[PATCH] Fix libstdc++/5708

[Benjamin Kosnik]

> What other implementation does it?

Dinkum, as shipped with icc.

I'd support changing 22.2.6.3 p2 to:

The value space indicates that at least one space is required
at that position. 

to

The value space indicates that at least one white space is required at
that position, determinted by the fill value.

-benjamin