[C++ Patch/RFC] PR 60389

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

this regression is just an ICE on invalid, but I'm finding it somewhat 
tricky. Let's see if I can explain clearly enough what I figured out so far.

The problem manifests itself when is_valid_constexpr_fn is called by 
explain_invalid_constexpr_fn with a TEMPLATE_DECL (the inherited 
template constructor) as the fun argument and FUNCTION_FIRST_USER_PARM 
cannot be used on it. This happens because explain_invalid_constexpr_fn 
(called from the second half of explain_non_literal_class) doesn't early 
return via:

/* Only diagnose defaulted functions or instantiations. */
if (!DECL_DEFAULTED_FN (fun)
&& !is_instantiation_of_constexpr (fun))
return;

because we internally represent inherited *template* constructors too as 
defaulted. Thus, the idea of handling those specially per the first 
draft I'm attaching. In fact, note that if we try to fix up 
is_valid_constexpr_fn (like in the second draft I'm attaching below), we 
get a final error message of the form

60389.C:13:12: note: ‘template<class ... T> B::B(T ...)’ is not usable 
as a constexpr function because:
using A::A;
^
60389.C:13:12: error: invalid type for parameter 1 of constexpr function 
‘template<class ... T> B::B(T ...)’

which I find quite confusing, because that parameter is a 
TYPE_PACK_EXPANSION (for which the literal_type_p check in 
is_valid_constexpr_fn returns false) but in fact a B::B() is needed... 
Note, moreover, that if the user tries to "force" the compilation by 
simply declaring the A::A(T...) constructor explicitly constexpr, the 
compilation in fact succeeds (at least, with GCC ;) and that doesn't 
appear to make much sense vs the latter error message. So, what shall we 
do? Just patch 1 (with a comment explaining the early return)? Or Patch 
2 and reconsider these issues later? Or something else?

Thanks!
Paolo.

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Attachment: patch_draft_60389
Description: Text document

Attachment: patch_draft_60389_2
Description: Text document