From: Marek Polacek <polacek@redhat.com>
To: Jason Merrill <jason@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: [PATCH v2] c++: P2280R4, Using unknown refs in constant expr [PR106650]
Date: Wed, 29 Nov 2023 13:56:32 -0500 [thread overview]
Message-ID: <ZWeJYOzSY21/ErRW@redhat.com> (raw)
In-Reply-To: <fc067ce1-240b-4b1d-b27d-86f974930e0c@redhat.com>
On Mon, Nov 20, 2023 at 04:29:33PM -0500, Jason Merrill wrote:
> On 11/17/23 16:46, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > This patch is an attempt to implement (part of?) P2280, Using unknown
> > pointers and references in constant expressions. (Note that R4 seems to
> > only allow References to unknown/Accesses via this, but not Pointers to
> > unknown.)
>
> Indeed. That seems a bit arbitrary to me, but there it is.
>
> We were rejecting the testcase before because cxx_bind_parameters_in_call
> was trying to perform an lvalue->rvalue conversion on the reference itself;
> this isn't really a thing in the language, but worked to implement the
> reference bullet that the paper removes. Your approach to fixing that makes
> sense to me.
>
> We should do the same for VAR_DECL references, e.g.
>
> extern int (&r)[42];
> constexpr int i = array_size (r);
Argh, right.
> You also need to allow (implict or explicit) use of 'this', as in:
>
> struct A
> {
> constexpr int f() { return 42; }
> void g() { constexpr int i = f(); }
> };
Ah, I thought that already worked, but not so. Apology apology.
> > This patch works to the extent that the test case added in [expr.const]
> > works as expected, as well as the test in
> > <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#the-this-pointer>
> >
> > Most importantly, the proposal makes this compile:
> >
> > template <typename T, size_t N>
> > constexpr auto array_size(T (&)[N]) -> size_t {
> > return N;
> > }
> >
> > void check(int const (¶m)[3]) {
> > constexpr auto s = array_size(param);
> > static_assert (s == 3);
> > }
> >
> > and I think it would be a pity not to have it in GCC 14.
> >
> > What still doesn't work (and I don't know if it should) is the test in $3.2:
> >
> > struct A2 { constexpr int f() { return 0; } };
> > struct B2 : virtual A2 {};
> > void f2(B2 &b) { constexpr int k = b.f(); }
> >
> > where we say
> > error: '* & b' is not a constant expression
>
> It seems like that is supposed to work, the problem is accessing the vtable
> to perform the conversion. I have WIP to recognize that conversion better
> in order to fix PR53288; this testcase can wait for that fix.
Great.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
This patch is an attempt to implement (part of?) P2280, Using unknown
pointers and references in constant expressions. (Note that R4 seems to
only allow References to unknown/Accesses via this, but not Pointers to
unknown.)
This patch works to the extent that the test case added in [expr.const]
works as expected, as well as the test in
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2280r4.html#the-this-pointer>
Most importantly, the proposal makes this compile:
template <typename T, size_t N>
constexpr auto array_size(T (&)[N]) -> size_t {
return N;
}
void check(int const (¶m)[3]) {
constexpr auto s = array_size(param);
static_assert (s == 3);
}
and I think it would be a pity not to have it in GCC 14.
What still doesn't work is the test in $3.2:
struct A2 { constexpr int f() { return 0; } };
struct B2 : virtual A2 {};
void f2(B2 &b) { constexpr int k = b.f(); }
where we say
error: '* & b' is not a constant expression
This will be fixed in the future.
PR c++/106650
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_constant_expression) <case PARM_DECL>: Allow
reference to unknown/this as per P2280.
<case VAR_DECL>: Allow reference to unknown as per P2280.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-array-ptr6.C: Remove dg-error.
* g++.dg/cpp0x/constexpr-ref12.C: Likewise.
* g++.dg/cpp0x/constexpr-ref2.C: Adjust dg-error.
* g++.dg/cpp0x/noexcept34.C: Remove dg-error.
* g++.dg/cpp1y/lambda-generic-const10.C: Likewise.
* g++.dg/cpp0x/constexpr-ref13.C: New test.
* g++.dg/cpp1z/constexpr-ref1.C: New test.
* g++.dg/cpp1z/constexpr-ref2.C: New test.
* g++.dg/cpp2a/constexpr-ref1.C: New test.
