cgen: fix interface casting in anon fn

vlib/v/gen/c/cgen.v

@@ -1157,6 +1157,7 @@ pub fn (mut g Gen) write_typeof_functions() {
 				g.writeln('\tif (sidx == _${sym.cname}_${sub_sym.cname}_index) return "${util.strip_main_name(sub_sym.name)}";')
 			}
 			g.writeln2('\treturn "unknown ${util.strip_main_name(sym.name)}";', '}')
+			g.definitions.writeln('int v_typeof_interface_idx_${sym.cname}(int sidx);')
 			g.writeln2('', 'int v_typeof_interface_idx_${sym.cname}(int sidx) {')
 			if g.pref.parallel_cc {
 				g.extern_out.writeln('extern int v_typeof_interface_idx_${sym.cname}(int sidx);')

vlib/v/tests/interfaces/interface_as_cast_in_anon_fn_test.v

@@ -0,0 +1,26 @@
+@[heap]
+struct Foo {
+mut:
+	val int
+}
+
+@[heap]
+struct Bar {
+mut:
+	val int
+}
+
+interface FooBar {
+mut:
+	val int
+}
+
+fn test_interface_as_cast_in_anon_fn() {
+	mut fbs := []&FooBar{}
+	fbs << &Foo{1}
+	do_something := fn [mut fbs] () {
+		_ := fbs.last() as Foo // this line works outside of anon fn
+	}
+	do_something()
+	assert true
+}