[Bug c++/126811] [14/15/16/17 Regression] c++: ICE in tsubst when a constexpr init contains an NSDMI

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126811

Drea Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=102990

--- Comment #4 from Drea Pinski <pinskia at gcc dot gnu.org> ---
the patch that fixed PR 102990 might give a hint of what is going on:
In a template, we should never create a FIX_TRUNC_EXPR (that's what
    conv_unsafe_in_template_p is for).  But in this test we are NOT in
    a template when we call digest_nsdmi_init which ends up calling
    convert_like, converting 1.0e+0 to int, so convert_to_integer_1
    gives us a FIX_TRUNC_EXPR.

    But then when we get to parsing f's parameters, we are in a template
    when processing decltype(Helpers{}), and since r268321, when the
    compound literal isn't instantiation-dependent and the type isn't
    type-dependent, finish_compound_literal falls back to the normal
    processing, so it calls digest_init, which does fold_non_dependent_init
    and since the FIX_TRUNC_EXPR isn't dependent, we instantiate and
    therefore crash in tsubst_copy_and_build.

    The fateful call to fold_non_dependent_init comes from massage_init_elt,
    We shouldn't be calling f_n_d_i on the result of get_nsdmi.  This we can
    avoid by eschewing calling f_n_d_i on CONSTRUCTORs; their elements have
    already been folded.