Fix dynamic narrow mode.

* Rename `cur_max` to `mb_cur_max` to avoid confusion with `curr_max`.
* Adjust some comments, drop reference to R bugzilla ticket.
* Fix one case where we were still checking the static `TRE_MB_CUR_MAX`
  instead of the dynamic `mb_cur_max`.
* Drop an invalid assertion.

Inspired by the R repository.

Fixes: 8e84229

lib/tre-compile.c

@@ -1880,8 +1880,8 @@ tre_compile(regex_t *preg, const tre_char_t *regex, size_t n, int cflags)
   parse_ctx.len = n;
   parse_ctx.cflags = cflags;
   parse_ctx.max_backref = -1;
-  /* workaround for PR#14408: use 8-bit optimizations in 8-bit mode */
-  parse_ctx.cur_max = (cflags & REG_USEBYTES) ? 1 : TRE_MB_CUR_MAX;
+  /* Use 8-bit optimizations in 8-bit mode */
+  parse_ctx.mb_cur_max = (cflags & REG_USEBYTES) ? 1 : TRE_MB_CUR_MAX;
   DPRINT(("tre_compile: parsing '%.*" STRF "'\n", (int)n, regex));
   errcode = tre_parse(&parse_ctx);
   if (errcode != REG_OK)
@@ -2021,7 +2021,7 @@ tre_compile(regex_t *preg, const tre_char_t *regex, size_t n, int cflags)
   /* If in eight bit mode, compute a table of characters that can be the
      first character of a match. */
   tnfa->first_char = -1;
-  if (TRE_MB_CUR_MAX == 1 && !tmp_ast_l->nullable)
+  if (parse_ctx.mb_cur_max == 1 && !tmp_ast_l->nullable)
     {
       int count = 0;
       tre_cint_t k;

lib/tre-parse.c

@@ -128,7 +128,6 @@ tre_expand_ctype(tre_mem_t mem, tre_ctype_t class, tre_ast_node_t ***items,
   reg_errcode_t status = REG_OK;
   tre_cint_t c;
   int j, min = -1, max = 0;
-  assert(TRE_MB_CUR_MAX == 1);
 
   DPRINT(("  expanding class to character ranges\n"));
   for (j = 0; (j < 256) && (status == REG_OK); j++)
@@ -332,7 +331,7 @@ tre_parse_bracket_items(tre_parse_ctx_t *ctx, int negate,
 		  if (!class)
 		    status = REG_ECTYPE;
 		  /* Optimize character classes for 8 bit character sets. */
-		  if (status == REG_OK && ctx->cur_max == 1)
+		  if (status == REG_OK && ctx->mb_cur_max == 1)
 		    {
 		      status = tre_expand_ctype(ctx->mem, class, items,
 						&i, &max_i, ctx->cflags);

lib/tre-parse.h

@@ -36,8 +36,8 @@ typedef struct {
   int nofirstsub;
   /* The currently set approximate matching parameters. */
   int params[TRE_PARAM_LAST];
-  /* the CUR_MAX in use */
-  int cur_max;
+  /* the MB_CUR_MAX in use */
+  int mb_cur_max;
 } tre_parse_ctx_t;
 
 /* Parses a wide character regexp pattern into a syntax tree.  This parser