pp_multiconcat: don't stringify LHS overload arg

RT #132385

In something like

    $a1 . $a2

where $a2 is overloaded, the concat overload method was being called
like

    concat($a2, "$a1", 1);

(The 1 indicated that the args are reversed).

This commit changes it so that it's called as

    concat($a2, $a1, 1);

i.e. that the original arg is passed in rather than a stringified copy
of it. This is important if for example $a1 is a ref.

1 files changed, 35 insertions, 0 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 2ce77b38ef..d980db837c 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -520,6 +520,41 @@ PP(pp_multiconcat)
                      * FAKE implies an optimised sprintf which doesn't use
                      * concat overloading, only "" overloading.
                      */
+
+                    if (   svpv_end == svpv_buf + 1
+                           /* no const string segments */
+                        && aux[PERL_MULTICONCAT_IX_LENGTHS].size     == -1
+                        && aux[PERL_MULTICONCAT_IX_LENGTHS + 1].size == -1
+                    ) {
+                        /* special case: if the overloaded sv is the
+                         * second arg in the concat chain, stop at the
+                         * first arg rather than this, so that
+                         *
+                         *   $arg1 . $arg2
+                         *
+                         * invokes overloading as
+                         *
+                         *    concat($arg2, $arg1, 1)
+                         *
+                         * rather than
+                         *
+                         *    concat($arg2, "$arg1", 1)
+                         *
+                         * This means that if for example arg1 is a ref,
+                         * it gets passed as-is to the concat method
+                         * rather than a stringified copy. If it's not the
+                         * first arg, it doesn't matter, as in $arg0 .
+                         * $arg1 .  $arg2, where the result of ($arg0 .
+                         * $arg1) will already be a string.
+                         * THis isn't perfect: we'll have already
+                         * done SvPV($arg1) on the previous iteration;
+                         * and are now throwing away that result and
+                         * hoping arg1 hasn;t been affected.
+                         */
+                        svpv_end--;
+                        SP--;
+                    }
+
                   setup_overload:
                     dsv = newSVpvn_flags("", 0, SVs_TEMP);