From 2d81ab2a365edd829341d7648607ba9ab18c01b6 Mon Sep 17 00:00:00 2001
From: CreeperFace <165158232+dy-tea@users.noreply.github.com>
Date: Tue, 4 Nov 2025 10:10:26 +0000
Subject: [PATCH] checker: allow cast from voidptr to sumtype in unsafe block
 (fix #25652) (#25657)

---
 vlib/v/checker/checker.v                      | 10 +++++++--
 .../tests/sumtype_from_voidptr_err.out        |  7 ++++++
 .../checker/tests/sumtype_from_voidptr_err.vv | 15 +++++++++++++
 .../sumtypes/sumtype_cast_from_voidptr_test.v | 22 +++++++++++++++++++
 4 files changed, 52 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/checker/tests/sumtype_from_voidptr_err.out
 create mode 100644 vlib/v/checker/tests/sumtype_from_voidptr_err.vv
 create mode 100644 vlib/v/tests/sumtypes/sumtype_cast_from_voidptr_test.v

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 4dbd75d74..a52957c96 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3511,9 +3511,15 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			from_type = node.expr_type
 		}
 		if !c.table.sumtype_has_variant(to_type, from_type, false) {
-			ft := c.table.type_to_str(from_type)
 			tt := c.table.type_to_str(to_type)
-			c.error('cannot cast `${ft}` to `${tt}`', node.pos)
+			if from_type == ast.voidptr_type_idx && to_type.is_ptr() {
+				if !c.inside_unsafe {
+					c.error('cannot cast voidptr to `${tt}` outside `unsafe`', node.pos)
+				}
+			} else {
+				ft := c.table.type_to_str(from_type)
+				c.error('cannot cast `${ft}` to `${tt}`', node.pos)
+			}
 		}
 	} else if mut to_sym.info is ast.Alias && !(final_to_sym.kind == .struct && final_to_is_ptr) {
 		if (!c.check_types(from_type, to_sym.info.parent_type) && !(final_to_sym.is_int()
diff --git a/vlib/v/checker/tests/sumtype_from_voidptr_err.out b/vlib/v/checker/tests/sumtype_from_voidptr_err.out
new file mode 100644
index 000000000..b5b3b2c66
--- /dev/null
+++ b/vlib/v/checker/tests/sumtype_from_voidptr_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/sumtype_from_voidptr_err.vv:13:11: error: cannot cast voidptr to `&Event` outside `unsafe`
+   11 | fn main() {
+   12 |     some_ptr := voidptr(1234)
+   13 |     event := &Event(some_ptr)
+      |              ~~~~~~~~~~~~~~~~
+   14 |     _ = event
+   15 | }
diff --git a/vlib/v/checker/tests/sumtype_from_voidptr_err.vv b/vlib/v/checker/tests/sumtype_from_voidptr_err.vv
new file mode 100644
index 000000000..173fa8fa6
--- /dev/null
+++ b/vlib/v/checker/tests/sumtype_from_voidptr_err.vv
@@ -0,0 +1,15 @@
+struct EventA {
+	a u32
+}
+
+struct EventB {
+	b u32
+}
+
+type Event = EventA | EventB
+
+fn main() {
+	some_ptr := voidptr(1234)
+	event := &Event(some_ptr)
+	_ = event
+}
diff --git a/vlib/v/tests/sumtypes/sumtype_cast_from_voidptr_test.v b/vlib/v/tests/sumtypes/sumtype_cast_from_voidptr_test.v
new file mode 100644
index 000000000..5f716d233
--- /dev/null
+++ b/vlib/v/tests/sumtypes/sumtype_cast_from_voidptr_test.v
@@ -0,0 +1,22 @@
+struct EventA {
+	a u32
+}
+
+struct EventB {
+	b u32
+}
+
+type Event = EventA | EventB
+
+fn test_main() {
+	some_ptr := voidptr(&EventA{
+		a: 1234
+	})
+	event := unsafe { &Event(some_ptr) }
+
+	d1 := &EventA(event)
+	assert d1.a == 1234
+
+	d2 := &EventB(event)
+	assert d2.b == 1234
+}
-- 
2.39.5