maint: fix: do not emit nested comments

I hope I can release 3.6.2 this weekend.

commit c9099a7d7b7f6656d9fabd3db3aacd49178efccf
Author: Akim Demaille <[email protected]>
Date:   Fri May 15 08:13:44 2020 +0200

    fix: do not emit nested comments
    
    With input such as
    
        %token<fl> yVL_CLOCK "/*verilator sc_clock*/"
    
    we generate
    
        yVL_CLOCK = 610,      /* "/*verilator sc_clock*/"  */
    
    which is invalid since the comment will actually be closed on the
    first */.  Let's turn "*/" into "*\/" to avoid this.  But GCC will
    also warn about "/*" inside a comment, so let's "escape" it too.
    
    Reported by Huang Rui.
    https://github.com/akimd/bison/issues/38
    
    * data/skeletons/c-like.m4 (_b4_comment): Escape comment delimiters in
    comments.
    * tests/input.at (Torturing the Scanner): Check thes cases.
    * tests/m4.at: New.

diff --git a/NEWS b/NEWS
index 43d09e8d..23a935fa 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,12 @@ GNU Bison NEWS
 
   Some tests were fixed.
 
+  When token aliases contain comment delimiters:
+
+    %token FOO "/* foo */"
+
+  bison used to emit "nested" comments, which is invalid C.
+
 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
 
 ** Bug fixes
diff --git a/data/skeletons/c-like.m4 b/data/skeletons/c-like.m4
index e0460d48..fb0dc53b 100644
--- a/data/skeletons/c-like.m4
+++ b/data/skeletons/c-like.m4
@@ -22,10 +22,14 @@
 # Put TEXT in comment.  Avoid trailing spaces: don't indent empty lines.
 # Avoid adding indentation to the first line, as the indentation comes
 # from OPEN.  That's why we don't patsubst([$1], [^\(.\)], [   \1]).
+# Turn "*/" in TEXT into "* /" so that we don't unexpectedly close
+# the comments before its end.
 #
 # Prefix all the output lines with PREFIX.
 m4_define([_b4_comment],
-[$2[]m4_bpatsubst(m4_expand([[$1]]), [
+[$2[]m4_bpatsubsts(m4_expand([$1]),
+                   [[*]/], [*\\/],
+                   [/[*]], [/\\*], [
 \(.\)], [
 $3\1])$4])
 
diff --git a/tests/input.at b/tests/input.at
index 77fa3e59..cc72ddc8 100644
--- a/tests/input.at
+++ b/tests/input.at
@@ -1424,6 +1424,11 @@ char quote[] = "@:>@@:>@,";
 /* Exercise quotes in strings.  */
 %token FAKE "fake @<:@@:>@ \a\b\f\n\r\t\v\"\'\?\\\u005B\U0000005c ??!??'??(??)??-??/??<??=??> \x1\1"
 
+/* Beware of the generated comments that embed string aliases that
+   might close the comment.  */
+%token COMMENT_CLOSE "*/"
+%token COMMENT       "/* comment */"
+
 %%
 /* Exercise M4 quoting: '@:>@@:>@', @<:@, 1.  */
 exp: '@<:@' '\1' two '$' '@' '{' oline output.or.oline.opt
diff --git a/tests/local.mk b/tests/local.mk
index fac8426f..a62fe267 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -56,6 +56,7 @@ TESTSUITE_AT =                                \
   %D%/java.at                                 \
   %D%/javapush.at                             \
   %D%/local.at                                \
+  %D%/m4.at                                   \
   %D%/named-refs.at                           \
   %D%/output.at                               \
   %D%/package.m4                              \
diff --git a/tests/m4.at b/tests/m4.at
new file mode 100644
index 00000000..5f59c9b2
--- /dev/null
+++ b/tests/m4.at
@@ -0,0 +1,46 @@
+# Basic m4 macros.                               -*- Autotest -*-
+
+# Copyright (C) 2020 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+AT_BANNER([[M4 Macros.]])
+
+
+AT_SETUP([Generating Comments])
+
+AT_DATA([input.y],
+[%%
+exp:
+])
+
+AT_DATA([input.m4],
+[[m4@&t@_include(b4_skeletonsdir/[c.m4])
+
+b4_output_begin([output.txt])
+b4_comment([["/* () */"]])
+b4_comment([["/* (  */"]])
+b4_comment([["/*  ) */"]])
+b4_output_end([output.txt])
+]])
+
+AT_BISON_CHECK([-S ./input.m4 input.y])
+
+AT_CHECK([cat output.txt], [],
+[/* "/\* () *\/"  */
+/* "/\* (  *\/"  */
+/* "/\*  ) *\/"  */
+])
+
+AT_CLEANUP
\ No newline at end of file
diff --git a/tests/testsuite.at b/tests/testsuite.at
index e2b72b84..44393db5 100644
--- a/tests/testsuite.at
+++ b/tests/testsuite.at
@@ -16,6 +16,8 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+# Basic M4 macros.
+m4_include([m4.at])
 
 # Resistance to user bugs.
 m4_include([input.at])