[perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail

I'd like to reopen this old bug as I've hit it in my own code... I've no
idea how to fix it properly, but at least before that is decided either
way, I believe issuing a warning gives some idea that there's undefined
behavior:

--- pp.c.0    2012-01-06 01:07:34.511988700 +0100
+++ pp.c    2012-01-06 19:04:06.311428300 +0100
@@ -515,9 +515,11 @@
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }
-    else if (SvPADTMP(sv) && !IS_PADGV(sv))
+    else if (SvPADTMP(sv) && !IS_PADGV(sv)) {
+        Perl_ck_warner(aTHX_ packWARN(WARN_MISC),
+         "Implicit copy of a read-only scalar due to aliasing");
         sv = newSVsv(sv);
-    else {
+    } else {
     SvTEMP_off(sv);
     SvREFCNT_inc_void_NN(sv);
     }




---
via perlbug:  queue: perl5 status: open
https://rt.perl.org:443/rt3/Ticket/Display.html?id=21979

• [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by perlbug-followup
• Re: [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by Enache Adrian
• Re: [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by Arthur Bergman
• Re: [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by Enache Adrian
• Re: [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by Arthur Bergman
• [perl #21979] Modifying an alias to a read-only item of an argument list of a subroutine works, while it's expected to fail by Dmitry Karasik via RT