zig/test/behavior/bugs/4328.zig

const expectEqual = @import("std").testing.expectEqual;

const FILE = extern struct {
    dummy_field: u8,
};

extern fn printf([*c]const u8, ...) c_int;
extern fn fputs([*c]const u8, noalias [*c]FILE) c_int;
extern fn ftell([*c]FILE) c_long;
extern fn fopen([*c]const u8, [*c]const u8) [*c]FILE;

const S = extern struct {
    state: c_short,

    extern fn s_do_thing([*c]S, b: c_int) c_short;
};

test "Extern function calls in @TypeOf" {
    const Test = struct {
        fn test_fn_1(a: anytype, b: anytype) @TypeOf(printf("%d %s\n", a, b)) {
            return 0;
        }

        fn test_fn_2(a: anytype) @TypeOf((S{ .state = 0 }).s_do_thing(a)) {
            return 1;
        }

        fn doTheTest() void {
            expectEqual(c_int, @TypeOf(test_fn_1(0, 42)));
            expectEqual(c_short, @TypeOf(test_fn_2(0)));
        }
    };

    Test.doTheTest();
    comptime Test.doTheTest();
}

test "Peer resolution of extern function calls in @TypeOf" {
    const Test = struct {
        fn test_fn() @TypeOf(ftell(null), fputs(null, null)) {
            return 0;
        }

        fn doTheTest() void {
            expectEqual(c_long, @TypeOf(test_fn()));
        }
    };

    Test.doTheTest();
    comptime Test.doTheTest();
}

test "Extern function calls, dereferences and field access in @TypeOf" {
    const Test = struct {
        fn test_fn_1(a: c_long) @TypeOf(fopen("test", "r").*) {
            return .{ .dummy_field = 0 };
        }

        fn test_fn_2(a: anytype) @TypeOf(fopen("test", "r").*.dummy_field) {
            return 255;
        }

        fn doTheTest() void {
            expectEqual(FILE, @TypeOf(test_fn_1(0)));
            expectEqual(u8, @TypeOf(test_fn_2(0)));
        }
    };

    Test.doTheTest();
    comptime Test.doTheTest();
}