---
gcc/cp/constexpr.cc | 8 ++-
.../g++.dg/cpp0x/constexpr-array-ptr6.C | 2 +-
gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C | 4 +-
gcc/testsuite/g++.dg/cpp0x/constexpr-ref13.C | 41 ++++++++++++++
gcc/testsuite/g++.dg/cpp0x/constexpr-ref2.C | 4 +-
gcc/testsuite/g++.dg/cpp0x/noexcept34.C | 8 +--
.../g++.dg/cpp1y/lambda-generic-const10.C | 2 +-
gcc/testsuite/g++.dg/cpp1z/constexpr-ref1.C | 26 +++++++++
gcc/testsuite/g++.dg/cpp1z/constexpr-ref2.C | 23 ++++++++
gcc/testsuite/g++.dg/cpp2a/constexpr-ref1.C | 54 +++++++++++++++++++
10 files changed, 161 insertions(+), 11 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-ref13.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-ref1.C
create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-ref2.C
create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-ref1.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 344107d494b..b17e176aded 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7336,7 +7336,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
if (TREE_CODE (r) == TARGET_EXPR
&& TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
r = TARGET_EXPR_INITIAL (r);
- if (DECL_P (r))
+ if (DECL_P (r)
+ /* P2280 allows references to unknown. */
+ && !(VAR_P (t) && TYPE_REF_P (TREE_TYPE (t))))
{
if (!ctx->quiet)
non_const_var_error (loc, r, /*fundef_p*/false);
@@ -7378,6 +7380,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
r = build_constructor (TREE_TYPE (t), NULL);
TREE_CONSTANT (r) = true;
}
+ else if (TYPE_REF_P (TREE_TYPE (t)))
+ /* P2280 allows references to unknown... */;
+ else if (is_this_parameter (t))
+ /* ...as well as the this pointer. */;
else
{
if (!ctx->quiet)
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-array-ptr6.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-array-ptr6.C
index 1c065120314..d212665e51f 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-array-ptr6.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-array-ptr6.C
@@ -12,7 +12,7 @@ constexpr auto sz_d = size(array_double);
static_assert(sz_d == 3, "Array size failure");
void f(bool (¶m)[2]) {
- static_assert(size(param) == 2, "Array size failure"); // { dg-error "" }
+ static_assert(size(param) == 2, "Array size failure");
short data[] = {-1, 2, -45, 6, 88, 99, -345};
static_assert(size(data) == 7, "Array size failure");
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C
index 7c3ce66b4c9..f4500144946 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref12.C
@@ -40,7 +40,7 @@ void f(a ap, a& arp)
static_assert (g(ar2),""); // { dg-error "constant" }
static_assert (h(ar2),""); // { dg-error "constant" }
- static_assert (arp.g(),""); // { dg-error "constant" }
- static_assert (g(arp),""); // { dg-error "constant" }
+ static_assert (arp.g(),"");
+ static_assert (g(arp),"");
static_assert (h(arp),""); // { dg-error "constant" }
}
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ref13.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref13.C
new file mode 100644
index 00000000000..f26027552a0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref13.C
@@ -0,0 +1,41 @@
+// P2280R4 - Using unknown pointers and references in constant expressions
+// PR c++/106650
+// { dg-do compile { target c++11 } }
+
+using size_t = decltype(sizeof(42));
+
+template <typename T, size_t N>
+constexpr auto array_size(T (&)[N]) -> size_t {
+ return N;
+}
+
+extern int (&r)[42];
+constexpr int i = array_size (r);
+
+void check(int const (¶m)[3]) {
+ int local[] = {1, 2, 3};
+ constexpr auto s0 = array_size(local);
+ constexpr auto s1 = array_size(param);
+}
+
+template <typename T, size_t N>
+constexpr size_t array_size_ptr(T (*)[N]) {
+ return N;
+}
+
+void check_ptr(int const (*param)[3]) {
+ constexpr auto s2 = array_size_ptr(param); // { dg-error "not a constant" }
+}
+
+struct A
+{
+ constexpr int f() { return 42; }
+ void g() { constexpr int i = f(); }
+ void g2() { constexpr int i = this->f(); }
+};
+
+struct B {
+ constexpr static bool b = false;
+ void g() noexcept(b) { }
+ void g2() noexcept(this->b) { }
+};
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ref2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref2.C
index 76973638d5f..d5327c2dcb2 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ref2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ref2.C
@@ -4,8 +4,8 @@
extern int *p;
constexpr int& ri = *p; // { dg-error "p" }
-extern constexpr int &er; // { dg-error "not a definition" }
-constexpr int& ri2 = er; // { dg-error "er" }
+extern constexpr int &er; // { dg-error "not a definition|not a constant" }
+constexpr int& ri2 = er;
void f(int j)
{
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept34.C b/gcc/testsuite/g++.dg/cpp0x/noexcept34.C
index 963881b5ad6..5cb99675fd5 100644
--- a/gcc/testsuite/g++.dg/cpp0x/noexcept34.C
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept34.C
