eval_sv: add a G_USEHINTS flag

Fixes Perl#21415

cop.h

@@ -1219,6 +1219,7 @@ struct context {
 #define G_RE_REPARSING  0x800   /* compiling a run-time /(?{..})/ */
 #define G_METHOD_NAMED 0x1000	/* calling named method, eg without :: or ' */
 #define G_RETHROW      0x2000	/* eval_sv(): re-throw any error */
+#define G_USEHINTS     0x4000   /* eval_sv(): use current hints/features */
 
 /* flag bits for PL_in_eval */
 #define EVAL_NULL	0	/* not in an eval */

ext/XS-APItest/Makefile.PL

@@ -24,7 +24,7 @@ my @names = (qw(HV_DELETE HV_DISABLE_UVAR_XKEY HV_FETCH_ISSTORE
 		HV_FETCH_ISEXISTS HV_FETCH_LVALUE HV_FETCH_JUST_SV
 		G_SCALAR G_LIST G_VOID G_DISCARD G_EVAL G_NOARGS
 		G_KEEPERR G_NODEBUG G_METHOD G_FAKINGEVAL G_RETHROW
-		GV_NOADD_NOINIT
+		GV_NOADD_NOINIT G_USEHINTS
 		SV_GMAGIC SV_SKIP_OVERLOAD
 		IS_NUMBER_IN_UV IS_NUMBER_GREATER_THAN_UV_MAX
 		IS_NUMBER_NOT_INT IS_NUMBER_NEG IS_NUMBER_INFINITY

ext/XS-APItest/t/call.t

@@ -11,7 +11,7 @@ use strict;
 
 BEGIN {
     require '../../t/test.pl';
-    plan(538);
+    plan(540);
     use_ok('XS::APItest')
 };
 use Config;
@@ -340,6 +340,16 @@ for my $fn_type (qw(eval_pv eval_sv call_sv)) {
     }
 }
 
+{
+    use feature "fc";
+    # the XS eval_sv() returns the count of results
+    is(eval_sv('fc("A") eq fc("a"); 1', G_LIST), 0,
+       "don't inherit hints by default (so the eval fails)");
+    is(eval_sv('fc("A") eq fc("a"); 1', G_LIST | G_USEHINTS), 1,
+       "inherit hints when requested (so the eval succeeds)")
+      or diag($@);
+}
+
 # DAPM 9-Aug-04. A taint test in eval_sv() could die after setting up
 # a new jump level but before pushing an eval context, leading to
 # stack corruption

perl.c

@@ -3229,7 +3229,12 @@ as C<call_sv>, with the obvious exception of C<G_EVAL>.  See L<perlcall>.
 The C<G_RETHROW> flag can be used if you only need eval_sv() to
 execute code specified by a string, but not catch any errors.
 
+By default the code is compiled and executed with the default hints,
+such as strict and features.  Set C<G_USEHINTS> in flags to use the
+current hints from C<PL_curcop>.
+
 =for apidoc Amnh||G_RETHROW
+=for apidoc Amnh||G_USEHINTS
 =cut
 */
 
@@ -3277,6 +3282,9 @@ Perl_eval_sv(pTHX_ SV *sv, I32 flags)
     if (flags & G_RE_REPARSING)
         myop.op_private |= (OPpEVAL_COPHH | OPpEVAL_RE_REPARSING);
 
+    if (flags & G_USEHINTS)
+        myop.op_private |= OPpEVAL_COPHH;
+
     /* fail now; otherwise we could fail after the JMPENV_PUSH but
      * before a cx_pusheval(), which corrupts the stack after a croak */
     TAINT_PROPER("eval_sv()");