Re: [C++ Patch] [PR c++/88146] do not crash synthesizing inherited ctor(...)

[Jason Merrill <jason@redhat.com>]

On 12/19/18 7:04 PM, Alexandre Oliva wrote:
> Christophe,
> 
> Thanks again for the report.  This was quite an adventure to figure
> out ;-)  See below.
> 
> 
> [PR88146] avoid diagnostics diffs if cdtor_returns_this
> 
> Diagnostics for testsuite/g++.dg/cpp0x/inh-ctor32.C varied across
> platforms.  Specifically, on ARM, the diagnostics within the subtest
> derived_ctor::inherited_derived_ctor::constexpr_noninherited_ctor did
> not match those displayed on other platforms, and the test failed.
> 
> The difference seemed to have to do with locations assigned to ctors,
> but it was more subtle: on ARM, the instantiation of bor's template
> ctor was nested within the instantiation of bar's template ctor
> inherited from bor.  The reason turned out to be related with the
> internal return type of ctors: arm_cxx_cdtor_returns_this is enabled
> for because of AAPCS, while cxx.cdtor_returns_this is disabled on most
> other platforms.  While convert_to_void returns early with a VOID
> expr, the non-VOID return type of the base ctor CALL_EXPR causes
> convert_to_void to inspect the called decl for nodiscard attributes:
> maybe_warn_nodiscard -> cp_get_fndecl_from_callee ->
> maybe_constant_init -> cxx_eval_outermost_constant_expr ->
> instantiate_constexpr_fns -> nested instantiation.

I think the bug is in calling instantiate_constexpr_fns in this case.  I 
think that should only happen when ctx->manifestly_const_eval.

Jason