Why (and when) do I need to use parentheses after sizeof?

According to 6.5.3, there are two forms for sizeof as the following:

sizeof unary-expressionsizeof ( type-name )

Since arr in your code is a type-name, it has to be parenthesized.

That's the way the language is specified, type names must be parenthesized here.

Suppose the grammar looked like this:

sizeof unary-expressionsizeof type-name

Now, e.g. the following expression would be ambiguous:

sizeof int * + 0

It could be either sizeof(int *) + 0 or sizeof(int) * +0. This ambiguity doesn't arise for unary expressions, as an asterisk appended to an expression isn't an expression (but for some type names, appending one, is again a type name).

Something had to be specified here and requiring type-names to be parenthesized is a way to solve the ambiguity.

I think it's because you have typedef. If you remove it, it should compile.

Example from wikipedia:

/* the following code fragment illustrates the use of sizeof      * with variables and expressions (no parentheses needed), * and with type names (parentheses needed)     */char c;printf("%zu,%zu\n", sizeof c, sizeof (int));