signatures: generate only one debug breakable line for signatures

With the debugger enabled the argcheck op and each argelem op
generated to process a function signature would have an associated
dbstate op, allowing the debugger to step on that op.

This can be confusing and/or onerous when stepping through code, since
for a signature with 7 arguments you'd have 9 steps to get past the
prologue of the sub.

Make most of these dbstates into nextstates to make them non-steppable.

Fixes #22602

lib/perl5db.t

@@ -3662,6 +3662,35 @@ EOS
     $wrapper->contents_like(qr/print "2\\n"/, "break immediately after defining problem");
 }
 
+{
+    my $wrapper = DebugWrap->new(
+        {
+            cmds =>
+            [
+                "s",
+                "s",
+                "s",
+                "s",
+                "s",
+                "s",
+                "s",
+                "q",
+            ],
+            prog => \<<'EOS',
+use v5.40.0;
+sub four ($x1, $x2, $x3, $x4) { # start sub four
+    print $x1, $x2, $x3, $x4;
+}
+four(1,2,3,4);
+EOS
+        }
+       );
+    my $content = $wrapper->_contents;
+    my $count = () = $content =~ /start sub four/g;
+    is($count, 1, "expect to step on sub four entry once")
+      or diag $content;
+}
+
 done_testing();
 
 END {

op.c

@@ -8896,7 +8896,8 @@ Perl_newSTATEOP(pTHX_ I32 flags, char *label, OP *o)
     flags &= ~SVf_UTF8;
 
     NewOp(1101, cop, 1, COP);
-    if (PERLDB_LINE && CopLINE(PL_curcop) && PL_curstash != PL_debstash) {
+    if (PERLDB_LINE && CopLINE(PL_curcop) && PL_curstash != PL_debstash &&
+        !(flags & OPf_FORCE_NEXTSTATE)) {
         OpTYPE_set(cop, OP_DBSTATE);
     }
     else {

op.h

@@ -171,6 +171,15 @@ Deprecated.  Use C<GIMME_V> instead.
    op_convert_list to set op_folded.  */
 #define OPf_FOLDED      (1<<16)
 
+/* In debugging mode newSTATEOP() normally makes an OP_DBSTATE, set
+   this to force creation of an OP_NEXTSTATE (which isn't
+   steppable/breakable) instead.
+
+   This is useful if you need to intro_my() but not make a breakable
+   op.
+ */
+#define OPf_FORCE_NEXTSTATE (1 << 17)
+
 /* old names; don't use in new code, but don't break them, either */
 #define OPf_LIST	OPf_WANT_LIST
 #define OPf_KNOW	OPf_WANT

perly.y

@@ -837,7 +837,7 @@ sigslurpelem: sigslurpsigil sigvarname sigdefault/* def only to catch errors */
                                 yyerror("A slurpy parameter may not have "
                                         "a default value");
 
-                            $$ = var ? newSTATEOP(0, NULL, var) : NULL;
+                            $$ = var ? newSTATEOP(OPf_FORCE_NEXTSTATE, NULL, var) : NULL;
                         }
 	;
 
@@ -913,7 +913,7 @@ sigscalarelem:
                                             "follows optional parameter");
                             }
 
-                            $$ = var ? newSTATEOP(0, NULL, var) : NULL;
+                            $$ = var ? newSTATEOP(OPf_FORCE_NEXTSTATE, NULL, var) : NULL;
                         }
 	;
 
@@ -985,7 +985,7 @@ subsigguts:
                                             (UNOP_AUX_item *)aux);
                             sigops = op_prepend_elem(OP_LINESEQ, check, sigops);
                             sigops = op_prepend_elem(OP_LINESEQ,
-                                                newSTATEOP(0, NULL, NULL),
+                                                newSTATEOP(OPf_FORCE_NEXTSTATE, NULL, NULL),
                                                 sigops);
                             /* a nextstate at the end handles context
                              * correctly for an empty sub body */