parser, checker: fix generic fn that returns a generic struct (#9469)

vlib/v/checker/checker.v

@@ -1944,9 +1944,39 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			if rts.info.generic_types.len > 0 {
 				gts := c.table.get_type_symbol(call_expr.generic_types[0])
 				nrt := '$rts.name<$gts.name>'
+				c_nrt := '${rts.name}_T_$gts.name'
 				idx := c.table.type_idxs[nrt]
-				c.ensure_type_exists(idx, call_expr.pos) or {}
-				call_expr.return_type = table.new_type(idx).derive(f.return_type)
+				if idx != 0 {
+					c.ensure_type_exists(idx, call_expr.pos) or {}
+					call_expr.return_type = table.new_type(idx).derive(f.return_type)
+				} else {
+					mut fields := rts.info.fields.clone()
+					if rts.info.generic_types.len == generic_types.len {
+						for i, _ in fields {
+							mut field := fields[i]
+							if field.typ.has_flag(.generic) {
+								for j, gp in rts.info.generic_types {
+									if gp == field.typ {
+										field.typ = generic_types[j].derive(field.typ).clear_flag(.generic)
+										break
+									}
+								}
+							}
+							fields[i] = field
+						}
+						mut info := rts.info
+						info.generic_types = []
+						info.fields = fields
+						stru_idx := c.table.register_type_symbol(table.TypeSymbol{
+							kind: .struct_
+							name: nrt
+							cname: util.no_dots(c_nrt)
+							mod: c.mod
+							info: info
+						})
+						call_expr.return_type = table.new_type(stru_idx)
+					}
+				}
 			}
 		}
 	} else {

vlib/v/parser/parse_type.v

@@ -502,6 +502,11 @@ pub fn (mut p Parser) parse_generic_struct_inst_type(name string) table.Type {
 			}
 		})
 		return table.new_type(idx)
+	} else {
+		idx := p.table.find_type_idx(name)
+		if idx != 0 {
+			return table.new_type(idx).set_flag(.generic)
+		}
 	}
 	return p.parse_enum_or_struct_type(name, .v)
 }

vlib/v/tests/generics_return_generics_struct_test.v

@@ -0,0 +1,31 @@
+pub struct Optional<T> {
+mut:
+	value T
+	some bool
+}
+
+pub fn new_some<T>(value T) Optional<T> {
+	return {value: value, some: true}
+}
+
+pub fn some<T>(opt Optional<T>) bool {
+	return opt.some
+}
+
+pub fn get<T>(opt Optional<T>) T {
+	return opt.value
+}
+
+pub fn set<T>(mut opt Optional<T>, value T) {
+	opt.value = value
+	opt.some = true
+}
+
+fn test_generics_return_generics_struct() {
+	mut o := new_some<int>(23)
+	println(some<int>(o))
+	assert some<int>(o) == true
+	set<int>(mut o, 42)
+	println(get<int>(o))
+	assert get<int>(o) == 42
+}