The new lvalue warning should not be turned on by default

A check for the warning category was missing from commit
885ef6f56b61fd750ef3b1fa614d11480baac635. Also, document
the warning category in perldiag.

1 files changed, 1 insertions, 1 deletions

diff --git a/op.c b/op.c
index a7205cb9f7..56196224ca 100644
--- a/op.c
+++ b/op.c
@@ -5714,7 +5714,7 @@ Perl_newATTRSUB(pTHX_ I32 floor, OP *o, OP *proto, OP *attrs, OP *block)
 		 )&& !attrs) {
 		if (CvFLAGS(PL_compcv)) {
 		    /* might have had built-in attrs applied */
-		    if (CvLVALUE(PL_compcv) && ! CvLVALUE(cv))
+		    if (CvLVALUE(PL_compcv) && ! CvLVALUE(cv) && ckWARN(WARN_MISC))
 			Perl_warner(aTHX_ packWARN(WARN_MISC), "lvalue attribute ignored after the subroutine has been defined");
 		    CvFLAGS(cv) |= (CvFLAGS(PL_compcv) & CVf_BUILTIN_ATTRS & ~CVf_LVALUE);
 		}