From 6787724420b4a9dcb65becb303ee74f3d95fe35c Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 23:11:18 +0300
Subject: [PATCH] cgen: fix recursive anonymous functions and closures (fixes
 #16349)

---
 vlib/builtin/closure/closure.c.v                  | 13 +++++++++++++
 vlib/v/gen/c/assign.v                             | 15 +++++++++++++++
 vlib/v/markused/markused.v                        |  1 +
 .../tests/fns/recursive_closure_assignment_test.v | 13 +++++++++++++
 4 files changed, 42 insertions(+)
 create mode 100644 vlib/v/tests/fns/recursive_closure_assignment_test.v

diff --git a/vlib/builtin/closure/closure.c.v b/vlib/builtin/closure/closure.c.v
index b17984fcd..ceb47d330 100644
--- a/vlib/builtin/closure/closure.c.v
+++ b/vlib/builtin/closure/closure.c.v
@@ -322,3 +322,16 @@ fn closure_create(func voidptr, data voidptr) voidptr {
 	}
 	return curr_closure
 }
+
+// closure_data returns the userdata pointer associated with a closure object.
+@[direct_array_access]
+fn closure_data(closure voidptr) voidptr {
+	unsafe {
+		mut p := &voidptr(&u8(closure) - assumed_page_size)
+		$if ppc64 {
+			return p[2]
+		} $else {
+			return p[0]
+		}
+	}
+}
diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 49f9d3b99..222980e25 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -7,6 +7,20 @@ import v.ast
 import v.util
 import v.token
 
+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
+	}
+	ident := left as ast.Ident
+	if ident.name !in anon_fn.inherited_vars.map(it.name) {
+		return
+	}
+	concrete_types := g.get_anon_fn_concrete_types(anon_fn)
+	ctx_struct := g.closure_ctx(anon_fn.decl, concrete_types)
+	left_expr := g.expr_string(left)
+	g.writeln('((${ctx_struct}*)builtin__closure__closure_data(${left_expr}))->${c_name(ident.name)} = ${left_expr};')
+}
+
 fn (mut g Gen) expr_with_opt_or_block(expr ast.Expr, expr_typ ast.Type, var_expr ast.Expr, ret_typ ast.Type,
 	in_heap bool) {
 	gen_or := expr is ast.Ident && expr.or_expr.kind != .absent
@@ -538,6 +552,7 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 					}
 					g.expr(val)
 					g.writeln(';')
+					g.gen_self_recursing_anon_fn_capture_patch(left, val)
 					if blank_assign {
 						g.write('}')
 					}
diff --git a/vlib/v/markused/markused.v b/vlib/v/markused/markused.v
index 963f686db..1f7453c80 100644
--- a/vlib/v/markused/markused.v
+++ b/vlib/v/markused/markused.v
@@ -116,6 +116,7 @@ pub fn mark_used(mut table ast.Table, mut pref_ pref.Preferences, ast_files []&a
 			core_fns << 'builtin.closure.closure_alloc'
 			core_fns << 'builtin.closure.closure_init'
 			core_fns << 'builtin.closure.closure_create'
+			core_fns << 'builtin.closure.closure_data'
 		}
 		if table.used_features.arr_map {
 			include_panic_deps = true
diff --git a/vlib/v/tests/fns/recursive_closure_assignment_test.v b/vlib/v/tests/fns/recursive_closure_assignment_test.v
new file mode 100644
index 000000000..4b5f1108a
--- /dev/null
+++ b/vlib/v/tests/fns/recursive_closure_assignment_test.v
@@ -0,0 +1,13 @@
+fn test_recursive_closure_assignment() {
+	one := 1
+	mut fact := fn (n int) int {
+		return 1
+	}
+	fact = fn [one, fact] (n int) int {
+		if n <= 1 {
+			return one
+		}
+		return n * fact(n - 1)
+	}
+	assert fact(5) == 120
+}
-- 
2.39.5