Make toDelegate safe for function pointers

[0xEAB]

Revamp of #8769.

While I’m not sure whether it covers all use-cases that @Bolpat’s patch had considered and that exist (also see Dennis’ unanswered question over there), it at least passes the testsuite on my machine.

As this is a @trusted hack, I’d appreciate my patch to be thoroughly reviewed.

[dlang-bot]

Thanks for your pull request, @0xEAB!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10599"

[Bolpat]

What’s that doing?
In static assert(!is(typeof(&s1.opCall) == delegate));, the local variable s1 isn’t in scope.
If you swap the lines, the static assert passes.

[0xEAB]

good catch

[0xEAB]

Apparently, it’s a copy&paste of this code:

phobos/std/functional.d

Lines 1891 to 1901 in 8c6fca9

	struct S1 { int opCall() { inc(myNum); return myNum; } }
	static assert(!is(typeof(&s1.opCall) == delegate));
	S1 s1;
	auto getvals1 = toDelegate(s1);
	assert(getvals1() == 4);
	struct S2 { static int opCall() { return 123456; } }
	static assert(!is(typeof(&S2.opCall) == delegate));
	S2 s2;
	auto getvals2 =&S2.opCall;
	assert(getvals2() == 123456);

https://github.com/dlang/phobos/blame/8c6fca9ccdb30b69b627808c698526a76f7c4248/std/functional.d#L1891-L1901