[MERGE] handle /(?{})/ with overload::constant qr

The reworking of the re_eval implementation for 5.17.1 made the assumption
that constant strings within literal patterns were, um, constant.

It turns out this this is an invalid assumption, because

    overload::constant qr => { sub return bless [], 'Foo' }

can cause the constant bits of a pattern, like foo, bar in

    /foo(?{...})bar/

to get replaced with (for example) blessed objects: so the 'constant' SV
attached to an OP_CONST is actually a blessed object, that could itself be
overloaded with string or concat methods say, or could be a qr// object
etc.

The commits in this merge (hopefully) fix the various problems this
assumption caused: chiefly with qr// objects containing compiled (?{})
code that were getting re-stringified and thus failing unless in the
presence of use re 'eval' (and sometimes failing even in its presence).
Also, runtime patterns could trigger a recursive call to the overload
method, and eventually stack overflow and SEGV.

See [perl #116823].

cop.h

@@ -1048,13 +1048,15 @@ L<perlcall>.
 				   Perl_magic_methcall().  */
 #define G_WRITING_TO_STDERR 1024 /* Perl_write_to_stderr() is calling
 				    Perl_magic_methcall().  */
+#define G_RE_REPARSING 0x800     /* compiling a run-time /(?{..})/ */
 
 /* flag bits for PL_in_eval */
 #define EVAL_NULL	0	/* not in an eval */
 #define EVAL_INEVAL	1	/* some enclosing scope is an eval */
 #define EVAL_WARNONLY	2	/* used by yywarn() when calling yyerror() */
 #define EVAL_KEEPERR	4	/* set by Perl_call_sv if G_KEEPERR */
 #define EVAL_INREQUIRE	8	/* The code is being required. */
+#define EVAL_RE_REPARSING 0x10	/* eval_sv() called with G_RE_REPARSING */
 
 /* Support for switching (stack and block) contexts.
  * This ensures magic doesn't invalidate local stack and cx pointers.

op.h

@@ -308,6 +308,7 @@ Deprecated.  Use C<GIMME_V> instead.
 #define OPpEVAL_UNICODE		4
 #define OPpEVAL_BYTES		8
 #define OPpEVAL_COPHH		16	/* Construct %^H from cop hints */
+#define OPpEVAL_RE_REPARSING	32	/* eval_sv(..., G_RE_REPARSING) */
 
 /* Private for OP_CALLER, OP_WANTARRAY and OP_RUNCV */
 #define OPpOFFBYONE		128	/* Treat caller(1) as caller(2) */

parser.h

@@ -71,7 +71,7 @@ typedef struct yy_parser {
     char	multi_open;	/* delimiter of said string */
     char	multi_close;	/* delimiter of said string */
     bool	preambled;
-    /*** 8-bit hole ***/
+    bool        lex_re_reparsing; /* we're doing G_RE_REPARSING */
     I32		lex_allbrackets;/* (), [], {}, ?: bracket count */
     SUBLEXINFO	sublex_info;
     LEXSHARED	*lex_shared;

perl.c

@@ -2808,8 +2808,9 @@ Perl_eval_sv(pTHX_ SV *sv, I32 flags)
     myop.op_flags |= OP_GIMME_REVERSE(flags);
     if (flags & G_KEEPERR)
 	myop.op_flags |= OPf_SPECIAL;
-    if (PL_reg_state.re_reparsing)
-	myop.op_private = OPpEVAL_COPHH;
+
+    if (flags & G_RE_REPARSING)
+	myop.op_private = (OPpEVAL_COPHH | OPpEVAL_RE_REPARSING);
 
     /* fail now; otherwise we could fail after the JMPENV_PUSH but
      * before a PUSHEVAL, which corrupts the stack after a croak */

pp_ctl.c

@@ -3358,7 +3358,9 @@ S_doeval(pTHX_ int gimme, CV* outside, U32 seq, HV *hh)
 
     PL_in_eval = (in_require
 		  ? (EVAL_INREQUIRE | (PL_in_eval & EVAL_INEVAL))
-		  : EVAL_INEVAL);
+		  : (EVAL_INEVAL |
+                        ((PL_op->op_private & OPpEVAL_RE_REPARSING)
+                            ? EVAL_RE_REPARSING : 0)));
 
     PUSHMARK(SP);
 
@@ -3420,6 +3422,15 @@ S_doeval(pTHX_ int gimme, CV* outside, U32 seq, HV *hh)
     else {
 	PL_hints = saveop->op_private & OPpEVAL_COPHH
 		     ? oldcurcop->cop_hints : saveop->op_targ;
+
+        /* making 'use re eval' not be in scope when compiling the
+         * qr/mabye_has_runtime_code_block/ ensures that we don't get
+         * infinite recursion when S_has_runtime_code() gives a false
+         * positive: the second time round, HINT_RE_EVAL isn't set so we
+         * don't bother calling S_has_runtime_code() */
+        if (PL_in_eval & EVAL_RE_REPARSING)
+            PL_hints &= ~HINT_RE_EVAL;
+
 	if (hh) {
 	    /* SAVEHINTS created a new HV in PL_hintgv, which we need to GC */
 	    SvREFCNT_dec(GvHV(PL_hintgv));