cgen: fix array auto cloning (#20078)

vlib/v/gen/c/index.v

@@ -65,11 +65,10 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 }
 
 fn (mut g Gen) index_range_expr(node ast.IndexExpr, range ast.RangeExpr) {
-	sym := g.table.final_sym(node.left_type)
+	sym := g.table.final_sym(g.unwrap_generic(node.left_type))
 	mut tmp_opt := ''
 	mut cur_line := ''
 	mut gen_or := node.or_expr.kind != .absent || node.is_option
-
 	if sym.kind == .string {
 		if node.is_gated {
 			g.write('string_substr_ni(')

vlib/v/tests/array_fixed_auto_clone_test.v

@@ -0,0 +1,32 @@
+fn test_main() {
+	mut buf := [100]u8{}
+	name(mut buf)
+
+	mut buf2 := []u8{}
+	name(mut buf2)
+}
+
+fn name[T](mut buf4 T) {
+	for idx in 0 .. 100 {
+		$if T is $array_fixed {
+			buf4[idx] = idx
+		} $else $if T is $array {
+			buf4 << idx
+		}
+	}
+	mut bp := buf4[0..5]
+	assert bp.len == 5
+	println(bp)
+
+	bp = buf4[..]
+	assert bp.len == 100
+	println(bp)
+
+	bp = buf4[4..]
+	assert bp.len == 96
+	println(bp)
+
+	bp = buf4[1..99]
+	assert bp.len == 98
+	println(bp)
+}