From 06dd19b7747c08bef4f4fcf582e13445dd0c31f4 Mon Sep 17 00:00:00 2001
From: Jesus Alvarez <jeezusjr@gmail.com>
Date: Sun, 25 Jan 2026 20:55:04 -0800
Subject: [PATCH] cgen: fix unused functions returning fixed arrays of custom
 structs (fix #26439) (#26440)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
---
 vlib/v/gen/c/cgen.v                           |  7 ++++
 .../fns/unused_fn_fixed_array_ret_test.v      | 37 +++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 vlib/v/tests/fns/unused_fn_fixed_array_ret_test.v

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 5391666ce..d70e0877d 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -2029,6 +2029,13 @@ pub fn (mut g Gen) write_array_fixed_return_types() {
 			// unresolved sizes e.g. [unknown_const]int
 			continue
 		}
+		// Skip if element type is a non-builtin struct that isn't marked as used
+		// This prevents generating wrapper structs that reference undefined types
+		elem_sym := g.table.sym(info.elem_type)
+		if g.pref.skip_unused && elem_sym.kind == .struct && !elem_sym.is_builtin()
+			&& elem_sym.idx !in g.table.used_features.used_syms {
+			continue
+		}
 		mut fixed_elem_name := g.styp(info.elem_type.set_nr_muls(0))
 		if info.elem_type.is_ptr() {
 			fixed_elem_name += '*'.repeat(info.elem_type.nr_muls())
diff --git a/vlib/v/tests/fns/unused_fn_fixed_array_ret_test.v b/vlib/v/tests/fns/unused_fn_fixed_array_ret_test.v
new file mode 100644
index 000000000..a5ed30a66
--- /dev/null
+++ b/vlib/v/tests/fns/unused_fn_fixed_array_ret_test.v
@@ -0,0 +1,37 @@
+// Test that unused functions returning fixed arrays of custom structs
+// don't cause C compilation errors when skip_unused is enabled.
+// Related to: functions returning [N]CustomStruct where the function is never called.
+
+struct QuadVertex {
+	pos [2]f32
+	uv  [2]f32
+}
+
+// This function is intentionally never called - it tests that skip_unused
+// correctly handles fn_ret ArrayFixed types with custom struct elements.
+pub fn make_quad_vertices(x f32, y f32, w f32, h f32) [4]QuadVertex {
+	return [
+		QuadVertex{
+			pos: [x, y]!
+			uv:  [f32(0), 0]!
+		},
+		QuadVertex{
+			pos: [x + w, y]!
+			uv:  [f32(1), 0]!
+		},
+		QuadVertex{
+			pos: [x + w, y + h]!
+			uv:  [f32(1), 1]!
+		},
+		QuadVertex{
+			pos: [x, y + h]!
+			uv:  [f32(0), 1]!
+		},
+	]!
+}
+
+fn test_unused_fn_fixed_array_ret() {
+	// The test passes if compilation succeeds.
+	// make_quad_vertices is intentionally not called.
+	assert true
+}
-- 
2.39.5