muscles: fix handling of the "@'" escape

[Akim Demaille]

Another reachable assertion uncovered by fuzzing.

commit 637636412037d1fc2079f81260b19de5d0dab199
Author: Akim Demaille <akim.demaille@gmail.com>
Date:   Sun Sep 4 08:34:11 2022 +0200

    muscles: fix handling of the "@'" escape
    
    When we use `b4_` or `m4_` somewhere in the input, it is escaped as
    `b4@'_`/`m4@'_` so that the warning about unexpanded b4_foo/m4_foo
    macros does not fire.
    
    But in the case of muscles, the `@'` escape was not recognized, and an
    assertion was triggered.
    
    Reported by Han Zheng.
    <https://github.com/akimd/bison/issues/91>
    
    * src/muscle-tab.c (COMMON_DECODE): Handle `@'`.
    * tests/skeletons.at (Suspicious sequences): Check that case.

diff --git a/src/muscle-tab.c b/src/muscle-tab.c
index 43911d3e..0945d609 100644
--- a/src/muscle-tab.c
+++ b/src/muscle-tab.c
@@ -294,10 +294,11 @@ muscle_location_grow (char const *key, location loc)
   case '@':                                                     \
     switch (*++(Value))                                         \
       {                                                         \
+      case '\'': /* Ignore "@'" */ break;                       \
       case '@': obstack_sgrow (&muscle_obstack, "@" ); break;   \
       case '{': obstack_sgrow (&muscle_obstack, "[" ); break;   \
       case '}': obstack_sgrow (&muscle_obstack, "]" ); break;   \
       default: aver (false); break;                             \
       }                                                         \
     break;                                                      \
   default:                                                      \
diff --git a/tests/skeletons.at b/tests/skeletons.at
index e1b86514..13f796b5 100644
--- a/tests/skeletons.at
+++ b/tests/skeletons.at
@@ -369,5 +369,20 @@ AT_SETUP([[Suspicious sequences]])
 input1.tab.c:13: warning: suspicious sequence in the output: m4@&t@_poison 
[-Wother]
 ]])
 
+# Regression test for <https://github.com/akimd/bison/issues/91>.
+AT_DATA([[input2.y]],
+[[%define parse.trace {m4@&t@_foo}
+%debug
+%%
+exp:
+]])
+
+AT_BISON_CHECK([[input2.y]], [1], [],
+[[input2.y:1.1-28: warning: %define variable 'parse.trace' requires keyword 
values [-Wdeprecated]
+input2.y:1.1-28: error: invalid value for %define Boolean variable 
'parse.trace'
+input2.y:2.1-6: error: %define variable 'parse.trace' redefined
+input2.y:1.1-28: note: previous definition
+input2.y: warning: fix-its can be applied.  Rerun with option '--update'. 
[-Wother]
+]])
 
 AT_CLEANUP