Re: [PATCH v3] c++: P2448 - Relaxing some constexpr restrictions [PR106649]

[Marek Polacek <polacek@redhat.com>]

On Fri, Nov 18, 2022 at 09:26:26PM -0500, Jason Merrill wrote:
> On 11/16/22 15:27, Jason Merrill wrote:
> > On 11/16/22 11:06, Marek Polacek wrote:
> > > On Wed, Nov 16, 2022 at 08:41:53AM -0500, Jason Merrill wrote:
> > > > On 11/15/22 19:30, Marek Polacek wrote:
> > > > > @@ -996,19 +1040,26 @@ register_constexpr_fundef (const
> > > > > constexpr_fundef &value)
> > > > >      **slot = value;
> > > > >    }
> > > > > -/* FUN is a non-constexpr function called in a context that requires a
> > > > > -   constant expression.  If it comes from a constexpr
> > > > > template, explain why
> > > > > -   the instantiation isn't constexpr.  */
> > > > > +/* FUN is a non-constexpr (or, with -Wno-invalid-constexpr,
> > > > > a constexpr
> > > > > +   function called in a context that requires a constant expression).
> > > > > +   If it comes from a constexpr template, explain why the
> > > > > instantiation
> > > > > +   isn't constexpr.  */
> > > > 
> > > > The "if it comes from a constexpr template" wording has needed
> > > > an update for
> > > > a while now.
> > > 
> > > Probably ever since r178519.  I've added "Otherwise, explain why the
> > > function
> > > cannot be used in a constexpr context."  Is that acceptable?
> > > > > --- /dev/null
> > > > > +++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit15.C
> > > > > @@ -0,0 +1,43 @@
> > > > > +// PR c++/106649
> > > > > +// P2448 - Relaxing some constexpr restrictions
> > > > > +// { dg-do compile { target c++23 } }
> > > > > +// { dg-options "-Winvalid-constexpr" }
> > > > > +// A copy/move assignment operator for a class X that is defaulted and
> > > > > +// not defined as deleted is implicitly defined when it is odr-used,
> > > > > +// when it is needed for constant evaluation, or when it is explicitly
> > > > > +// defaulted after its first declaration.
> > > > > +// The implicitly-defined copy/move assignment operator is constexpr.
> > > > > +
> > > > > +struct S {
> > > > > +  constexpr S() {}
> > > > > +  S& operator=(const S&) = default; // #1
> > > > > +  S& operator=(S&&) = default; // #2
> > > > > +};
> > > > > +
> > > > > +struct U {
> > > > > +  constexpr U& operator=(const U&) = default;
> > > > > +  constexpr U& operator=(U&&) = default;
> > > > > +};
> > > > > +
> > > > > +/* FIXME: If we only declare #1 and #2, and default them here:
> > > > > +
> > > > > +   S& S::operator=(const S&) = default;
> > > > > +   S& S::operator=(S&&) = default;
> > > > > +
> > > > > +then they aren't constexpr.  This sounds like a bug:
> > > > > +<https://gcc.gnu.org/PR107598>.  */
> > > > 
> > > > As I commented on the PR, I don't think this is actually a bug, so let's
> > > > omit this FIXME.
> > > 
> > > I'm glad I didn't really attempt to "fix" it (the inform message is
> > > flawed
> > > and should be improved).  Thanks for taking a look.
> > > 
> > > Here's a version with the two comments updated.
> > > 
> > > Ok?
> > 
> > OK.
> 
> Since this patch I'm seeing these failures:
> 
> FAIL: g++.dg/cpp0x/constexpr-ex1.C  -std=c++23 -fimplicit-constexpr  at line
> 91 (test for errors, line 89)
> FAIL: g++.dg/cpp23/constexpr-nonlit10.C  -std=gnu++23 -fimplicit-constexpr
> (test for warnings, line 14)
> FAIL: g++.dg/cpp23/constexpr-nonlit10.C  -std=gnu++23 -fimplicit-constexpr
> (test for warnings, line 20)
> FAIL: g++.dg/cpp23/constexpr-nonlit11.C  -std=gnu++23 -fimplicit-constexpr
> (test for warnings, line 28)
> FAIL: g++.dg/cpp23/constexpr-nonlit11.C  -std=gnu++23 -fimplicit-constexpr
> (test for warnings, line 31)
> FAIL: g++.dg/cpp2a/spaceship-eq3.C  -std=c++23 -fimplicit-constexpr (test
> for excess errors)

Ah, sorry.  The following patch fixes those fails.

