On 4/14/25 5:22 PM, Patrick Palka wrote:
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for trunk and later 14?
OK.
-- >8 -- When remapping existing specializations of a hidden template friend from a previous declaration to the new definition, we must remap only those specializations that match this new definition, but currently we remap all specializations since they're all contained in the DECL_TEMPLATE_INSTANTIATIONS list of the most general template of f. Concretely, in the first testcase below, we form two specializations of the friend A::f, one with arguments {{0},{bool}} and another with arguments {{1},{bool}}. Later when instantiating B, we need to remap these specializations. During the B<0> instantiation we only want to remap the first specialization, and during the B<1> instantiation only the second specialization, but currently we remap both specializations twice. tsubst_friend_function needs to determine if an existing specialization matches the shape of the new definition, which is tricky in general, e.g. the outer template parameters may not match up. Fortunately we don't have to reinvent the wheel here since is_specialization_of_friend seems to do exactly what we need. We can check this unconditionally, but I think it's only necessary when dealing with existing specializations formed from a class template scope previous declaration, hence TMPL_ARGS_HAVE_MULTIPLE_LEVELS check. PR c++/119807 PR c++/112288 gcc/cp/ChangeLog: * pt.cc (tsubst_friend_function): Skip remapping an existing specialization if it doesn't match the shape of the new friend definition. gcc/testsuite/ChangeLog: * g++.dg/template/friend86.C: New test. * g++.dg/template/friend87.C: New test. --- gcc/cp/pt.cc | 4 +++ gcc/testsuite/g++.dg/template/friend86.C | 25 ++++++++++++++ gcc/testsuite/g++.dg/template/friend87.C | 42 ++++++++++++++++++++++++ 3 files changed, 71 insertions(+) create mode 100644 gcc/testsuite/g++.dg/template/friend86.C create mode 100644 gcc/testsuite/g++.dg/template/friend87.C diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc index b7060b4c5aa6..4349b19119b7 100644 --- a/gcc/cp/pt.cc +++ b/gcc/cp/pt.cc @@ -11772,6 +11772,10 @@ tsubst_friend_function (tree decl, tree args) elt.args = DECL_TI_ARGS (spec); elt.spec = NULL_TREE;+ if (TMPL_ARGS_HAVE_MULTIPLE_LEVELS (DECL_TI_ARGS (spec))+ && !is_specialization_of_friend (spec, new_template)) + continue; + decl_specializations->remove_elt (&elt);tree& spec_args = DECL_TI_ARGS (spec);diff --git a/gcc/testsuite/g++.dg/template/friend86.C b/gcc/testsuite/g++.dg/template/friend86.C new file mode 100644 index 000000000000..9e2c1afb351c --- /dev/null +++ b/gcc/testsuite/g++.dg/template/friend86.C @@ -0,0 +1,25 @@ +// PR c++/119807 +// { dg-do run } + +template<int N> +struct A { + template<class T> friend int f(A<N>, T); +}; + +template struct A<0>; +template struct A<1>; + +int main() { + A<0> x; + A<1> y; + if (f(x, true) != 0) __builtin_abort(); + if (f(y, true) != 1) __builtin_abort(); +} + +template<int N> +struct B { + template<class T> friend int f(A<N>, T) { return N; } +}; + +template struct B<0>; +template struct B<1>; diff --git a/gcc/testsuite/g++.dg/template/friend87.C b/gcc/testsuite/g++.dg/template/friend87.C new file mode 100644 index 000000000000..94c0dfc52924 --- /dev/null +++ b/gcc/testsuite/g++.dg/template/friend87.C @@ -0,0 +1,42 @@ +// PR c++/119807 +// { dg-do compile { target c++20 } } + +using size_t = decltype(sizeof(0)); + +template<auto tag, size_t current> +struct CounterReader { + template<typename> + friend auto counterFlag(CounterReader<tag, current>) noexcept; +}; + +template<auto tag, size_t current> +struct CounterWriter { + static constexpr size_t value = current; + + template<typename> + friend auto counterFlag(CounterReader<tag, current>) noexcept {} +}; + +template<auto tag, auto unique, size_t current = 0, size_t mask = size_t(1) << (sizeof(size_t) * 8 - 1)> +[[nodiscard]] constexpr size_t counterAdvance() noexcept { + if constexpr (!mask) { + return CounterWriter<tag, current + 1>::value; + } else if constexpr (requires { counterFlag<void>(CounterReader<tag, current | mask>()); }) { + return counterAdvance<tag, unique, current | mask, (mask >> 1)>(); + } + else { + return counterAdvance<tag, unique, current, (mask >> 1)>(); + } +} + +constexpr auto defaultCounterTag = [] {}; + +template<auto tag = defaultCounterTag, auto unique = [] {}> +constexpr size_t counter() noexcept { + return counterAdvance<tag, unique>(); +} + +int main() { + static_assert(counter() == 1); + static_assert(counter() == 2); +}
