checker: make negative literal index a checker error (#8242)

vlib/v/checker/checker.v

@@ -4804,7 +4804,7 @@ pub fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) table.Type {
 	return right_type
 }
 
-fn (mut c Checker) check_index_type(typ_sym &table.TypeSymbol, index_type table.Type, pos token.Position) {
+fn (mut c Checker) check_index(typ_sym &table.TypeSymbol, index ast.Expr, index_type table.Type, pos token.Position) {
 	index_type_sym := c.table.get_type_symbol(index_type)
 	// println('index expr left=$typ_sym.name $node.pos.line_nr')
 	// if typ_sym.kind == .array && (!(table.type_idx(index_type) in table.number_type_idxs) &&
@@ -4818,6 +4818,14 @@ fn (mut c Checker) check_index_type(typ_sym &table.TypeSymbol, index_type table.
 			}
 			c.error('$type_str', pos)
 		}
+		if index is ast.PrefixExpr {
+			if index.op == .minus && index.right is ast.IntegerLiteral {
+				val := (index.right as ast.IntegerLiteral).val
+				if val.int() > 0 {
+					c.error('invalid index `-$val` (index must be non-negative)', index.pos)
+				}
+			}
+		}
 		if index_type.has_flag(.optional) {
 			type_str := if typ_sym.kind in [.string, .ustring] {
 				'(type `$typ_sym.name`)'
@@ -4859,11 +4867,11 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) table.Type {
 	if mut node.index is ast.RangeExpr { // [1..2]
 		if node.index.has_low {
 			index_type := c.expr(node.index.low)
-			c.check_index_type(typ_sym, index_type, node.pos)
+			c.check_index(typ_sym, node.index.low, index_type, node.pos)
 		}
 		if node.index.has_high {
 			index_type := c.expr(node.index.high)
-			c.check_index_type(typ_sym, index_type, node.pos)
+			c.check_index(typ_sym, node.index.high, index_type, node.pos)
 		}
 		// array[1..2] => array
 		// fixed_array[1..2] => array
@@ -4883,7 +4891,7 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) table.Type {
 				c.error('invalid key: $err', node.pos)
 			}
 		} else {
-			c.check_index_type(typ_sym, index_type, node.pos)
+			c.check_index(typ_sym, node.index, index_type, node.pos)
 		}
 		value_type := c.table.value_type(typ)
 		if value_type != table.void_type {

vlib/v/checker/tests/index_expr.out

@@ -1,41 +1,69 @@
-vlib/v/checker/tests/index_expr.vv:3:7: error: type `int` does not support indexing 
+vlib/v/checker/tests/index_expr.vv:3:7: error: type `int` does not support indexing
     1 | fn test_invalid_index() {
     2 |     v := 4
     3 |     _ = v[0]
       |          ~~~
-    4 |     
+    4 |
     5 |     a := [2]
-vlib/v/checker/tests/index_expr.vv:6:7: error: non-integer index `[]int` (array type `[]int`) 
+vlib/v/checker/tests/index_expr.vv:6:7: error: non-integer index `[]int` (array type `[]int`)
-    4 |     
+    4 |
     5 |     a := [2]
     6 |     _ = a[a]
       |          ~~~
-    7 | }
+    7 |     _ = a[-1]
-    8 |
+    8 | }
-vlib/v/checker/tests/index_expr.vv:11:7: error: type `int` does not support indexing 
+vlib/v/checker/tests/index_expr.vv:7:8: error: invalid index `-1` (index must be non-negative)
-    9 | fn test_invalid_slice() {
+    5 |     a := [2]
-   10 |     v := 4
+    6 |     _ = a[a]
-   11 |     _ = v[1..]
+    7 |     _ = a[-1]
+      |           ^
+    8 | }
+    9 |
+vlib/v/checker/tests/index_expr.vv:12:7: error: type `int` does not support indexing
+   10 | fn test_invalid_slice() {
+   11 |     v := 4
+   12 |     _ = v[1..]
       |          ~~~~~
-   12 |     _ = v[..1]
+   13 |     _ = v[..1]
-   13 |
+   14 |
-vlib/v/checker/tests/index_expr.vv:12:7: error: type `int` does not support indexing 
+vlib/v/checker/tests/index_expr.vv:13:7: error: type `int` does not support indexing
-   10 |     v := 4
+   11 |     v := 4
-   11 |     _ = v[1..]
+   12 |     _ = v[1..]
-   12 |     _ = v[..1]
+   13 |     _ = v[..1]
       |          ~~~~~
-   13 |     
+   14 |
-   14 |     a := [2]
+   15 |     a := [2]
-vlib/v/checker/tests/index_expr.vv:15:7: error: non-integer index `[]int` (array type `[]int`) 
+vlib/v/checker/tests/index_expr.vv:16:7: error: non-integer index `[]int` (array type `[]int`)
-   13 |     
+   14 |
-   14 |     a := [2]
+   15 |     a := [2]
-   15 |     _ = a[a..]
+   16 |     _ = a[a..]
       |          ~~~~~
-   16 |     _ = a[..a]
+   17 |     _ = a[..a]
-   17 | }
+   18 |     _ = a[-1..]
-vlib/v/checker/tests/index_expr.vv:16:7: error: non-integer index `[]int` (array type `[]int`) 
+vlib/v/checker/tests/index_expr.vv:17:7: error: non-integer index `[]int` (array type `[]int`)
-   14 |     a := [2]
+   15 |     a := [2]
-   15 |     _ = a[a..]
+   16 |     _ = a[a..]
-   16 |     _ = a[..a]
+   17 |     _ = a[..a]
       |          ~~~~~
-   17 | }
+   18 |     _ = a[-1..]
+   19 |     _ = a[..-1]
+vlib/v/checker/tests/index_expr.vv:18:8: error: invalid index `-1` (index must be non-negative)
+   16 |     _ = a[a..]
+   17 |     _ = a[..a]
+   18 |     _ = a[-1..]
+      |           ^
+   19 |     _ = a[..-1]
+   20 |     _ = a[-1..-2]
+vlib/v/checker/tests/index_expr.vv:19:10: error: invalid index `-1` (index must be non-negative)
+   17 |     _ = a[..a]
+   18 |     _ = a[-1..]
+   19 |     _ = a[..-1]
+      |             ^
+   20 |     _ = a[-1..-2]
+   21 | }
+vlib/v/checker/tests/index_expr.vv:20:8: error: invalid index `-1` (index must be non-negative)
+   18 |     _ = a[-1..]
+   19 |     _ = a[..-1]
+   20 |     _ = a[-1..-2]
+      |           ^
+   21 | }

vlib/v/checker/tests/index_expr.vv

@@ -1,17 +1,21 @@
 fn test_invalid_index() {
 	v := 4
 	_ = v[0]
-	
+
 	a := [2]
 	_ = a[a]
+	_ = a[-1]
 }
 
 fn test_invalid_slice() {
 	v := 4
 	_ = v[1..]
 	_ = v[..1]
-	
+
 	a := [2]
 	_ = a[a..]
 	_ = a[..a]
+	_ = a[-1..]
+	_ = a[..-1]
+	_ = a[-1..-2]
 }