Make warn handle magic vars (fixes [perl #97480])

pp_warn was checking flags before calling get-magic, resulting in sev-
eral bugs that I fixed all at once::
• warn now calls get-magic exactly once on its argument, when there
  is just one argument (it always worked correctly for multiple)
  [perl #97480].
• warn calls get-magic exactly once on $@ when falling back to it,
  instead of zero times.
• A tied variable returning an object that stringifies as an empty
  string is no longer ignored if the tied variable was not ROK
  before FETCH.
• A tied $@ containing a string, or $@ aliased to $1, is no
  longer ignored.
• A tied $@ that last returned a reference but will return a string on
  the next FETCH now gets "\t...caught" appended.

1 files changed, 16 insertions, 7 deletions

diff --git a/pp_sys.c b/pp_sys.c
index 3ddf5e2d70..994bd6ce9c 100644
--- a/pp_sys.c
+++ b/pp_sys.c
@@ -438,20 +438,29 @@ PP(pp_warn)
     }
     else {
 	exsv = TOPs;
+	if (SvGMAGICAL(exsv)) exsv = sv_mortalcopy(exsv);
     }
 
     if (SvROK(exsv) || (SvPV_const(exsv, len), len)) {
 	/* well-formed exception supplied */
     }
-    else if (SvROK(ERRSV)) {
-	exsv = ERRSV;
-    }
-    else if (SvPOK(ERRSV) && SvCUR(ERRSV)) {
-	exsv = sv_mortalcopy(ERRSV);
-	sv_catpvs(exsv, "\t...caught");
-    }
     else {
+      SvGETMAGIC(ERRSV);
+      if (SvROK(ERRSV)) {
+	if (SvGMAGICAL(ERRSV)) {
+	    exsv = sv_newmortal();
+	    sv_setsv_nomg(exsv, ERRSV);
+	}
+	else exsv = ERRSV;
+      }
+      else if (SvPOKp(ERRSV) && SvCUR(ERRSV)) {
+	exsv = sv_newmortal();
+	sv_setsv_nomg(exsv, ERRSV);
+	sv_catpvs(exsv, "\t...caught");
+      }
+      else {
 	exsv = newSVpvs_flags("Warning: something's wrong", SVs_TEMP);
+      }
     }
     if (SvROK(exsv) && !PL_warnhook)
 	 Perl_warn(aTHX_ "%"SVf, SVfARG(exsv));