checker: support ?T, &T generic return types (#6772)

vlib/v/checker/checker.v

@@ -1565,7 +1565,13 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 		// Handle `foo<T>() T` => `foo<int>() int` => return int
 		return_sym := c.table.get_type_symbol(f.return_type)
 		if return_sym.source_name == 'T' {
-			return call_expr.generic_type
+			mut typ := call_expr.generic_type
+			typ = typ.set_nr_muls(f.return_type.nr_muls())
+			if f.return_type.has_flag(.optional) {
+				typ = typ.set_flag(.optional)
+			}
+			call_expr.return_type = typ
+			return typ
 		} else if return_sym.kind == .array {
 			elem_info := return_sym.info as table.Array
 			elem_sym := c.table.get_type_symbol(elem_info.elem_type)

vlib/v/tests/generics_test.v

@@ -102,6 +102,30 @@ fn test_return_array() {
 	assert a4 == [true, false, true]
 }
 
+fn opt<T>(v T) ?T {
+	if sizeof(T) > 1 {return v}
+	return none
+}
+
+fn test_optional() {
+	s := opt('hi') or { '' }
+	assert s == 'hi'
+	i := opt(5) or {0}
+	assert i == 5
+	b := opt(s[0]) or {99}
+	assert b == 99
+}
+
+fn ptr<T>(v T) &T {
+	a := [v]
+	return a.data
+}
+
+fn test_ptr() {
+	assert *ptr(4) == 4
+	assert *ptr('aa') == 'aa'
+}
+
 /*
 fn map_f<T,U>(l []T, f fn(T)U) []U {
 	mut r := []U{}