From f4f6f6e0f7dfdc14f720d1af0ec8e82bbffa5225 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 14 Apr 2026 12:45:33 +0300
Subject: [PATCH] cgen: Instantiating struct with global on heap causes c error
 (fixes #22970)

---
 vlib/v/gen/c/assign.v                         | 46 +++++++++++++++++++
 ...truct_field_default_shadowed_global_test.v | 38 +++++++++++++++
 2 files changed, 84 insertions(+)
 create mode 100644 vlib/v/tests/structs/struct_field_default_shadowed_global_test.v

diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index d7eed6781..7957e08ff 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -118,6 +118,34 @@ fn assign_expr_unwraps_option_or_result(expr ast.Expr) bool {
 	}
 }
 
+fn (mut g Gen) decl_assign_struct_init_needs_tmp(expr ast.Expr) bool {
+	node := match expr {
+		ast.StructInit {
+			expr
+		}
+		ast.ParExpr {
+			if expr.expr is ast.StructInit {
+				expr.expr
+			} else {
+				return false
+			}
+		}
+		else {
+			return false
+		}
+	}
+	sym := g.table.final_sym(g.unwrap_generic(g.recheck_concrete_type(node.typ)))
+	if sym.info !is ast.Struct {
+		return false
+	}
+	info := sym.info as ast.Struct
+	if node.no_keys {
+		return node.init_fields.len < info.fields.len
+	}
+	init_field_names := node.init_fields.map(it.name)
+	return info.fields.any(it.name !in init_field_names)
+}
+
 fn (mut g Gen) gen_self_recursing_anon_fn_capture_patch(left ast.Expr, anon_fn ast.AnonFn) {
 	if left !is ast.Ident || anon_fn.inherited_vars.len == 0 {
 		return
@@ -1665,6 +1693,19 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				g.expr(left)
 				g.write(',')
 			}
+			mut decl_tmp_var := ''
+			mut decl_stmt_str := ''
+			if is_decl && g.inside_ternary == 0 && !node.is_static && !var_type.has_flag(.shared_f)
+				&& g.decl_assign_struct_init_needs_tmp(val) {
+				decl_stmt_str = g.go_before_last_stmt().trim_space()
+				g.empty_line = true
+				decl_tmp_var = g.new_tmp_var()
+				g.write('${styp} ')
+				if is_auto_heap && !(val_type.is_ptr() && val_type.has_flag(.option)) {
+					g.write('*')
+				}
+				g.write('${decl_tmp_var} ${op} ')
+			}
 			mut cloned := false
 			if g.is_autofree {
 				if right_sym.kind in [.array, .string] && !unwrapped_val_type.has_flag(.shared_f) {
@@ -1946,6 +1987,11 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				g.write(' })')
 				g.is_arraymap_set = g.cur_indexexpr.len > 0
 			}
+			if decl_tmp_var != '' {
+				g.writeln(';')
+				g.write2(decl_stmt_str, ' ')
+				g.write(decl_tmp_var)
+			}
 			g.is_shared = false
 		}
 		g.right_is_opt = false
diff --git a/vlib/v/tests/structs/struct_field_default_shadowed_global_test.v b/vlib/v/tests/structs/struct_field_default_shadowed_global_test.v
new file mode 100644
index 000000000..59d36d78a
--- /dev/null
+++ b/vlib/v/tests/structs/struct_field_default_shadowed_global_test.v
@@ -0,0 +1,38 @@
+@[has_globals]
+module main
+
+struct Game {
+mut:
+	object_id u32
+}
+
+__global (
+	g Game
+)
+
+struct StackGameObject {
+mut:
+	id u32 = g.object_id++
+}
+
+@[heap]
+struct HeapGameObject {
+mut:
+	id u32 = g.object_id++
+}
+
+fn reset_game_state() {
+	g = Game{}
+}
+
+fn test_stack_struct_field_default_can_use_shadowed_global() {
+	reset_game_state()
+	mut g := StackGameObject{}
+	assert g.id == 0
+}
+
+fn test_heap_struct_field_default_can_use_shadowed_global() {
+	reset_game_state()
+	mut g := HeapGameObject{}
+	assert g.id == 0
+}
-- 
2.39.5