From a5e1f7430eaab15973e026bd2c7592a7b40bc3ab Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 01:40:05 +0300
Subject: [PATCH] checker: fix error for same function signature passed as
 argument (fixes #13965)

---
 vlib/v/checker/check_types.v                  | 13 ++++++++++++-
 .../same_function_signature_nested_fn_test.v  | 19 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/tests/fns/same_function_signature_nested_fn_test.v

diff --git a/vlib/v/checker/check_types.v b/vlib/v/checker/check_types.v
index 24b47628a..acbcffea1 100644
--- a/vlib/v/checker/check_types.v
+++ b/vlib/v/checker/check_types.v
@@ -704,13 +704,24 @@ fn (mut c Checker) check_matching_function_symbols(got_type_sym &ast.TypeSymbol,
 			|| c.type_has_unresolved_generic_parts(exp_arg_typ) {
 			continue
 		}
-		if c.table.unaliased_type(got_arg_typ).idx() != c.table.unaliased_type(exp_arg_typ).idx() {
+		if !c.have_identical_fn_arg_type(got_arg_typ, exp_arg_typ) {
 			return false
 		}
 	}
 	return true
 }
 
+fn (mut c Checker) have_identical_fn_arg_type(got ast.Type, expected ast.Type) bool {
+	got_unaliased := c.table.unaliased_type(got)
+	expected_unaliased := c.table.unaliased_type(expected)
+	got_final_sym := c.table.final_sym(got_unaliased)
+	expected_final_sym := c.table.final_sym(expected_unaliased)
+	if got_final_sym.kind == .function && expected_final_sym.kind == .function {
+		return c.check_matching_function_symbols(got_final_sym, expected_final_sym)
+	}
+	return got_unaliased.idx() == expected_unaliased.idx()
+}
+
 fn (mut c Checker) check_shift(mut node ast.InfixExpr, left_type_ ast.Type, right_type_ ast.Type) ast.Type {
 	left_type := c.unwrap_generic(left_type_)
 	right_type := c.unwrap_generic(right_type_)
diff --git a/vlib/v/tests/fns/same_function_signature_nested_fn_test.v b/vlib/v/tests/fns/same_function_signature_nested_fn_test.v
new file mode 100644
index 000000000..558682ca4
--- /dev/null
+++ b/vlib/v/tests/fns/same_function_signature_nested_fn_test.v
@@ -0,0 +1,19 @@
+type Transform = fn (string) string
+
+fn take_transform(cb fn (Transform) string) string {
+	return cb(to_upper)
+}
+
+fn make_transform_callback() fn (fn (string) string) string {
+	return fn (cb fn (string) string) string {
+		return cb('x')
+	}
+}
+
+fn to_upper(s string) string {
+	return s.to_upper()
+}
+
+fn test_same_function_signature_nested_fn() {
+	assert take_transform(make_transform_callback()) == 'X'
+}
-- 
2.39.5