From eaefe4d941e458c7959fa928efea4370d08c7d53 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:24 +0300
Subject: [PATCH] cgen: Subtle runtime segfault on Linux x86_64 in GitHub
 actions CI (fixes #26121)

---
 vlib/v/gen/c/cgen.v                           | 13 +++++
 .../tests/sumtype_default_common_field_test.v | 49 +++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 vlib/v/tests/sumtype_default_common_field_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 660bd0c73..4f6723cc3 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -9746,6 +9746,19 @@ fn (mut g Gen) type_default_sumtype(typ_ ast.Type, sym ast.TypeSymbol) string {
 	} else {
 		g.type_default_no_sumtype(first_typ)
 	}
+	sumtype_info := sym.info as ast.SumType
+	if sumtype_info.fields.len > 0 && !g.inside_global_decl && !g.inside_const {
+		// Route local defaults through the cast helper so common-field pointers stay valid.
+		fname := g.get_sumtype_casting_fn(first_typ, typ_)
+		mut first_default := g.type_default(first_typ)
+		if first_default[0] == `{` {
+			first_default = '(${first_styp})${first_default}'
+		}
+		if first_sym.info is ast.FnType {
+			return '${fname}(${first_default})'
+		}
+		return '${fname}(HEAP(${first_styp}, (${first_default})), true)'
+	}
 	if default_str[0] == `{` {
 		return '(${g.styp(typ_)}){._${first_field}=HEAP(${first_styp}, ((${first_styp})${default_str})),._typ=${u32(first_typ)}}'
 	} else {
diff --git a/vlib/v/tests/sumtype_default_common_field_test.v b/vlib/v/tests/sumtype_default_common_field_test.v
new file mode 100644
index 000000000..aaff3b272
--- /dev/null
+++ b/vlib/v/tests/sumtype_default_common_field_test.v
@@ -0,0 +1,49 @@
+module main
+
+struct DefaultAlpha {
+	x int
+}
+
+struct DefaultBeta {
+	x int
+}
+
+type DefaultSum = DefaultAlpha | DefaultBeta
+
+struct DefaultHolder {
+	s DefaultSum
+}
+
+struct NestedAlpha {
+	x int
+}
+
+struct NestedBeta {
+	x int
+}
+
+struct NestedGamma {
+	x int
+}
+
+struct NestedDelta {
+	x int
+}
+
+type NestedLeft = NestedAlpha | NestedBeta
+type NestedRight = NestedDelta | NestedGamma
+type NestedOuter = NestedLeft | NestedRight
+
+struct NestedHolder {
+	o NestedOuter
+}
+
+fn test_default_sumtype_common_field_is_accessible() {
+	holder := DefaultHolder{}
+	assert holder.s.x == 0
+}
+
+fn test_default_nested_sumtype_common_field_is_accessible() {
+	holder := NestedHolder{}
+	assert holder.o.x == 0
+}
-- 
2.39.5