Nasty corner case behaviour

[H. S. Teoh]

Got bitten by this today:

	import std.algorithm : filter, map;
	import std.stdio;
	
	struct S {
		bool flag;
		auto method() {
			return [ 1 ].filter!(e => flag);
		}
	}
	
	void main() {
		auto s = S(true);
		writeln(s.method);
	
		auto arr = [ s ];
		writeln(arr[0].method);
	
		auto mappedArray = arr.map!(e => e.method);
		writeln(mappedArray.front);
		writeln(mappedArray);
	}

Expected output:

	[1]
	[1]
	[1]
	[[1]]

Actual output:

	[1]
	[1]
	[1]
	[[]]

Exercise for the reader: explain the problem.

Hint: it's not a bug in Phobos.

If method() is changed to the following, the problem goes away:

	auto method() {
		auto _flag = flag;
		return [ 1 ].filter!(e => _flag);
	}

It should be obvious by now that the problem is caused by the lambda
incorrectly closing over a member variable in a struct instance that
subsequently goes out of scope, while the returned closure lives on.
Unfortunately, neither -dip25 nor -dip1000 catches this leakage, even if
main() is marked @safe.


T

-- 
Тише едешь, дальше будешь.