Per the ongoing work in #1502, basic_string cannot be made to work in constant expressions without additional changes to the character traits class. Specifically, we've had to modify char_traits::assign(s,n,c) (for all library-defined specializations of char_traits) to create new objects with value c rather than assigning that value to existing objects. (This may not be the only necessary change, we need to audit #1502 for any other fallout.)
Per the ongoing work in #1502,
basic_stringcannot be made to work in constant expressions without additional changes to the character traits class. Specifically, we've had to modifychar_traits::assign(s,n,c)(for all library-defined specializations ofchar_traits) to create new objects with valuecrather than assigning that value to existing objects. (This may not be the only necessary change, we need to audit #1502 for any other fallout.)