From 4a4f703299c356e36ee08804bb8a65b91be64e0e Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 15:58:24 +0300
Subject: [PATCH] cgen: fix c backend bug on tuple reassign to struct local
 (fixes #26594)

---
 vlib/v/gen/c/assign.v                         |  7 +-
 ...tuple_reassign_to_heap_struct_local_test.v | 83 +++++++++++++++++++
 2 files changed, 87 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/tests/assign/tuple_reassign_to_heap_struct_local_test.v

diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 7dd7ffd9a..37eb4ce1e 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -2213,6 +2213,7 @@ fn (mut g Gen) gen_multi_return_assign(node &ast.AssignStmt, return_type ast.Typ
 		}
 		left_type := if recompute_types { mr_types[i] } else { node.left_types[i] }
 		styp := if ident.name in g.defer_vars { '' } else { g.styp(left_type) }
+		needs_auto_heap_alloc := is_auto_heap && node.op == .decl_assign
 		if node.op == .decl_assign {
 			g.write('${styp} ')
 		}
@@ -2229,7 +2230,7 @@ fn (mut g Gen) gen_multi_return_assign(node &ast.AssignStmt, return_type ast.Typ
 		}
 		if left_type.has_flag(.option) {
 			base_typ := g.base_type(left_type)
-			tmp_var := if is_auto_heap {
+			tmp_var := if needs_auto_heap_alloc {
 				if aligned != 0 {
 					'HEAP_align(${styp}, ${mr_var_name}.arg${i}, ${aligned})'
 				} else {
@@ -2261,7 +2262,7 @@ fn (mut g Gen) gen_multi_return_assign(node &ast.AssignStmt, return_type ast.Typ
 					'memcpy(&${g.expr_string(lx)}, &${mr_var_name}.arg${i}, sizeof(${styp}));')
 			} else {
 				if cur_indexexpr != -1 {
-					if is_auto_heap {
+					if needs_auto_heap_alloc {
 						if aligned != 0 {
 							g.writeln('HEAP_align(${styp}, ${mr_var_name}.arg${i}, ${aligned}) });')
 						} else {
@@ -2274,7 +2275,7 @@ fn (mut g Gen) gen_multi_return_assign(node &ast.AssignStmt, return_type ast.Typ
 					}
 					g.cur_indexexpr.delete(cur_indexexpr)
 				} else {
-					if is_auto_heap {
+					if needs_auto_heap_alloc {
 						if aligned != 0 {
 							g.writeln(' = HEAP_align(${styp}, ${mr_var_name}.arg${i}, ${aligned});')
 						} else {
diff --git a/vlib/v/tests/assign/tuple_reassign_to_heap_struct_local_test.v b/vlib/v/tests/assign/tuple_reassign_to_heap_struct_local_test.v
new file mode 100644
index 000000000..3228cec5c
--- /dev/null
+++ b/vlib/v/tests/assign/tuple_reassign_to_heap_struct_local_test.v
@@ -0,0 +1,83 @@
+@[heap; minify]
+struct Cfg {
+	id   string
+	rows []int
+	meta map[string]string
+}
+
+struct State {
+	value int
+}
+
+struct CrudState {
+	value int
+}
+
+struct Caps {}
+
+struct Store {
+mut:
+	data map[string]State
+}
+
+fn (store &Store) get(id string) ?State {
+	if id in store.data {
+		return store.data[id]
+	}
+	return none
+}
+
+@[heap]
+struct Window {
+mut:
+	store Store
+}
+
+fn resolve_a(cfg Cfg, mut _ Window) (Cfg, State, bool, Caps) {
+	return cfg, State{
+		value: 1
+	}, true, Caps{}
+}
+
+fn resolve_b(cfg Cfg, mut _ Window) (Cfg, CrudState) {
+	return cfg, CrudState{
+		value: 2
+	}
+}
+
+fn (mut window Window) trigger(cfg Cfg) (Cfg, State, CrudState) {
+	resolved_cfg0, source_state0, has_source, _ := resolve_a(cfg, mut window)
+	mut resolved_cfg := resolved_cfg0
+	mut source_state := source_state0
+	mut crud_state := CrudState{}
+	crud_enabled := true
+	if crud_enabled {
+		resolved_cfg, crud_state = resolve_b(resolved_cfg, mut window)
+		if has_source {
+			source_state = window.store.get(resolved_cfg.id) or { source_state }
+		}
+	}
+	return resolved_cfg, source_state, crud_state
+}
+
+fn test_tuple_reassign_to_heap_struct_local() {
+	mut window := &Window{
+		store: Store{
+			data: {
+				'x': State{
+					value: 99
+				}
+			}
+		}
+	}
+	resolved_cfg, source_state, crud_state := window.trigger(Cfg{
+		id:   'x'
+		rows: [1, 2, 3]
+		meta: {
+			'a': 'b'
+		}
+	})
+	assert resolved_cfg.id == 'x'
+	assert source_state.value == 99
+	assert crud_state.value == 2
+}
-- 
2.39.5