From a88dc82d935061dd90b93334d59e2cb39899bfae Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 11 Mar 2026 14:03:47 +0300
Subject: [PATCH] checker(js): allows you to pass a numeric value to JS.Number
 without a cast (fixes #24677)

---
 vlib/v/checker/checker.v                                  | 7 +++++++
 .../checker/tests/js_number_requires_explicit_cast.js.out | 6 ++++++
 vlib/v/checker/tests/js_number_requires_explicit_cast.vv  | 8 ++++++++
 vlib/v/compiler_errors_test.v                             | 5 ++++-
 4 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/checker/tests/js_number_requires_explicit_cast.js.out
 create mode 100644 vlib/v/checker/tests/js_number_requires_explicit_cast.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 6cd4f8c88..7b3c894ef 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1505,6 +1505,13 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 	if typ_sym.name == 'JS.Any' {
 		return true
 	}
+	if inter_sym.kind == .interface && c.table.get_attrs(inter_sym).any(it.name == 'single_impl') {
+		if pos.file_idx != -1 {
+			c.error('cannot use `${styp}` as `${inter_sym.name}` without an explicit cast (e.g. `${inter_sym.name}(value)`)',
+				pos)
+		}
+		return false
+	}
 	if typ_sym.kind == .function && inter_sym.name != 'JS.Any' {
 		c.error('cannot implement interface `${inter_sym.name}` using function', pos)
 		return false
diff --git a/vlib/v/checker/tests/js_number_requires_explicit_cast.js.out b/vlib/v/checker/tests/js_number_requires_explicit_cast.js.out
new file mode 100644
index 000000000..f4d5b56e1
--- /dev/null
+++ b/vlib/v/checker/tests/js_number_requires_explicit_cast.js.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/js_number_requires_explicit_cast.vv:7:4: error: cannot use `int` as `JS.Number` without an explicit cast (e.g. `JS.Number(value)`)
+    5 | fn main() {
+    6 |     d := 1
+    7 |     x(d)
+      |       ^
+    8 | }
diff --git a/vlib/v/checker/tests/js_number_requires_explicit_cast.vv b/vlib/v/checker/tests/js_number_requires_explicit_cast.vv
new file mode 100644
index 000000000..2341695e7
--- /dev/null
+++ b/vlib/v/checker/tests/js_number_requires_explicit_cast.vv
@@ -0,0 +1,8 @@
+fn x(a JS.Number) {
+	println(a)
+}
+
+fn main() {
+	d := 1
+	x(d)
+}
diff --git a/vlib/v/compiler_errors_test.v b/vlib/v/compiler_errors_test.v
index 8016e1ddb..643322b48 100644
--- a/vlib/v/compiler_errors_test.v
+++ b/vlib/v/compiler_errors_test.v
@@ -95,8 +95,10 @@ fn test_all() {
 	run_dir := '${checker_dir}/run'
 	su_dir := 'vlib/v/tests/skip_unused'
 	no_closures_dir := 'vlib/v/tests/no_closures'
+	js_checker_tests := ['js_number_requires_explicit_cast.vv']
 
-	checker_tests := get_tests_in_dir(checker_dir, false).filter(!it.contains('with_check_option'))
+	checker_tests := get_tests_in_dir(checker_dir, false).filter(!it.contains('with_check_option')
+		&& it !in js_checker_tests)
 	parser_tests := get_tests_in_dir(parser_dir, false)
 	scanner_tests := get_tests_in_dir(scanner_dir, false)
 	global_tests := get_tests_in_dir(global_dir, false)
@@ -171,6 +173,7 @@ fn test_all() {
 	}
 	tasks.add('', parser_dir, '', '.out', parser_tests, false)
 	tasks.add('', checker_dir, '', '.out', checker_tests, false)
+	tasks.add('', checker_dir, '-b js', '.js.out', js_checker_tests, false)
 	tasks.add('', scanner_dir, '', '.out', scanner_tests, false)
 	tasks.add('', checker_dir, '-enable-globals run', '.run.out', ['globals_error.vv'],
 		false)
-- 
2.39.5