compiler: fix panic when dealing with variadic functions

vlib/compiler/fn.v

@@ -1308,6 +1308,8 @@ fn (p mut Parser) fn_call_args(f mut Fn, generic_param_types []string) {
 				p.check(.comma)
 				p.fspace()
 				p.gen(',')
+			} else if p.tok != .comma {
+				p.gen(',')
 			}
 		}
 	}
@@ -1499,9 +1501,17 @@ fn (p mut Parser) fn_call_vargs(f Fn) (string,[]string) {
 			p.error_with_token_index('variadic arg index out of range: $va.index/${values.len-1}, vargs are 0 indexed', va.tok_idx)
 		}
 	}
-	if !f.is_method && f.args.len > 1 {
+
+	if types.len == 0 {
+		return last_arg.typ,[]string
+	}
+
+	insert_comma_after_arg := if f.is_method { 2 } else { 1 }
+
+	if f.args.len > insert_comma_after_arg {
 		p.cgen.gen(',')
 	}
+
 	return types[0],values
 }
 
@@ -1509,7 +1519,12 @@ fn (p mut Parser) fn_gen_caller_vargs(f &Fn, varg_type string, values []string)
 	is_varg := varg_type.starts_with('varg_')
 	if is_varg {
 		// forwarding varg
-		p.cgen.gen('${values[0]}')
+		if values.len == 0 {
+			vargs_struct := p.register_vargs_stuct(varg_type, 1)
+			p.cgen.gen('&($vargs_struct){.len=0}')
+		} else {
+			p.cgen.gen('${values[0]}')
+		}
 	}
 	else {
 		vargs_struct := p.register_vargs_stuct(varg_type, values.len)

vlib/compiler/tests/fn_variadic_test.v

@@ -41,3 +41,36 @@ fn variadic_forward_b(a ...string) string {
 fn test_fn_variadic_forward() {
 	assert variadic_forward_a('a', 'b', 'c') == 'abc'
 }
+
+fn variadic_test_no_args(name string, groups ...VaTestGroup) {
+	assert groups.len == 0
+}
+
+fn test_fn_variadic_no_args() {
+	variadic_test_no_args('marko')
+}
+
+struct VaTestStruct {
+}
+
+fn (a VaTestStruct) variadic_method(name string, groups ...VaTestGroup) {
+	assert groups.len == 2
+	assert groups[0].name == 'users'
+	assert groups[1].name == 'admins'
+}
+
+fn (a VaTestStruct) variadic_method_no_args(name string, groups ...VaTestGroup) {
+	assert groups.len == 0
+}
+
+fn test_fn_variadic_method() {
+	a := VaTestStruct{}
+	group1 := VaTestGroup{name: 'users'}
+	group2 := VaTestGroup{name: 'admins'}
+	a.variadic_method('marko', group1, group2)
+}
+
+fn test_fn_variadic_method_no_args() {
+	a := VaTestStruct{}
+	a.variadic_method_no_args('marko')
+}