Re: C++ PATCH for c++/78244 - narrowing conversion in template not detected, part 2

[Jason Merrill <jason@redhat.com>]

On 1/24/19 7:17 PM, Marek Polacek wrote:
> On Wed, Jan 23, 2019 at 03:34:04PM -0500, Jason Merrill wrote:
>> On Wed, Jan 23, 2019 at 12:57 PM Marek Polacek <polacek@redhat.com> wrote:
>>>
>>> On Wed, Jan 23, 2019 at 09:00:36AM -0500, Jason Merrill wrote:
>>>> I was talking about digest_init, not reshape_init.  digest_init calls
>>>> convert_for_initialization.
>>>
>>> /facepalm
>>>
>>> So yes, digest_init calls convert_for_initialization which will end up
>>> calling perform_implicit_conversion_flags which could call convert_like_real
>>> where the narrowing warnings are given, but it doesn't, we go to this case:
>>>
>>>    else if (processing_template_decl && conv->kind != ck_identity)
>>>      {
>>>        /* In a template, we are only concerned about determining the
>>>           type of non-dependent expressions, so we do not have to
>>>           perform the actual conversion.  But for initializers, we
>>>           need to be able to perform it at instantiation
>>>           (or instantiate_non_dependent_expr) time.  */
>>>        expr = build1 (IMPLICIT_CONV_EXPR, type, expr);
>>>
>>> finish_decltype_type throws away the expression because it's not dependent, and
>>> only uses its type.  So narrowing remains undetected.  Not sure if I should mess
>>> with perform_implicit_conversion_flags.
>>
>> Let's try that; this is a situation where the comment is incorrect.
>> Perhaps just call check_narrowing here if appropriate, rather than go
>> through the whole conversion machinery.
> 
> I have not been successful.
> 
> First, I modified perform_implicit_conversion_flags to go the convert_like
> route when dealing with something non-dependent.  That breaks e.g. in
> build_value_init:
>   346   /* The AGGR_INIT_EXPR tweaking below breaks in templates.  */
>   347   gcc_assert (!processing_template_decl
>   348               || (SCALAR_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE));
> Even if I restrict the convert_like way for non-dependent exprs in a template
> for scalars, it still breaks elsewhere, e.g. constexpr-template3.C where it
> complains about taking the address of an rvalue.
> 
> Second, I added check_narrowing to the processing_template_decl case in
> perform_implicit_conversion_flags.  That works except it breaks
> constexpr-inst1.C -- we no longer get the error.  That's because currently
> check_narrowing in finish_compound_literal calls maybe_constant_init, which
> calls instantiate_constexpr_fns and we get the desired diagnostic.  But if
> I move check_narrowing to perform_implicit_conversion_flags, we no longer
> call it in this case -- processing_template_decl is 0 so we call convert_like
> but that doesn't do the trick.
> 
> So, back to the patch that leaves check_narrowing in finish_compound_literal?

That patch still needs a test for the aggregate case.

Jason