On Fri, Aug 15, 2025 at 12:38:01PM +0200, Jakub Jelinek wrote:
> Hi!
>
> The following patch adds some testcases for the default argument (function
> and template) part of the paper, making sure we diagnose multiple defargs
> in the same TU and when visible in modules and DTRT when some aren't visible
> and some are visible and they are equal. Not testing when they are
> different since that is IFNDR.
>
> Tested on x86_64-linux and i686-linux, ok for trunk?
>
> 2025-08-15 Jakub Jelinek <ja...@redhat.com>
>
> PR c++/121552
> * g++.dg/parse/defarg21.C: New test.
> * g++.dg/template/defarg24.C: New test.
> * g++.dg/modules/default-arg-4_a.C: New test.
> * g++.dg/modules/default-arg-4_b.C: New test.
> * g++.dg/modules/default-arg-5_a.C: New test.
> * g++.dg/modules/default-arg-5_b.C: New test.
>
> --- gcc/testsuite/g++.dg/parse/defarg21.C.jj 2025-08-15 11:17:39.446828030
> +0200
> +++ gcc/testsuite/g++.dg/parse/defarg21.C 2025-08-15 11:17:33.679900471
> +0200
> @@ -0,0 +1,38 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-do compile }
> +
> +int f1 (int);
> +int f1 (int = 42);
> +int f2 (int);
> +int f2 (int = 42); // { dg-message "previous specification in 'int
> f2\\\(int\\\)' here" }
> +int f2 (int = 42); // { dg-error "default argument given for
> parameter 1 of 'int f2\\\(int\\\)'" }
> +int f3 (int = 42); // { dg-message "previous specification in 'int
> f3\\\(int\\\)' here" }
> +int f3 (int = 43); // { dg-error "default argument given for
> parameter 1 of 'int f3\\\(int\\\)'" }
> +namespace A
> +{
> + int f4 (int = 1); // { dg-message "previous specification in 'int
> A::f4\\\(int\\\)' here" }
> + int f5 (int = 1); // { dg-message "previous specification in 'int
> A::f5\\\(int\\\)' here" }
> +}
> +namespace A
> +{
> + int f4 (int = 1); // { dg-error "default argument given for
> parameter 1 of 'int A::f4\\\(int\\\)'" }
> + int f5 (int = 2); // { dg-error "default argument given for
> parameter 1 of 'int A::f5\\\(int\\\)'" }
> +}
> +template <int N>
> +int f6 (long = 42L);
> +template <int N>
> +int f6 (long = 42L); // { dg-error "redeclaration of 'template<int
> N> int f6\\\(long int\\\)' may not have default arguments" }
> +
> +void
> +foo ()
> +{
> + int f7 (int = 42); // { dg-message "previous specification in 'int
> f7\\\(int\\\)' here" }
> + int f7 (int = 42); // { dg-error "default argument given for
> parameter 1 of 'int f7\\\(int\\\)'" }
> + int f8 (int = 42);
> + {
> + int f8 (int = 42);
> + {
> + int f8 (int = 43);
> + }
> + }
> +}
> --- gcc/testsuite/g++.dg/template/defarg24.C.jj 2025-08-15
> 11:19:33.886390518 +0200
> +++ gcc/testsuite/g++.dg/template/defarg24.C 2025-08-15 11:31:12.396664456
> +0200
> @@ -0,0 +1,37 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-do compile { target c++11 } }
> +
> +template <int N>
> +int f1 (int);
> +template <int N = 42>
> +int f1 (int);
> +template <int N> // { dg-message "original definition
> appeared here" }
> +int f2 (int);
> +template <int N = 42>
> +int f2 (int);
> +template <int N = 42> // { dg-error "redefinition of
> default argument for 'int N'" }
> +int f2 (int);
> +template <int N = 42> // { dg-message "original
> definition appeared here" }
> +int f3 (int);
> +template <int N = 43> // { dg-error "redefinition of
> default argument for 'int N'" }
> +int f3 (int);
> +template <typename T>
> +int f4 (int);
> +template <typename T = int>
> +int f4 (int);
> +namespace A
> +{
> + template <typename T> // { dg-message "original
> definition appeared here" }
> + int f5 (int);
> + template <typename T = int>
> + int f5 (int);
> + template <typename T = int> // { dg-message "original
