ast,cgen: fallback to field type when default_expr_typ is 0 (fix #25891) (#25903)

Author: dy-tea

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 {

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 {

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 }
+}