make a new stub to clone into when pushing a new pad

This previously put the same CV into the inner pad, so on a
recursive call into the owning sub, from this sub, this CV would
still be active, and the attempt to clone into the still busy CV
would throw an error.

Fixes Perl#18606

pad.c

@@ -2468,8 +2468,19 @@ Perl_pad_push(pTHX_ PADLIST *padlist, int depth)
                         || PadnameIsSTATE(names[ix])
                         || sigil == '&')
                 {
-                    /* outer lexical or anon code */
-                    sv = SvREFCNT_inc(oldpad[ix]);
+                    SV *tmp = oldpad[ix];
+                    if (sigil == '&' && SvTYPE(tmp) == SVt_PVCV
+                        && !PadnameOUTER(names[ix])
+                        && CvLEXICAL(tmp) && CvCLONED(tmp)
+                        && !PadnameIsOUR(names[ix])
+                        && !PadnameIsSTATE(names[ix])) {
+                        /* lexical sub that needs cloning */
+                        sv = newSV_type(SVt_PVCV);
+                    }
+                    else {
+                        /* outer non-cloning lexical or anon code */
+                        sv = SvREFCNT_inc(tmp);
+                    }
                 }
                 else {		/* our own lexical */
                     if (sigil == '@')

t/op/lexsub.t

@@ -7,7 +7,8 @@ BEGIN {
     *bar::is = *is;
     *bar::like = *like;
 }
-plan 152;
+
+plan 156;
 
 # -------------------- our -------------------- #
 
@@ -979,3 +980,59 @@ is join("-", qw(aa bb), do { my sub lleexx; 123 }, qw(cc dd)),
     }
     nested();
 }
+
+SKIP:
+{
+    # github #18606
+    sub rec1 {
+        my $x = shift;
+        my sub b { $x };
+        rec1(2) if $x == 1;
+        return b();
+    }
+    is(rec1(1), 1, "check recursively defined lexical sub is lexical");
+
+    sub rec2 {
+        my ($x, $name) = @_;
+
+        my sub b;
+        if ($x) {
+            # provides definition for my sub b
+            sub b { $name }
+        }
+        rec2($x-1, "inner") if $x;
+
+        b();
+    }
+    is(rec2(0, "outer"), "outer", "check again");
+    is(rec2(1, "outer"), "outer", "check some more");
+
+    fresh_perl_like(<<'EOS', qr/result=103/, {}, "test code from #18606");
+use strict; use warnings; use feature qw/say state/;
+say "Perl version $^V";
+
+sub process($;$) {
+  my ($value, $callback) = @_;
+  say "### process($value,", ($callback//"undef"), ")";
+
+  my sub inner {
+    if ($callback) {
+      return $callback->($value+1);
+    } else {
+      return $value+1;
+    }
+  }
+
+  inner();
+}
+
+my $result =
+  process(1, sub{
+                 my $arg = shift;
+                 say "#   callback called with arg $arg";
+                 process($arg+100);
+              });
+say "result=", ($result//"undef");
+EOS
+
+}