[flang] [llvm] [flang][runtime] Handle null list-directed fields in child input (PR #155707)

[Peter Klausler via llvm-commits]

klausler wrote:

I've added documentation to `flang/docs/Extensions.md` to describe how null values in list-directed input are interpreted here (13.10.3.2p2).  It's the plain reading of "A null value has no effect on the definition status of the next effective item." and the portable interpretation across the most commonly-used compilers.

https://github.com/llvm/llvm-project/pull/155707