenly rejects
	it, but the fixed version accepts it.

Zoltan.

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Index: compiler/prog_io.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/compiler/prog_io.m,v
retrieving revision 1.296
diff -u -b -r1.296 prog_io.m
--- compiler/prog_io.m	3 Dec 2008 05:01:40 -0000	1.296
+++ compiler/prog_io.m	3 Dec 2008 05:02:50 -0000
@@ -2197,15 +2197,14 @@
 
 desugar_field_access(Term) = DesugaredTerm :-
     (
-        Term = functor(atom("^"), [A, RHS], _),
-        RHS  = functor(atom(FieldName), Bs, Context)
+        Term = functor(atom("^"), [A, RHS], Context),
+        RHS  = functor(atom(FieldName), Bs, _)
     ->
         DesugaredTerm = functor(atom(FieldName), Bs ++ [A], Context)
     ;
-        % XXX We shouldn't insist on the context of LHS and RHS being the same.
         Term = functor(atom(":="), [LHS, X], _),
         LHS  = functor(atom("^"), [A, RHS], Context),
-        RHS  = functor(atom(FieldName), Bs, Context)
+        RHS  = functor(atom(FieldName), Bs, _)
     ->
         FunctionName = FieldName ++ " :=",
         DesugaredTerm = functor(atom(FunctionName), Bs ++ [A, X], Context)
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Index: tests/hard_coded/field_syntax.m
===================================================================
RCS file: /home/mercury/mercury1/repository/tests/hard_coded/field_syntax.m,v
retrieving revision 1.1
diff -u -b -r1.1 field_syntax.m
--- tests/hard_coded/field_syntax.m	16 Jan 2003 02:34:35 -0000	1.1
+++ tests/hard_coded/field_syntax.m	3 Dec 2008 01:00:41 -0000
@@ -35,7 +35,20 @@
 :- func (array(T) ^ elem_from_end(int) := T)  = array(T).
 :- mode (array_di ^ elem_from_end(in)  := in) = array_uo is det.
 
-(A ^ elem_from_end(I) := X) = (A ^ elem(max(A) - I) := X).
+% This formatting gives each part of the term a different context.
+% This is a regression test for a bug in which the code that recognized
+% field accesses insisted on the same context for parts of the term.
+(A
+    ^
+        elem_from_end(I)
+            :=
+                X)
+                    =
+                        (A
+                            ^
+                                elem(max(A) - I)
+                                    :=
+                                        X).
 
 %-----------------------------------------------------------------------------%
 
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