From 19532b79d362dcf11c5de780e1fef87e14ab13ec Mon Sep 17 00:00:00 2001
From: wenxuanjun <41050170+wenxuanjun@users.noreply.github.com>
Date: Sat, 28 Mar 2026 21:06:59 +0800
Subject: [PATCH] cgen: fix C error array of incomplete type for fixed-size
 optional array (#26780)

---
 vlib/v/gen/c/cgen.v                           | 37 +++++++++++++------
 ...ption_array_fixed_interface_sumtype_test.v | 37 +++++++++++++++++++
 2 files changed, 62 insertions(+), 12 deletions(-)
 create mode 100644 vlib/v/tests/options/option_array_fixed_interface_sumtype_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 2b69d19e1..dab1e0ab3 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7174,6 +7174,26 @@ fn (mut g Gen) write_sorted_types() {
 	}
 }
 
+fn (mut g Gen) ensure_fixed_array_option_definition(elem_type ast.Type) bool {
+	if !elem_type.has_flag(.option) || elem_type.has_flag(.generic) {
+		return false
+	}
+	elem_sym := g.table.final_sym(elem_type.clear_option_and_result())
+	if elem_sym.info !is ast.Struct && elem_sym.info !is ast.Interface
+		&& elem_sym.info !is ast.SumType {
+		return false
+	}
+	styp, base := g.option_type_name(elem_type)
+	lock g.done_options {
+		if base !in g.done_options {
+			g.done_options << base
+			g.typedefs.writeln('typedef struct ${styp} ${styp};')
+			g.type_definitions.writeln('${g.option_type_text(styp, base)};')
+		}
+	}
+	return true
+}
+
 fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 	mut struct_names := map[string]bool{}
 	for sym in symbols {
@@ -7200,7 +7220,8 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 			ast.Struct {
 				if !struct_names[name]
 					&& (!g.pref.skip_unused || sym.idx in g.table.used_features.used_syms) {
-					// generate field option types for fixed array of option struct before struct declaration
+					// generate field option types for fixed array of option struct/interface/sumtype
+					// before the struct declaration
 					opt_fields := sym.info.fields.filter(g.table.final_sym(it.typ).kind == .array_fixed)
 					for opt_field in opt_fields {
 						field_sym := g.table.final_sym(opt_field.typ)
@@ -7208,6 +7229,7 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 						if !arr.elem_type.has_flag(.option) || arr.elem_type.has_flag(.generic) {
 							continue
 						}
+						g.ensure_fixed_array_option_definition(arr.elem_type)
 						styp := field_sym.cname
 						mut fixed_elem_name := g.styp(arr.elem_type.set_nr_muls(0))
 						if arr.elem_type.is_ptr() {
@@ -7323,17 +7345,8 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 							g.type_definitions.writeln(def_str)
 						} else if elem_sym.info !is ast.ArrayFixed
 							|| (elem_sym.info as ast.ArrayFixed).size > 0 {
-							// fixed array of option struct must be defined backwards
-							if sym.info.elem_type.has_flag(.option) && elem_sym.info is ast.Struct {
-								styp_elem, base := g.option_type_name(sym.info.elem_type)
-								lock g.done_options {
-									if base !in g.done_options {
-										g.done_options << base
-										g.typedefs.writeln('typedef struct ${styp_elem} ${styp_elem};')
-										g.type_definitions.writeln('${g.option_type_text(styp_elem,
-											base)};')
-									}
-								}
+							// fixed array of option struct/interface/sumtype must be defined backwards
+							if g.ensure_fixed_array_option_definition(sym.info.elem_type) {
 								g.type_definitions.writeln('typedef ${fixed_elem_name} ${styp} [${len}];')
 							} else if !(elem_sym.info is ast.ArrayFixed && elem_sym.info.is_fn_ret) {
 								g.type_definitions.writeln('typedef ${fixed_elem_name} ${styp} [${len}];')
diff --git a/vlib/v/tests/options/option_array_fixed_interface_sumtype_test.v b/vlib/v/tests/options/option_array_fixed_interface_sumtype_test.v
new file mode 100644
index 000000000..3af873b45
--- /dev/null
+++ b/vlib/v/tests/options/option_array_fixed_interface_sumtype_test.v
@@ -0,0 +1,37 @@
+interface Speaker {}
+
+struct SpeakerA {}
+
+struct SpeakerHolder {
+	speakers [2]?Speaker
+}
+
+struct EventA {
+	a u32
+}
+
+struct EventB {
+	b u32
+}
+
+type Event = EventA | EventB
+
+struct EventHolder {
+	events [2]?Event
+}
+
+fn test_fixed_array_of_option_interface_and_sumtype() {
+	holder := SpeakerHolder{
+		speakers: [?Speaker(SpeakerA{}), none]!
+	}
+	assert holder.speakers[0] != none
+	assert holder.speakers[1] == none
+
+	event_holder := EventHolder{
+		events: [?Event(EventA{
+			a: 1
+		}), none]!
+	}
+	assert event_holder.events[0] != none
+	assert event_holder.events[1] == none
+}
-- 
2.39.5