patch 8.2.4103: Vim9: variable declared in for loop not initialzed

Problem:    Vim9: variable declared in for loop not initialzed.
Solution:   Always initialze the variable. (closes #9535)

src/proto/vim9instr.pro

@@ -63,6 +63,7 @@ int generate_UNPACK(cctx_T *cctx, int var_count, int semicolon);
 int generate_cmdmods(cctx_T *cctx, cmdmod_T *cmod);
 int generate_undo_cmdmods(cctx_T *cctx);
 int generate_store_var(cctx_T *cctx, assign_dest_T dest, int opt_flags, int vimvaridx, int scriptvar_idx, int scriptvar_sid, type_T *type, char_u *name);
+int inside_loop_scope(cctx_T *cctx);
 int generate_store_lhs(cctx_T *cctx, lhs_T *lhs, int instr_count, int is_decl);
 void may_generate_prof_end(cctx_T *cctx, int prof_lnum);
 void delete_instr(isn_T *isn);

src/testdir/test_vim9_assign.vim

@@ -587,6 +587,41 @@ def Test_assign_index()
   CheckDefFailure(lines, 'E1012: Type mismatch; expected list<number> but got dict<unknown>', 2)
 enddef
 
+def Test_init_in_for_loop()
+  var lines =<< trim END
+      var l: list<number> = []
+      for i in [3, 4]
+        var n: number
+        add(l, n)
+        n = 123
+      endfor
+      assert_equal([0, 0], l)
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      var l: list<number> = []
+      for i in [3, 4]
+        var n: number = 0
+        add(l, n)
+        n = 123
+      endfor
+      assert_equal([0, 0], l)
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      var l: list<number> = []
+      for i in [3, 4]
+        var n: number = 3
+        add(l, n)
+        n = 123
+      endfor
+      assert_equal([3, 3], l)
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_extend_list()
   var lines =<< trim END
       var l1: list<number>

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4103,
 /**/
     4102,
 /**/

src/vim9compile.c

@@ -2256,12 +2256,17 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		    case VAR_VOID:
 		    case VAR_INSTR:
 		    case VAR_SPECIAL:  // cannot happen
-			// This is skipped for local variables, they are
+			// This is skipped for local variables, they are always
-			// always initialized to zero.
+			// initialized to zero.  But in a "for" or "while" loop
-			if (lhs.lhs_dest == dest_local)
+			// the value may have been changed.
+			if (lhs.lhs_dest == dest_local
+						   && !inside_loop_scope(cctx))
 			    skip_store = TRUE;
 			else
+			{
+			    instr_count = instr->ga_len;
 			    generate_PUSHNR(cctx, 0);
+			}
 			break;
 		}
 	    }

src/vim9instr.c

@@ -1845,6 +1845,25 @@ generate_store_var(
     return FAIL;
 }
 
+/*
+ * Return TRUE when inside a "for" or "while" loop.
+ */
+    int
+inside_loop_scope(cctx_T *cctx)
+{
+    scope_T	*scope = cctx->ctx_scope;
+
+    for (;;)
+    {
+	if (scope == NULL)
+	    break;
+	if (scope->se_type == FOR_SCOPE || scope->se_type == WHILE_SCOPE)
+	    return TRUE;
+	scope = scope->se_outer;
+    }
+    return FALSE;
+}
+
     int
 generate_store_lhs(cctx_T *cctx, lhs_T *lhs, int instr_count, int is_decl)
 {
@@ -1869,8 +1888,9 @@ generate_store_lhs(cctx_T *cctx, lhs_T *lhs, int instr_count, int is_decl)
 	    varnumber_T val = isn->isn_arg.number;
 	    garray_T    *stack = &cctx->ctx_type_stack;
 
-	    if (val == 0 && is_decl)
+	    if (val == 0 && is_decl && !inside_loop_scope(cctx))
 	    {
+		// zero is the default value, no need to do anything
 		--instr->ga_len;
 	    }
 	    else