From 74d55eff945851f941ce98b8c5ded99a28101c44 Mon Sep 17 00:00:00 2001
From: CreeperFace <165158232+dy-tea@users.noreply.github.com>
Date: Sat, 6 Dec 2025 09:11:10 +0000
Subject: [PATCH] ast,cgen: fallback to field type when default_expr_typ is 0
 (fix #25891) (#25903)

---
 vlib/v/ast/table.v                         | 15 +++++++++++++++
 vlib/v/gen/c/cgen.v                        | 11 +++++++++--
 vlib/v/tests/global_generic_sumtype_test.v | 15 +++++++++++++++
 3 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/tests/global_generic_sumtype_test.v

diff --git a/vlib/v/ast/table.v b/vlib/v/ast/table.v
index 142cb810e..6a5434418 100644
--- a/vlib/v/ast/table.v
+++ b/vlib/v/ast/table.v
@@ -2219,6 +2219,21 @@ pub fn (mut t Table) unwrap_generic_type_ex(typ Type, generic_names []string, co
 							}
 						}
 					}
+					if fields[i].has_default_expr {
+						if fields[i].default_expr_typ.has_flag(.generic) {
+							if t_typ := t.convert_generic_type(fields[i].default_expr_typ,
+								t_generic_names, t_concrete_types)
+							{
+								fields[i].default_expr_typ = t_typ
+							}
+						} else if fields[i].default_expr_typ == 0
+							|| fields[i].default_expr_typ == nil_type {
+							if fields[i].default_expr.is_nil()
+								&& fields[i].typ.is_any_kind_of_pointer() {
+								fields[i].default_expr_typ = fields[i].typ
+							}
+						}
+					}
 				}
 				// update concrete types
 				for i in 0 .. ts.info.generic_types.len {
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index eaf59806d..af4911b78 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7689,8 +7689,15 @@ fn (mut g Gen) type_default_impl(typ_ ast.Type, decode_sumtype bool) string {
 						if field.has_default_expr {
 							mut expr_str := ''
 							if field_sym.kind in [.sum_type, .interface] {
+								default_expr_typ := if field.default_expr_typ == 0
+									&& field.default_expr.is_nil()
+									&& field.typ.is_any_kind_of_pointer() {
+									field.typ
+								} else {
+									field.default_expr_typ
+								}
 								expr_str = g.expr_string_with_cast(field.default_expr,
-									field.default_expr_typ, field.typ)
+									default_expr_typ, field.typ)
 							} else if field_sym.is_array_fixed() && g.inside_global_decl {
 								array_info := field_sym.array_fixed_info()
 								match field.default_expr {
@@ -7739,7 +7746,7 @@ fn (mut g Gen) type_default_impl(typ_ ast.Type, decode_sumtype bool) string {
 							zero_str := if field_sym.language == .v && field_sym.info is ast.Struct
 								&& field_sym.info.is_empty_struct() {
 								'{E_STRUCT}'
-							} else if field_sym.kind == .sum_type {
+							} else if field_sym.kind == .sum_type && !field.typ.is_ptr() {
 								if decode_sumtype {
 									g.type_default_sumtype(field.typ, field_sym)
 								} else {
diff --git a/vlib/v/tests/global_generic_sumtype_test.v b/vlib/v/tests/global_generic_sumtype_test.v
new file mode 100644
index 000000000..965cfbfc5
--- /dev/null
+++ b/vlib/v/tests/global_generic_sumtype_test.v
@@ -0,0 +1,15 @@
+@[has_globals]
+module main
+
+__global queue2 Queue[Event]
+
+type Event = int | u32
+
+struct Queue[T] {
+mut:
+	data &T
+}
+
+fn test_main() {
+	assert queue2.data == unsafe { nil }
+}
-- 
2.39.5