[Bug c++/82336] [6/7/8 Regression] GCC requires but does not emit defaulted constructors in certain cases

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82336

--- Comment #3 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
When checking the default arg during parsing, we perform an implicit conversion
inside an unevaluated_operand context.  That's needed to fix 54198, where an
ill-formed instantiation occurs otherwise.  

However, here, that implicit conversion is altering arg itself, via
reshape_init, and we lose an enclosing target_expr.