Clean up type error generation

[nikic]

This extracts most of the logic for the formatting of type errors into a single function, instead of repeating part of it all over the place.

/cc @TazeTSchnitzel

[nikic]

I've extended this a bit and deduplicated the type checking code for non-internal argument/return types.

[hikari-no-yume]

Why do we need to check if the default value is null here? Don't we always set allow_null in that case?

[hikari-no-yume]

If not, my "or null" code needs changing.

[nikic]

This handles cases like the following:

function test(Foo $bar = ABC) {}
define('ABC', null);
test(null);

Here we cannot determine at compile-time that the default value is actually null.

So yeah, at least theoretically the "or null" handling would have to be adjusted for this. However, I feel like we should instead drop this special case altogether. IIRC it was introduced as a bug "fix" sometime in 5.6 without anyone properly evaluating the consequences. For example, this means that allowsNull() in reflection lies. It also means that our LSP checks do not work correctly. Now that PHP 7.1 has an explicit way to mark that argument as nullable using ?Foo $bar = ABC, this should be dropped.

[hikari-no-yume]

Ah, right, that makes sense. I agree, dropping it would make sense.

[nikic]

Merged via ba09a52.