checker: allow explicit sumtype to option casts (fix #25796) (#25812)

Author: imperivox

vlib/v/checker/checker.v

@@ -3547,8 +3547,15 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	if from_type == ast.void_type {
 		c.error('expression does not return a value so it cannot be cast', node.expr.pos())
 	}
+	inner_to_type := if to_type.has_flag(.option) {
+		to_type.clear_flag(.option)
+	} else {
+		ast.void_type
+	}
 	if to_type.has_flag(.option) && from_type == ast.none_type {
 		// allow conversion from none to every option type
+	} else if to_type.has_flag(.option) && from_type == inner_to_type {
+		return to_type
 	} else if to_sym.kind == .sum_type {
 		to_sym_info := to_sym.info as ast.SumType
 		if c.pref.skip_unused && to_sym_info.concrete_types.len > 0 {

vlib/v/tests/options/option_sumtype_explicit_cast_test.v

@@ -0,0 +1,19 @@
+type SumType = u16 | u32
+
+fn wrap[T](x T) ?T {
+	return ?T(x)
+}
+
+fn test_sumtype_explicit_to_option_sumtype() {
+	value := SumType(u16(0))
+	a := ?SumType(value)
+	assert a != none
+	assert a? == SumType(u16(0))
+}
+
+fn test_sumtype_explicit_to_option_sumtype_generic() {
+	value := SumType(u16(1))
+	a := wrap[SumType](value)
+	assert a != none
+	assert a? == SumType(u16(1))
+}