From b17ac3384a13af1ee0a02aa18d8170e05e8db8aa Mon Sep 17 00:00:00 2001
From: imperivox <imperivox@gmail.com>
Date: Sun, 23 Nov 2025 19:37:32 +0100
Subject: [PATCH] checker: allow explicit sumtype to option casts (fix #25796)
 (#25812)

---
 vlib/v/checker/checker.v                      |  7 +++++++
 .../option_sumtype_explicit_cast_test.v       | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 vlib/v/tests/options/option_sumtype_explicit_cast_test.v

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index c6f39130d..691bd3a4d 100644
--- a/vlib/v/checker/checker.v
+++ b/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 {
diff --git a/vlib/v/tests/options/option_sumtype_explicit_cast_test.v b/vlib/v/tests/options/option_sumtype_explicit_cast_test.v
new file mode 100644
index 000000000..5eb9ffd5e
--- /dev/null
+++ b/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))
+}
-- 
2.39.5