@@ -7,13 +7,13 @@ template<typename> struct A
{
constexpr int f () { return 0; }
bool b = true;
- void g () noexcept (f()) { } // { dg-error ".this. is not a constant" }
- void g2 () noexcept (this->f()) { } // { dg-error ".this. is not a constant" }
+ void g () noexcept (f()) { }
+ void g2 () noexcept (this->f()) { }
void g3 () noexcept (b) { } // { dg-error "use of .this. in a constant expression|use of parameter|.this. is not a constant" }
void g4 (int i) noexcept (i) { } // { dg-error "use of parameter" }
- void g5 () noexcept (A::f()) { } // { dg-error ".this. is not a constant" }
+ void g5 () noexcept (A::f()) { }
void g6 () noexcept (foo(b)) { } // { dg-error "use of .this. in a constant expression|use of parameter|.this. is not a constant" }
- void g7 () noexcept (int{f()}) { } // { dg-error ".this. is not a constant" }
+ void g7 () noexcept (int{f()}) { }
};
int main ()
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-const10.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-const10.C
index 2f48dae4746..47a49f58419 100644
--- a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-const10.C
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-const10.C
@@ -11,7 +11,7 @@ int main()
constexpr auto x = f(); //ok, call constexpr const non-static method
[](auto const &f) {
- constexpr auto x = f(); // { dg-error "" }
+ constexpr auto x = f();
}(f);
[&]() {
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-ref1.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-ref1.C
new file mode 100644
index 00000000000..82771814a80
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-ref1.C
@@ -0,0 +1,26 @@
+// P2280R4 - Using unknown pointers and references in constant expressions
+// PR c++/106650
+// { dg-do compile { target c++17 } }
+
+#include <type_traits>
+
+template <typename T, typename U>
+constexpr bool is_type(U &&)
+{
+ return std::is_same_v<T, std::decay_t<U>>;
+}
+
+auto visitor = [](auto&& v) {
+ if constexpr(is_type<int>(v)) {
+ // ...
+ } else if constexpr(is_type<char>(v)) {
+ // ...
+ }
+};
+
+void
+g (int i)
+{
+ visitor (i);
+ constexpr bool b = is_type<int>(i);
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-ref2.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-ref2.C
new file mode 100644
index 00000000000..ca734378141
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-ref2.C
@@ -0,0 +1,23 @@
+// P2280R4 - Using unknown pointers and references in constant expressions
+// PR c++/106650
+// { dg-do compile { target c++17 } }
+
+template <bool V>
+struct Widget {
+ struct Config {
+ static constexpr bool value = V;
+ } config;
+
+ void f() {
+ if constexpr (config.value) {
+ // ...
+ }
+ }
+};
+
+void
+g ()
+{
+ Widget<false> w;
+ w.f();
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-ref1.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-ref1.C
new file mode 100644
index 00000000000..2ea865f8d5a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-ref1.C
@@ -0,0 +1,54 @@
+// P2280R4 - Using unknown pointers and references in constant expressions
+// PR c++/106650
+// { dg-do compile { target c++20 } }
+
+#include <typeinfo>
+
+using size_t = decltype(sizeof(42));
+
+template <typename T, size_t N>
+constexpr size_t array_size(T (&)[N]) {
+ return N;
+}
+
+void use_array(int const (&gold_medal_mel)[2]) {
+ constexpr auto gold = array_size(gold_medal_mel); // OK
+}
+
+constexpr auto olympic_mile() {
+ const int ledecky = 1500;
+ return []{ return ledecky; };
+}
+static_assert(olympic_mile()() == 1500); // OK
+
+struct Swim {
+ constexpr int phelps() { return 28; }
+ virtual constexpr int lochte() { return 12; }
+ int coughlin = 12;
+};
+
+constexpr int how_many(Swim& swam) {
+ Swim* p = &swam;
+ return (p + 1 - 1)->phelps();
+}
+
+void splash(Swim& swam) {
+ static_assert(swam.phelps() == 28); // OK
+ static_assert((&swam)->phelps() == 28); // OK
+
+ Swim* pswam = &swam;
+ static_assert(pswam->phelps() == 28); // { dg-error "non-constant|not usable" }
+
+ static_assert(how_many(swam) == 28); // OK
+ static_assert(Swim().lochte() == 12); // OK
+
+ static_assert(swam.lochte() == 12); // { dg-error "non-constant|not a constant" }
+
+ static_assert(swam.coughlin == 12); // { dg-error "non-constant|not a constant" }
+}
+
+extern Swim dc;
+extern Swim& trident;
+
+constexpr auto& sandeno = typeid(dc); // OK, can only be typeid(Swim)
+constexpr auto& gallagher = typeid(trident); // { dg-error "not a constant" }
base-commit: 634cf26c94de620e66aa124b8ec4d6c2be4b74b2
--
2.42.0
next prev parent reply other threads:[~2023-11-29 18:56 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-17 21:46 [PATCH] " Marek Polacek
2023-11-20 21:29 ` Jason Merrill
2023-11-29 18:56 ` Marek Polacek [this message]
2023-11-29 20:40 ` [PATCH v2] " Jason Merrill
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