Proposed resolution:

In Table 44 of [meta.unary.prop], third row, column "Value", change the cell content:

Table 44 -- Type property queries

Template	Value
template <class T, unsigned I = 0> struct extent;	If T is not an array type (8.3.4), or if it has rank less than or equal to I, or if I is 0 and T has type "array of unknown bound of U", then 0; otherwise, the size bound (8.3.4) of the I'th dimension of T, where indexing of I is zero-based.