Add propagation of badflag with .= (Ops::assgn) for sf.net bug 3543056.

This is a start for testing of the issue at <https://sourceforge.net/p/pdl/bugs/308/>, <#29>.

Basic/Ops/ops.pd

@@ -373,14 +373,26 @@ sub PDL::log10 {
 #
 pp_def(
        'assgn',
-#       HandleBad => 1,
+       HandleBad => 1,
        Pars => 'a(); [o]b();',
        Code =>
        '$b() = $a();',
-#       BadCode =>
-#       'if ( $ISBAD(a()) ) { $SETBAD(b()); } else { $b() = $a(); }',
+       BadCode =>
+       'if ( $ISBAD(a()) ) { $SETBAD(b()); } else { $b() = $a(); }',
        Doc =>
 'Plain numerical assignment. This is used to implement the ".=" operator',
+		BadDoc =>
+'If C<a> is a child piddle (e.g., the result of a slice) and bad values are generated in C<b>,
+the bad value flag is set in C<b>, but it is B<NOT> automatically propagated back to the parent of C<a>.
+The following idiom ensures that the badflag is propagated back to the parent of C<a>:
+
+ $pdl->slice(":,(1)") .= PDL::Bad_aware_func();
+ $pdl->badflag(1);
+ $pdl->check_badflag();
+
+This is unnecessary if $pdl->badflag is known to be 1 before the slice is performed.
+
+See http://pdl.perl.org/PDLdocs/BadValues.html#dataflow_of_the_badflag for details.'
 ); # pp_def assgn
 
 #pp_export_nothing(); 

t/ops.t

@@ -1,5 +1,6 @@
-use Test::More tests => 44;
+use Test::More tests => 45;
 use PDL::LiteF;
+use PDL::Config;
 kill INT,$$ if $ENV{UNDER_DEBUGGER}; # Useful for debugging.
 
 sub tapprox {
@@ -148,3 +149,20 @@ $b = $a->qsorti;
 $c = $b % 3;
 ok(all($c->double==pdl("0 1 2 " x 5)));
 ok(longlong(10)%longlong(5)==0);
+
+TODO: {
+	# Check badflag propagation with .= (Ops::assgn) sf.net bug 3543056
+	local $TODO = "Badflag needs to propagate via assignment";
+	todo_skip 'No BADVAL', 1 if !$PDL::Config{WITH_BADVAL};
+
+	$a = sequence(10);
+	$b = sequence(5);
+	$b->inplace->setvaltobad(3);
+	$a->slice('0:4') .= $b;
+	# NOTE TODO
+	# if you set the badflag manually, this will work, but to Do The Right
+	# Thing, this should be automatic:
+	#     $a->badflag(1);
+	#     $a->check_badflag();
+	ok($a->badflag == 1 && $a->nbad == 1);
+}