[m-rev.] for review: announce the fix for bug #496

[Julien Fischer]

For review by anyone.

Branches: 20.01, master

----------------------------

Announce the fix for bug #496.

NEWS:
     Add a news section for 20.01.1.

     Announce the fix.

     Add a better illustration of the problem that the fix
     addresses.

Julien.

diff --git a/NEWS b/NEWS
index 47c566a..23ded3b 100644
--- a/NEWS
+++ b/NEWS
@@ -24,6 +24,40 @@ Changes to the Mercury standard library

      - func `unsafe_new/2`

+NEWS for Mercury 20.01.1
+========================
+
+This is a bug-fix release.
+
+* [Mantis bug #496]. The compiler now emits a reminder about the limitation
+  of common subexpression elimination in the presence of uniqueness when
+  this is a possible cause of determinsim errors.
+
+  As of version 20.01, common subexpression elimination does not transform
+
+         (
+             X = f(A1, ..., An),
+             goal A
+         ;
+             X = f(B1, ..., Bn),
+             goal B
+         )
+
+  into
+
+         X = f(X1, ..., Xn),
+         (
+             A1 = X1, ..., An = Xn,
+             goal A
+         ;
+             B1 = X1, ..., Bn = Xn,
+             goal B
+         )
+
+  when the insts of some of `X`'s arguments are at least partially unique.
+  This is because the current mode analysis cannot track uniqueness through
+  the extra unifications that this transformation introduces.
+
  NEWS for Mercury 20.01
  ======================