Add compiler warning/error for duplicate variable declaration in Lua 5.1.5

compiler-warning-lua-5.1.5.patch

--- lparser.c	2018-11-30 11:47:05.053864317 +0100
+++ lparser.c	2018-12-27 09:28:55.536952662 +0100
@@ -6,6 +6,7 @@
 
 
 #include <string.h>
+#include <stdio.h>
 
 #define lparser_c
 #define LUA_CORE
@@ -61,6 +62,14 @@
   }
 }
 
+#define MAXSRC          80
+static void parser_warning (LexState *ls, const char *msg) {
+  char buff[MAXSRC];
+  luaO_chunkid(buff, getstr(ls->source), MAXSRC);
+  msg = luaO_pushfstring(ls->L, "%s:%d: warning %s", buff, ls->linenumber, msg);
+  fprintf(stderr, "%s\n", msg);
+  fflush(stderr);
+}
 
 static void error_expected (LexState *ls, int token) {
   luaX_syntaxerror(ls,
@@ -159,8 +168,30 @@
 
 static void new_localvar (LexState *ls, TString *name, int n) {
   FuncState *fs = ls->fs;
-  luaY_checklimit(fs, fs->nactvar+n+1, LUAI_MAXVARS, "local variables");
-  fs->actvar[fs->nactvar+n] = cast(unsigned short, registerlocalvar(ls, name));
+  int nactvar_n = fs->nactvar+n;
+
+  /* allow '_' and '(for...' duplicates */
+  const char *str_name = getstr(name);
+  if(!(str_name[0] == '(' || (name->tsv.len == 1 && str_name[0] == '_'))) {
+    int vidx = 0;
+    for (; vidx < nactvar_n; ++vidx) {
+      if (name == getlocvar(fs, vidx).varname) {
+        if(fs->bl && vidx < fs->bl->nactvar) {
+          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));
+        }
+      }
+    }
+  }
+
+  luaY_checklimit(fs, nactvar_n+1, LUAI_MAXVARS, "local variables");
+  fs->actvar[nactvar_n] = cast(unsigned short, registerlocalvar(ls, name));
 }
 

$ patch --merge --verbose -u -p0 < ../../compiler-warning-lua-5.1.5.patch
Hmm... Looks like a unified diff to me...
(Stripping trailing CRs from patch; use --binary to disable.)
The text leading up to this was:

|--- lparser.c 2018-11-30 11:47:05.053864317 +0100
|+++ lparser.c 2018-12-27 09:28:55.536952662 +0100

patching file lparser.c
Using Plan A...
Hunk #1 merged at 9.
Hunk #2 merged at 65-72.
patch unexpectedly ends in middle of line
Hunk #3 merged at 171-194.
done

min.lua:7: Name [a] already declared near 'local'

works very nice!