patch 8.2.3423: Vim9: list += list creates a new list in :def function

Problem:    Vim9: list += list creates a new list in :def function.
Solution:   Append to the existing list.

src/structs.h

@@ -4106,6 +4106,9 @@ typedef enum
     EXPR_MULT,		// *
     EXPR_DIV,		// /
     EXPR_REM,		// %
+    // used with ISN_ADDLIST
+    EXPR_COPY,		// create new list
+    EXPR_APPEND,	// append to first list
 } exprtype_T;
 
 /*

src/testdir/test_vim9_assign.vim

@@ -557,20 +557,21 @@ enddef
 
 def Test_extend_list()
   var lines =<< trim END
-      vim9script
-      var l: list<number>
-      l += [123]
-      assert_equal([123], l)
+      var l1: list<number>
+      var l2 = l1
+      assert_true(l1 is l2)
+      l1 += [123]
+      assert_equal([123], l1)
+      assert_true(l1 is l2)
   END
-  CheckScriptSuccess(lines)
+  CheckDefAndScriptSuccess(lines)
 
   lines =<< trim END
-      vim9script
       var list: list<string>
       extend(list, ['x'])
       assert_equal(['x'], list)
   END
-  CheckScriptSuccess(lines)
+  CheckDefAndScriptSuccess(lines)
 
   # appending to NULL list from a function
   lines =<< trim END

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3423,
 /**/
     3422,
 /**/

src/vim9compile.c

@@ -690,12 +690,16 @@ check_number_or_float(vartype_T type1, vartype_T type2, char_u *op)
     return OK;
 }
 
+/*
+ * Generate instruction for "+".  For a list this creates a new list.
+ */
     static int
 generate_add_instr(
 	cctx_T *cctx,
 	vartype_T vartype,
 	type_T *type1,
-	type_T *type2)
+	type_T *type2,
+	exprtype_T expr_type)
 {
     garray_T	*stack = &cctx->ctx_type_stack;
     isn_T	*isn = generate_instr_drop(cctx,
@@ -715,7 +719,12 @@ generate_add_instr(
 	return FAIL;
 
     if (isn != NULL)
-	isn->isn_arg.op.op_type = EXPR_ADD;
+    {
+	if (isn->isn_type == ISN_ADDLIST)
+	    isn->isn_arg.op.op_type = expr_type;
+	else
+	    isn->isn_arg.op.op_type = EXPR_ADD;
+    }
 
     // When concatenating two lists with different member types the member type
     // becomes "any".
@@ -769,7 +778,8 @@ generate_two_op(cctx_T *cctx, char_u *op)
     switch (*op)
     {
 	case '+':
-		  if (generate_add_instr(cctx, vartype, type1, type2) == FAIL)
+		  if (generate_add_instr(cctx, vartype, type1, type2,
+							    EXPR_COPY) == FAIL)
 		      return FAIL;
 		  break;
 
@@ -7186,7 +7196,8 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    {
 		if (generate_add_instr(cctx,
 			    operator_type(lhs.lhs_member_type, stacktype),
-				       lhs.lhs_member_type, stacktype) == FAIL)
+				       lhs.lhs_member_type, stacktype,
+							  EXPR_APPEND) == FAIL)
 		    goto theend;
 	    }
 	    else if (generate_two_op(cctx, op) == FAIL)

src/vim9execute.c

@@ -3677,7 +3677,14 @@ exec_instructions(ectx_T *ectx)
 
 		    // add two lists or blobs
 		    if (iptr->isn_type == ISN_ADDLIST)
-			eval_addlist(tv1, tv2);
+		    {
+			if (iptr->isn_arg.op.op_type == EXPR_APPEND
+						   && tv1->vval.v_list != NULL)
+			    list_extend(tv1->vval.v_list, tv2->vval.v_list,
+									 NULL);
+			else
+			    eval_addlist(tv1, tv2);
+		    }
 		    else
 			eval_addblob(tv1, tv2);
 		    clear_tv(tv2);