> definition appeared here" }
> + int f6 (int);
> +}
> +namespace A
> +{
> + template <typename T = int> // { dg-error "redefinition of
> default argument for 'class T'" }
> + int f5 (int);
> + template <typename T = long> // { dg-error "redefinition of
> default argument for 'class T'" }
> + int f6 (int);
> +}
> --- gcc/testsuite/g++.dg/modules/default-arg-4_a.C.jj 2025-08-15
> 12:16:07.521139287 +0200
> +++ gcc/testsuite/g++.dg/modules/default-arg-4_a.C 2025-08-15
> 12:31:11.313828020 +0200
> @@ -0,0 +1,19 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-additional-options "-fmodules-ts -Wno-global-module" }
> +// { dg-module-cmi M }
> +
> +module;
> +
> +int foo (int i = 42);
> +template <typename T, typename U = int>
> +int bar ();
> +template <typename T, int N = 42>
> +int baz ();
> +
> +export module M;
> +
> +export inline int
> +qux ()
> +{
> + return foo () + bar <int> () + baz <int> ();
> +}
> --- gcc/testsuite/g++.dg/modules/default-arg-4_b.C.jj 2025-08-15
> 12:16:12.027082865 +0200
> +++ gcc/testsuite/g++.dg/modules/default-arg-4_b.C 2025-08-15
> 12:29:25.186152954 +0200
> @@ -0,0 +1,36 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-do run }
> +// { dg-additional-options "-fmodules-ts" }
> +
> +import M;
> +
> +int
> +foo (int i = 42)
> +{
> + return i;
> +}
> +
> +template <typename T, typename U = int>
> +int
> +bar ()
> +{
> + return sizeof (U);
> +}
> +
> +template <typename T, int N = 42>
> +int
> +baz ()
> +{
> + return N;
> +}
> +
> +int
> +main ()
> +{
> + if (foo () + bar <int> () + baz <int> () != qux ())
> + __builtin_abort ();
> + if (foo () != foo (42)
> + || bar <int> () != bar <int, int> ()
> + || baz <int> () != baz <int, 42> ())
> + __builtin_abort ();
> +}
> --- gcc/testsuite/g++.dg/modules/default-arg-5_a.C.jj 2025-08-15
> 12:16:07.521139287 +0200
> +++ gcc/testsuite/g++.dg/modules/default-arg-5_a.C 2025-08-15
> 12:31:20.149717714 +0200
> @@ -0,0 +1,23 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-additional-options "-fmodules-ts -Wno-global-module" }
> +// { dg-module-cmi M }
> +
> +module;
> +
> +int foo (int i = 42);
> +template <typename T, typename U = int>
> +int bar ();
> +template <typename T, int N = 42>
> +int baz ();
> +
> +export module M;
> +
> +export inline int
> +qux ()
> +{
> + return foo () + bar <int> () + baz <int> ();
> +}
> +
> +export using ::foo;
> +export using ::bar;
> +export using ::baz;
> --- gcc/testsuite/g++.dg/modules/default-arg-5_b.C.jj 2025-08-15
> 12:16:12.027082865 +0200
> +++ gcc/testsuite/g++.dg/modules/default-arg-5_b.C 2025-08-15
> 12:28:39.750720402 +0200
> @@ -0,0 +1,35 @@
> +// C++20 P1766R1 - Mitigating minor modules maladies
> +// { dg-additional-options "-fmodules-ts" }
> +
> +import M;
> +
> +int
> +foo (int i = 42) // { dg-error "default argument given
> for parameter 1 of 'int foo\\\(int\\\)'" }
I don't think this (or the following in this file) should be errors; we
error currently because we don't implement textual deduplication
(PR99000), but since the default arguments are the same, from the
current wording this should be OK as far as I understand it.
In particular, the default argument definition here is in a different TU
from that of the definition in M, so [basic.def.odr] p2 doesn't apply.
We test this the other way around in modules/default-arg-3.
(Note that the wording in the C++20 WD is not the same as the wording
diff given in p1766r1, it seems to have been adjusted before C++20 was
published: https://timsong-cpp.github.io/cppwp/n4861/basic.def.odr#1)
Let me know if I've misunderstood something here.
Nathaniel