Ok?

-- >8 --
Some of the new tests were failing with -fimplicit-constexpr.  This
patch adjusts the expected diagnostic.  Tested with

GXX_TESTSUITE_STDS=98,11,14,17,20,23 make check-c++ RUNTESTFLAGS="--target_board=unix\{,-fimplicit-constexpr\} dg.exp=spaceship-eq3.C"

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ex1.C: Adjust dg-error.
	* g++.dg/cpp23/constexpr-nonlit10.C: Adjust dg-warning.
	* g++.dg/cpp23/constexpr-nonlit11.C: Likewise.
	* g++.dg/cpp2a/spaceship-eq3.C: Add dg-error.
---
 gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C      | 6 +++---
 gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C | 4 ++--
 gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C | 4 ++--
 gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C      | 1 +
 4 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
index 48281a47784..383d38a42d4 100644
--- a/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-ex1.C
@@ -87,8 +87,8 @@ struct resource {
   }
 };
 constexpr resource f(resource d)
-{ return d; }                  // { dg-error "non-.constexpr." "" { target { { ! implicit_constexpr } && c++20_down } } }
-// { dg-error "non-.constexpr." "" { target c++23 } .-2 }
-constexpr resource d = f(9);   // { dg-message ".constexpr." "" { target { ! implicit_constexpr } } }
+{ return d; }                  // { dg-error "non-.constexpr." "" { target { { { ! implicit_constexpr } && c++20_down } || c++11_only } } }
+// { dg-error "non-.constexpr." "" { target { c++23 && { ! implicit_constexpr } } } .-2 }
+constexpr resource d = f(9);   // { dg-message ".constexpr." "" { target { { ! implicit_constexpr } || c++11_only } } }
 
 // 4.4 floating-point constant expressions
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
index 48706f7b66e..31d4b873bbf 100644
--- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit10.C
@@ -11,13 +11,13 @@ struct NonLiteral {
 // C++23: It is possible to write a constexpr function for which no
 // invocation satisfies the requirements of a core constant expression.
 constexpr NonLiteral
-fn0 (int) // { dg-warning "invalid return type" }
+fn0 (int) // { dg-warning "invalid return type" "" { target { ! implicit_constexpr } } }
 {
   return NonLiteral{};
 }
 
 constexpr int
-fn1 (NonLiteral) // { dg-warning "invalid type" }
+fn1 (NonLiteral) // { dg-warning "invalid type" "" { target { ! implicit_constexpr } } }
 {
   return 42;
 }
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
index a7114bc66cb..e08809f873c 100644
--- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit11.C
@@ -25,10 +25,10 @@ struct X {
 struct S {
   X x;
   // Calls a non-constexpr constructor X::X(int).
-  constexpr S(int i) : x(i) { } // { dg-warning "call to" }
+  constexpr S(int i) : x(i) { } // { dg-warning "call to" "" { target { ! implicit_constexpr } } }
   S(int, int) { }
   // Target constructor isn't constexpr.
-  constexpr S() : S(42, 42) { } // { dg-warning "call to" }
+  constexpr S() : S(42, 42) { } // { dg-warning "call to" "" { target { ! implicit_constexpr } } }
 };
 
 namespace N1 {
diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
index 69eaa7b9b20..246839f1f95 100644
--- a/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq3.C
@@ -9,6 +9,7 @@ struct D
   A i;
   bool operator==(const D& x) const = default; // { dg-error "A::operator==" "" { target c++20_down } }
   bool operator!=(const D& z) const = default; // { dg-error "D::operator==" "" { target c++20_down } }
+// { dg-error "called" "" { target { c++23 && implicit_constexpr } } .-1 }
 };
 
 constexpr D d{A()};

base-commit: d19aa6af6634b1e97f38431ad091f3b3f12baf2f
-- 
2.38.1