Add compiler warning/error for duplicate variable declaration in Lua 5.2.4

compiler-warning-lua-5.2.4.patch

--- lparser.c
+++ lparser.c
@@ -6,6 +6,7 @@
 
 
 #include <string.h>
+#include <stdio.h>
 
 #define lparser_c
 #define LUA_CORE
@@ -67,6 +68,15 @@
 }
 
 
+static void parser_warning (LexState *ls, const char *msg) {
+  char buff[LUA_IDSIZE];
+  luaO_chunkid(buff, getstr(ls->source), LUA_IDSIZE);
+  msg = luaO_pushfstring(ls->L, "%s:%d: warning %s", buff, ls->linenumber, msg);
+  fprintf(stderr, "%s\n", msg);
+  fflush(stderr);
+}
+
+
 /* semantic error */
 static l_noret semerror (LexState *ls, const char *msg) {
   ls->t.token = 0;  /* remove 'near to' from final message */
@@ -178,6 +188,29 @@
 static void new_localvar (LexState *ls, TString *name) {
   FuncState *fs = ls->fs;
   Dyndata *dyd = ls->dyd;
+  /* allow '_' and '(for...' duplicates */
+  const char *str_name = getstr(name);
+  if(!(str_name[0] == '(' || (name->tsv.len == 1 && str_name[0] == '_'))) {
+    int vidx, nactvar_n, first_block_local;
+    vidx = fs->firstlocal;
+    nactvar_n = dyd->actvar.n;
+    first_block_local = fs->bl ? fs->bl->nactvar+fs->firstlocal : 0;
+    for (; vidx < nactvar_n; ++vidx) {
+      LocVar *lv = &fs->f->locvars[dyd->actvar.arr[vidx].idx];
+      if (lv && name == lv->varname) {
+        if(vidx < first_block_local) {
+          int saved_top = lua_gettop(ls->L);
+          parser_warning(ls, luaO_pushfstring(ls->L,
+                 "Name [%s] already declared will be shadowed", str_name));
+          lua_settop(ls->L, saved_top);
+        }
+        else {
+          luaX_syntaxerror(ls, luaO_pushfstring(ls->L,
+                 "Name [%s] already declared", str_name));
+        }
+      }
+    }
+  }
   int reg = registerlocalvar(ls, name);
   checklimit(fs, dyd->actvar.n + 1 - fs->firstlocal,
                   MAXVARS, "local variables");