[perl #31946] Warn when assigning to a TEMP

This is the first step in downgrading a fatal error (Can't return a
temporary from lvalue subroutine) to a warning. Currently only XS
lvalue routines that return TEMPs and pure-Perl lvalue routines that
use explicit return (which don’t quite work properly yet anyway,
despite commit fa1e92c) are affected by this.

This is implemented in pp_sassign and pp_aassign, rather than
pp_leavesublv, so it will affect explicit returns and so it will
be skipped for overloaded ‘.=’, etc.

Thanks to Craig DeForest for suggesting how to do this.

2 files changed, 13 insertions, 0 deletions

diff --git a/pod/perldiag.pod b/pod/perldiag.pod
index b90a141961..39393d8fd2 100644
--- a/pod/perldiag.pod
+++ b/pod/perldiag.pod
@@ -4974,6 +4974,12 @@ See L<POSIX/FUNCTIONS> for more information.
 (F) You called a Win32 function with incorrect arguments.
 See L<Win32> for more information.
 
+=item Useless assignment to a temporary
+
+(W misc) You assigned to an lvalue subroutine, but what
+the subroutine returned was a temporary scalar about to
+be discarded, so the assignment had no effect.
+
 =item Useless (?-%s) - don't use /%s modifier in regex; marked by <-- HERE in m/%s/
 
 (W regexp) You have used an internal modifier such as (?-o) that has no
diff --git a/pp_hot.c b/pp_hot.c
index 531a33e219..04de368d18 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -197,6 +197,13 @@ PP(pp_sassign)
 	}
 
     }
+    if (
+      SvTEMP(right) && !SvSMAGICAL(right) && SvREFCNT(right) == 1 &&
+      (!isGV_with_GP(right) || SvFAKE(right)) && ckWARN(WARN_MISC)
+    )
+	Perl_warner(aTHX_
+	    packWARN(WARN_MISC), "Useless assignment to a temporary"
+	);
     SvSetMagicSV(right, left);
     SETs(right);
     RETURN;