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?
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
+}
--
2.40.0
