The hyphenation of the header causes it to appear twice in the index of library headers, with an erroneous appearance sorted out of order, as-if it were a header named type:

\libheaderref{tuple},
\libheaderrefx{type-index}{type.index.synopsis},
\libheaderrefx{type_traits}{meta.type.synop},

->

<tuple>, 516–520, 689, 699, 710, 1968, 1993, 1994
<typeindex>, 1968
<type_traits>, 516–520, 657, 1968, 1992
<typeindex>, 803
<typeinfo>, 517, 538