From b7a9d7d3b08068ef1f8e87a211937b7144e0aa13 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 11:28:20 +0300
Subject: [PATCH] checker: array returned by function bypasses illegal sumtype
 conversion checker error (fixes #23271)

---
 vlib/v/checker/containers.v                   |  4 ++-
 .../tests/array_of_sumtype_append_err.out     | 29 ++++++++++++-------
 .../tests/array_of_sumtype_append_err.vv      |  5 ++++
 3 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/vlib/v/checker/containers.v b/vlib/v/checker/containers.v
index b58ecde20..cf1fbb587 100644
--- a/vlib/v/checker/containers.v
+++ b/vlib/v/checker/containers.v
@@ -856,7 +856,9 @@ fn (mut c Checker) check_append(mut node ast.InfixExpr, left_type ast.Type, righ
 			}
 		} else {
 			right_value_type := c.table.value_type(c.unwrap_generic(right_type))
-			if !c.table.is_sumtype_or_in_variant(left_value_type, ast.mktyp(right_value_type)) {
+			left_sumtype := c.table.unaliased_type(c.unwrap_generic(left_value_type))
+			right_sumtype := c.table.unaliased_type(c.unwrap_generic(right_value_type))
+			if left_sumtype != right_sumtype {
 				c.error('cannot append `${right_sym.name}` to `${left_sym.name}`', right_pos)
 			}
 		}
diff --git a/vlib/v/checker/tests/array_of_sumtype_append_err.out b/vlib/v/checker/tests/array_of_sumtype_append_err.out
index db60a7772..586fda8b8 100644
--- a/vlib/v/checker/tests/array_of_sumtype_append_err.out
+++ b/vlib/v/checker/tests/array_of_sumtype_append_err.out
@@ -1,13 +1,20 @@
-vlib/v/checker/tests/array_of_sumtype_append_err.vv:9:9: error: cannot append `[]string` to `[]Bar`
-    7 | fn main() {
-    8 |     mut bar := []Bar{}
-    9 |     bar << ['hey']
+vlib/v/checker/tests/array_of_sumtype_append_err.vv:13:9: error: cannot append `[]string` to `[]Bar`
+   11 | fn main() {
+   12 |     mut bar := []Bar{}
+   13 |     bar << ['hey']
       |            ~~~~~~~
-   10 |     bar << 'hey'
-   11 | }
-vlib/v/checker/tests/array_of_sumtype_append_err.vv:10:9: error: cannot append `string` to `[]Bar`
-    8 |     mut bar := []Bar{}
-    9 |     bar << ['hey']
-   10 |     bar << 'hey'
+   14 |     bar << 'hey'
+   15 |     bar << produce_bar_a_array()
+vlib/v/checker/tests/array_of_sumtype_append_err.vv:14:9: error: cannot append `string` to `[]Bar`
+   12 |     mut bar := []Bar{}
+   13 |     bar << ['hey']
+   14 |     bar << 'hey'
       |            ~~~~~
-   11 | }
+   15 |     bar << produce_bar_a_array()
+   16 | }
+vlib/v/checker/tests/array_of_sumtype_append_err.vv:15:9: error: cannot append `[]BarA` to `[]Bar`
+   13 |     bar << ['hey']
+   14 |     bar << 'hey'
+   15 |     bar << produce_bar_a_array()
+      |            ~~~~~~~~~~~~~~~~~~~~~
+   16 | }
diff --git a/vlib/v/checker/tests/array_of_sumtype_append_err.vv b/vlib/v/checker/tests/array_of_sumtype_append_err.vv
index 5239a1ab7..90216a85a 100644
--- a/vlib/v/checker/tests/array_of_sumtype_append_err.vv
+++ b/vlib/v/checker/tests/array_of_sumtype_append_err.vv
@@ -4,8 +4,13 @@ struct BarA {}
 
 struct BarB {}
 
+fn produce_bar_a_array() []BarA {
+	return [BarA{}]
+}
+
 fn main() {
 	mut bar := []Bar{}
 	bar << ['hey']
 	bar << 'hey'
+	bar << produce_bar_a_array()
 }
-- 
2.39.5