cgen: fix go call fn with anon fn argument (fix #10351, fix #10270) (#15446)

vlib/v/gen/c/fn.v

@@ -1826,8 +1826,15 @@ fn (mut g Gen) go_expr(node ast.GoExpr) {
 			g.type_definitions.writeln('EMPTY_STRUCT_DECLARATION;')
 		} else {
 			for i, arg in expr.args {
-				styp := g.typ(arg.typ)
-				g.type_definitions.writeln('\t$styp arg${i + 1};')
+				arg_sym := g.table.sym(arg.typ)
+				if arg_sym.info is ast.FnType {
+					sig := g.fn_var_signature(arg_sym.info.func.return_type, arg_sym.info.func.params,
+						'arg${i + 1}')
+					g.type_definitions.writeln('\t' + sig + ';')
+				} else {
+					styp := g.typ(arg.typ)
+					g.type_definitions.writeln('\t$styp arg${i + 1};')
+				}
 			}
 		}
 		if need_return_ptr {

vlib/v/tests/go_call_fn_with_anon_fn_arg_test.v

@@ -0,0 +1,14 @@
+fn start(f fn ()) string {
+	f()
+	return 'ok!!'
+}
+
+fn on_connect() {
+	println('fn ok!!')
+}
+
+fn test_go_call_fn_with_anon_fn_arg() {
+	g := go start(on_connect)
+	ret := g.wait()
+	assert ret == 'ok!!'
+}