In [rand.eng.mers] p4 the expression (1u<<w) - 1u is used repeatedly. If w==numeric_limits<unsigned>::digits that is ill-formed.

In the synopsis above we use { return 2w - 1; } despite that being actual C++ code, so why can't we do the same in the prose that follows it?

i.e. replace the code font expressions like a <= (1u<<w) - 1u with a ≤ 2^w - 1 to be clear that the rules of C++ unsigned integer arithmetic do not apply.