Add experimental::qt warning

MANIFEST

@@ -6135,6 +6135,7 @@ t/lib/warnings/pp_ctl			Tests for pp_ctl.c for warnings.t
 t/lib/warnings/pp_hot			Tests for pp_hot.c for warnings.t
 t/lib/warnings/pp_pack			Tests for pp_pack.c for warnings.t
 t/lib/warnings/pp_sys			Tests for pp_sys.c for warnings.t
+t/lib/warnings/qt			Tests for qt() syntax for warnings.t
 t/lib/warnings/regcomp			Tests for regcomp.c for warnings.t
 t/lib/warnings/regexec			Tests for regexec.c for warnings.t
 t/lib/warnings/run			Tests for run.c for warnings.t

pod/perldiag.pod

@@ -5535,6 +5535,12 @@ and then thrown away. Any prototypes or attributes are therefore
 meaningless and are ignored. You should remove them from the C<BEGIN> block.
 Note this also means you cannot create a constant called C<BEGIN>.
 
+=item qt() is experimental
+
+(S experimental::qt) This warning is emitted if you use the C<qt()>
+templated quoting syntax.  This quoting form is currently experimental and
+its behaviour may change in future releases of Perl.
+
 =item Quantifier follows nothing in regex; marked by S<<-- HERE> in m/%s/
 
 (F) You started a regular expression with a quantifier.  Backslash it if

regen/warnings.pl

@@ -16,7 +16,7 @@
 #
 # This script is normally invoked from regen.pl.
 
-$VERSION = '1.67';
+$VERSION = '1.68';
 
 BEGIN {
     require './regen/regen_lib.pl';
@@ -159,6 +159,8 @@ BEGIN
                                     [ 5.035, DEFAULT_ON],
                                 'experimental::class' =>
                                     [ 5.037, DEFAULT_ON ],
+                                'experimental::qt' =>
+                                    [ 5.039, DEFAULT_ON ],
                         }],
 
         'missing'       => [ 5.021, DEFAULT_OFF],

t/lib/warnings/qt

@@ -0,0 +1,7 @@
+toke.c		warnings from parsing qt() syntax
+__END__
+# experimental warnings
+use feature 'qt';
+my $str = qt{this is a quoted template string};
+EXPECT
+qt() is experimental at - line 3.

t/op/qt.t

@@ -7,6 +7,7 @@ BEGIN {
 }
 
 use feature 'qt';
+no warnings 'experimental::qt';
 
 # Literal quoting
 is( qt{some literal text}, "some literal text", 'qt{} as literal' );

toke.c

@@ -8554,6 +8554,9 @@ yyl_word_or_keyword(pTHX_ char *s, STRLEN len, I32 key, I32 orig_keyword, struct
         TERM(sublex_start());
 
     case KEY_qt:
+        Perl_ck_warner_d(aTHX_
+                         packWARN(WARN_EXPERIMENTAL__QT),
+                         "qt() is experimental");
         // For now parse purely as a single q(). Code entirely stolen from KEY_q
         s = scan_str(s,FALSE,FALSE,FALSE,NULL);
         if (!s)