Pre-increment on prvalues is not a requirement of WeaklyIncrementable

[ericniebler]

See [iterator.concept.random.access]/2.6 and also the exposition-only Advanceable concept in iota_view.

Proposed Resolution

Change [iterator.concept.random.access]/2.6 as follows:

-(2.6) — If (a + D(n - 1)) is valid, then (a + n) is equal to ++(a + D(n - 1)).
+(2.6) — If (a + D(n - 1)) is valid, then (a + n) is equal to
+        [](I c){ return ++c; }(a + D(n - 1))`.

Make a similar change to [range.iota.view]/4.6.

[CaseyCarter]

The lambda's a bit nasty. It might be clearer to define another iterator c in the front matter, and use ++(c = (a + D(n - 1))).