parser: check generic struct init using multi return type (#15472)

vlib/v/parser/parse_type.v

@@ -675,6 +675,9 @@ pub fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 			is_instance = true
 		}
 		gts := p.table.sym(gt)
+		if gts.kind == .multi_return {
+			p.error_with_pos('cannot use multi return as generic concrete type', type_pos)
+		}
 		if !is_instance && gts.name.len > 1 {
 			p.error_with_pos('generic struct parameter name needs to be exactly one char',
 				type_pos)

vlib/v/parser/tests/generic_struct_type_using_multi_return_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/generic_struct_type_using_multi_return_err.vv:6:18: error: cannot use multi return as generic concrete type
+    4 |
+    5 | fn main() {
+    6 |     sample := Tuple<(int, int)>{}
+      |                     ~~~~~~~~~~
+    7 |     println(sample)
+    8 | }

vlib/v/parser/tests/generic_struct_type_using_multi_return_err.vv

@@ -0,0 +1,8 @@
+struct Tuple<T> {
+	data T
+}
+
+fn main() {
+	sample := Tuple<(int, int)>{}
+	println(sample)
+}