[Adopted at the February/March, 2017 meeting.]

According to 9.3.3 [dcl.ambig.res] paragraph 3,

Another ambiguity arises in a parameter-declaration-clause of a function declaration, or in a type-id that is the operand of a sizeof or typeid operator, when a type-name is nested in parentheses.

There are two problems here: first, a parameter-declaration-clause appears in a lambda-expression, not just in a function declaration. Second, the ambiguity can arise in a type-id appearing in any context, not just in a sizeof or typeid expression.