Update behavior of augmented null coalescing operator

x ??= y now behaves like x ?? (x = y) rather than x = x ?? y

These are equivalent when x is null.

When x is non-null, we avoid an unnecessary re-assignment step, which
also allows for the possibility of using this operator with immutable
hash tables without raising errors when keys already exist.

Closes: #3612

M2/Macaulay2/d/evaluate.d

@@ -1236,16 +1236,37 @@ tryEval(c:Code):Expr := (
 	else tryEvalSuccess = true);
     p);
 
+nullify(c:Code):Expr := (
+    e := tryEval(c);
+    if tryEvalSuccess
+    then (
+	when e
+	is Nothing do e
+	else (
+	    f := lookup(Class(e), QuestionQuestionS);
+	    if f == nullE then e
+	    else applyEE(f, e)))
+    else nullE);
+
+nullCoalescion(lhs:Code,rhs:Code):Expr := (
+    e := nullify(lhs);
+    when e
+    is Nothing do eval(rhs)
+    else e);
+setup(QuestionQuestionS, nullify, nullCoalescion);
+
 augmentedAssignmentFun(x:augmentedAssignmentCode):Expr := (
     when lookup(x.oper.word, augmentedAssignmentOperatorTable)
     is null do buildErrorPacket("unknown augmented assignment operator")
     is s:Symbol do (
 	-- evaluate the left-hand side first
 	lexpr := nullE;
-	if s.word.name === "??" -- null coalescion; ignore errors
+	if s.word.name === "??" -- x ??= y is treated like x ?? (x = y)
 	then (
-	    e := tryEval(x.lhs);
-	    if tryEvalSuccess then lexpr = e)
+	    e := nullify(x.lhs);
+	    when e
+	    is Nothing do nothing
+	    else return e)
 	else lexpr = eval(x.lhs);
 	left := evaluatedCode(lexpr, dummyPosition);
 	when left.expr is e:Error do return Expr(e) else nothing;
@@ -2091,25 +2112,6 @@ export notFun(a:Expr):Expr := if a == True then False else if a == False then Tr
 -- to evaluate methods in hashtables.dd before it is defined.
 applyEEEpointer = applyEEE;
 
-nullify(c:Code):Expr := (
-    e := tryEval(c);
-    if tryEvalSuccess
-    then (
-	when e
-	is Nothing do e
-	else (
-	    f := lookup(Class(e), QuestionQuestionS);
-	    if f == nullE then e
-	    else applyEE(f, e)))
-    else nullE);
-
-nullCoalescion(lhs:Code,rhs:Code):Expr := (
-    e := nullify(lhs);
-    when e
-    is Nothing do eval(rhs)
-    else e);
-setup(QuestionQuestionS, nullify, nullCoalescion);
-
 -- Local Variables:
 -- compile-command: "echo \"make: Entering directory \\`$M2BUILDDIR/Macaulay2/d'\" && make -C $M2BUILDDIR/Macaulay2/d evaluate.o "
 -- End:

M2/Macaulay2/packages/Macaulay2Doc/doc_augmented_assignment.m2

@@ -112,7 +112,10 @@ doc ///
       The null coalescing operator can be combined with
       @TO "augmented assignment"@ as a shortcut for
       @M2CODE "if x === null then x = y"@ and
-      @M2CODE "if not x#?i then x#i = y"@.
+      @M2CODE "if not x#?i then x#i = y"@.  Note that it behaves
+      slightly differently than other augmented assignment operators,
+      as @CODE "x ??= y"@ is treated like @CODE "x ?? (x = y)"@ rather
+      than @CODE "x = x ?? y"@.
     Example
       x = null
       x ??= 2

M2/Macaulay2/tests/normal/augmented-assignment.m2

@@ -157,3 +157,7 @@ assert BinaryOperation(symbol ===, y ?? x, y)
 assert BinaryOperation(symbol ===, x ?? y, y)
 x ??= y
 assert BinaryOperation(symbol ===, x, y)
+
+-- issue #3612
+h = new HashTable from { symbol cache => new CacheTable };
+h.cache ??= new CacheTable