Bug 17661 – New isInputRange rejects valid input range

Code (compile with -dip25 -dip1000): -------- import std.range.primitives; struct C { private S* impl; } struct S { bool empty; @property C front() return { return C(&this); } void popFront() { } static assert(isInputRange!S); // FAILS } -------- Prior to git commit 82c3371d85154b7f98b1c9187b79805a50fc59dd, the assert passes. However, the new isInputRange checks introduced by this commit causes the above breakage. The problem is the new clause `typeof((R r) => r.front` in the new isInputRange. Because S.front returns a reference to `this` it carries the `return` annotation, so the compiler does not reject `front`. However, the check in isInputRange does not have this annotation, so the compiler rejects it as escaping a reference to `this`, thus causing isInputRange to fail to recognize S as an input range.

Can you try replacing the clause is(typeof((R r) => r.front)) with is(typeof((R r) => r.front)) || is(typeof((R r) return => r.front)) ? If that works you may want to create a PR with it. Thanks!

Haha, I tried doing that but ran into another bug: ------ struct C { private S* impl; } struct S { bool empty; @property C front() return { return C(&this); } void popFront() { } pragma(msg, typeof((S r) return => r.front)); } ------ Compiler output (dmd -dip25 -dip1000): ------ test.d(10): Error: returning r.front() escapes a reference to parameter r, perhaps annotate with return _error_ ------ Because of this, isInputRange still doesn't work, with your proposed change. Apparently dmd fails to pick up the `return` annotation for some reason. Which is actually rather strange, because: ------- pragma(msg, typeof((int* r) return => r)); ------- prints: ------- int* function(return int* r) pure nothrow @nogc return @safe ------- so obviously the syntax is correct, and at least some of the time dmd is able to process it correctly. I've no idea why it doesn't pick up the annotation and/or process it correctly when in the first code snippet above.

(In reply to Andrei Alexandrescu from comment #1) > is(typeof((R r) => r.front)) || is(typeof((R r) return => r.front)) I'm new to the "return" annotation, but is that right? I mean, I thought return was supposed to be an annotation on the parameter, not the function itself (which in this case is actually not a member function). I would have expected: (return R r) => r.front

hsteoh, could you please submit those as a separate bug report for dmd and create a phobos PR using the simplest workaround you can find? That PR would close this bug, and the other bug will be for the compiler. Thanks!

Well in, (R r) return => r.front, the return applies to the delegate context, but you're escaping a reference to the argument. What you want to check is `(return ref R r) => r.front`. Also rewriting your front methods to a free functions helps understanding. C front(return ref S _this) { return C(&_this); } Obviously allowing (R r) => front(r) would return a dangling reference to the value parameter. NB, if your front methods leaked a field of the range (e.g. a pointer), you'd need a `return scope` front methods, but the `(R r) => r.front` check is fine, since the parameter `r` isn't scoped, i.e. it might leak any pointer fields.

According to what Martin Nowak just said, there is no dmd bug, is that right? So we only need to fix Phobos.

Commits pushed to master at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/be9ad6a311a53e9ab8d2a8b69777b73d33b279c3 Fix issue 17661: isInputRange should work with .front that returns reference to parameter. https://github.com/dlang/phobos/commit/6b460ab71750ae4d405ec392581d781a7d4f4e2a Merge pull request #5688 from quickfur/issue17661 Issue 17661: isInputRange should accept .front that returns reference to range merged-on-behalf-of: Petar Kirov <[email protected]>

Commits pushed to stable at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/be9ad6a311a53e9ab8d2a8b69777b73d33b279c3 Fix issue 17661: isInputRange should work with .front that returns reference to parameter. https://github.com/dlang/phobos/commit/6b460ab71750ae4d405ec392581d781a7d4f4e2a Merge pull request #5688 from quickfur/issue17661

Commits pushed to dmd-cxx at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/be9ad6a311a53e9ab8d2a8b69777b73d33b279c3 Fix issue 17661: isInputRange should work with .front that returns reference to parameter. https://github.com/dlang/phobos/commit/6b460ab71750ae4d405ec392581d781a7d4f4e2a Merge pull request #5688 from quickfur/issue17661