Re: [PATCH] c++: noexcept and copy elision [PR109030]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 16 Mar 2023, Jason Merrill wrote:

> On 3/16/23 10:09, Patrick Palka wrote:
> > On Wed, 15 Mar 2023, Patrick Palka wrote:
> > 
> > > On Thu, 9 Mar 2023, Jason Merrill wrote:
> > > 
> > > > On 3/9/23 14:32, Patrick Palka wrote:
> > > > > On Mon, 6 Mar 2023, Marek Polacek via Gcc-patches wrote:
> > > > > 
> > > > > > When processing a noexcept, constructors aren't elided:
> > > > > > build_over_call
> > > > > > has
> > > > > > 	 /* It's unsafe to elide the constructor when handling
> > > > > > 	    a noexcept-expression, it may evaluate to the wrong
> > > > > > 	    value (c++/53025).  */
> > > > > > 	 && (force_elide || cp_noexcept_operand == 0))
> > > > > > so the assert I added recently needs to be relaxed a little bit.
> > > > > > 
> > > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > > > > 
> > > > > > 	PR c++/109030
> > > > > > 
> > > > > > gcc/cp/ChangeLog:
> > > > > > 
> > > > > > 	* constexpr.cc (cxx_eval_call_expression): Relax assert.
> > > > > > 
> > > > > > gcc/testsuite/ChangeLog:
> > > > > > 
> > > > > > 	* g++.dg/cpp0x/noexcept77.C: New test.
> > > > > > ---
> > > > > >    gcc/cp/constexpr.cc                     | 6 +++++-
> > > > > >    gcc/testsuite/g++.dg/cpp0x/noexcept77.C | 9 +++++++++
> > > > > >    2 files changed, 14 insertions(+), 1 deletion(-)
> > > > > >    create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept77.C
> > > > > > 
> > > > > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > > > > > index 364695b762c..5384d0e8e46 100644
> > > > > > --- a/gcc/cp/constexpr.cc
> > > > > > +++ b/gcc/cp/constexpr.cc
> > > > > > @@ -2869,7 +2869,11 @@ cxx_eval_call_expression (const constexpr_ctx
> > > > > > *ctx,
> > > > > > tree t,
> > > > > >        /* We used to shortcut trivial constructor/op= here, but
> > > > > > nowadays
> > > > > >         we can only get a trivial function here with
> > > > > > -fno-elide-constructors.  */
> > > > > > -  gcc_checking_assert (!trivial_fn_p (fun) ||
> > > > > > !flag_elide_constructors);
> > > > > > +  gcc_checking_assert (!trivial_fn_p (fun)
> > > > > > +		       || !flag_elide_constructors
> > > > > > +		       /* We don't elide constructors when processing
> > > > > > +			  a noexcept-expression.  */
> > > > > > +		       || cp_noexcept_operand);
> > > > > 
> > > > > It seems weird that we're performing constant evaluation within an
> > > > > unevaluated operand.  Would it make sense to also fix this a second
> > > > > way
> > > > > by avoiding constant evaluation from maybe_constant_init when
> > > > > cp_unevaluated_operand && !manifestly_const_eval, like in
> > > > > maybe_constant_value?
> > > > 
> > > > Sounds good.
> > > 
> > > Hmm, while working on this I noticed we currently don't reject a version
> > > of
> > > g++.dg/cpp2a/constexpr-inst1.C that list initializes an aggregate instead
> > > of
> > > int (ever since r12-4425-g1595fe44e11a96):
> > > 
> > >    struct A { int m; };
> > >    template<typename T> constexpr int f() { return T::value; }
> > >    template<bool B, typename T> void h(decltype(A{B ? f<T>() : 0})); //
> > > was int{...}
> > >    template<bool B, typename T> void h(...);
> > >    void x() {
> > >      h<false, int>(0); // OK?
> > >    }
> > > 
> > > ISTM we should instantiate f<int> here for the same reason we do in the
> > > original version of the testcase, and for that to happen we need to
> > > pass manifestly_const_eval=true in massage_init_elt.  Does that seem
> > > reasonable?
> > > 
> > 
> > FWIW the reason this came up is because I tried contriving a testcase
> > for the aforementioned maybe_constant_init change, and I came up with:
> > 
> >    struct __as_receiver {
> >      int empty_env;
> >    };
> > 
> >    template<class T>
> >    constexpr int f(T t) {
> >      return t.fail;
> >    };
> > 
> >    using type = decltype(__as_receiver{f(0)}); // OK, f<int> no longer
> > instantiated
> > 
> > which we used to reject and afterwards accept.  But since the elements
> > of an initializer list are potentially constant evaluated, I wonder if
> > that that means f<int> should be instantiated here after all despite the
> > unevaluated context?
> 
> The relevant section of the standard would seem to be
> https://eel.is/c++draft/expr.const#20 ; an immediate subexpression of a
> braced-init-list is potentially constant-evaluated even though it isn't
> potentially-evaluated or manifestly constant-evaluated.
> 
> It seems like the call to fold_non_dependent_expr in check_narrowing ought to
> cause instantiation in this case, why doesn't it?

Looks like check_narrowing isn't called at all in this aggr init case.
The call from e.g. convert_like_internal isn't reached because the
conversion for the initializer element is ck_identity, and don't ever
set conversion::check_narrowing for ck_identity conversions I think.

Yet for using 'type = decltype(int{f(0)});' (similar to the example in
[temp.inst]/8) we do call check_narrowing directly from
finish_compound_literal, despite the conversion effectively being an
identity conversion.

> 
> > Here's the full patch for reference:
> > 
> > -- >8 --
> > 
> > Subject: [PATCH] c++: maybe_constant_init and unevaluated operands
> > [PR109030]
> > 
> > This testcase in this PR (already fixed by r13-6526-ge4692319fd5fc7)
> > illustrates that maybe_constant_init can be called on an unevaluated
> > operand (from massage_init_elt), so this entry point should limit
> > constant evaluation in that case, like maybe_constant_value does.
> > 
> > 	PR c++/109030
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constexpr.cc (maybe_constant_init_1): For an unevaluated
> > 	non-manifestly-constant operand, don't constant evaluate
> > 	and instead call fold_to_constant.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp0x/decltype83.C: New test.
> > ---
> >   gcc/cp/constexpr.cc                     |  2 ++
> >   gcc/testsuite/g++.dg/cpp0x/decltype83.C | 14 ++++++++++++++
> >   2 files changed, 16 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype83.C
> > 
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 8683c00596a..f325af375c8 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -8795,6 +8795,8 @@ maybe_constant_init_1 (tree t, tree decl, bool
> > allow_non_constant,
> >   			&& (TREE_STATIC (decl) || DECL_EXTERNAL (decl)));
> >         if (is_static)
> >   	manifestly_const_eval = true;
> > +      if (cp_unevaluated_operand && !manifestly_const_eval)
> > +	return fold_to_constant (t);
> >         t = cxx_eval_outermost_constant_expr (t, allow_non_constant,
> > !is_static,
> >   					    mce_value (manifestly_const_eval),
> >   					    false, decl);
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype83.C
> > b/gcc/testsuite/g++.dg/cpp0x/decltype83.C
> > new file mode 100644
> > index 00000000000..17005a92eb5
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/decltype83.C
> > @@ -0,0 +1,14 @@
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct __as_receiver {
> > +  int empty_env;
> > +};
> > +
> > +template<class T>
> > +constexpr int f(T t) {
> > +  return t.fail;
> > +};
> > +
> > +int main() {
> > +  using type = decltype(__as_receiver{f(0)}); // OK, f<int> not
> > instantiated
> > +}
> 
>