On 3/16/23 11:48, Patrick Palka wrote:
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.
Ah, yes, that makes sense; an identity conversion can never be
narrowing, so we don't care about the constant value. So not
instantiating seems correct, and the patch is OK.
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.
Hmm, maybe check_narrowing should defer constant evaluation until after
deciding that the target type is not a superset of the source type...
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
+}
