Originally discovered by cigien on StackOverflow. See https://stackoverflow.com/questions/64655958/can-a-class-containing-a-function-pointer-be-used-as-a-non-type-template-parameter GCC does not like this program: ```` struct S { void (*f)(); }; constexpr S s {[]{}}; template<S> struct X {}; using x = X<s>; ```` It complains with: ```` error: '<lambda()>::_FUN' is not a valid template argument of type 'void (*)()' because '<lambda()>::_FUN' is not a variable using x = X<s>; ^ ```` As mentioned in the StackOverflow post, it appears that `invalid_tparm_referent_p` (called by `get_template_parm_object`) fails to account for the pointer-to-function case.
The lambda is not needed to reproduce the bug. This also triggers the error: void fun() {} struct S { void (*f)(); }; template<S> struct X {}; using x = X<S{ fun }>;
Confirmed.
This seems to be a duplicate of Bug 83258.
*** Bug 109168 has been marked as a duplicate of this bug. ***
One part of this seems to be resolved with version 13. Below, only the lambda use case fails to compile, whereas prior to version 13, both type aliases are rejected: ``` void fun() {} struct S { void (*f)(); }; template<S> struct X {}; using works = X<S{ fun }>; using does_not_work = X<S{ []{}}>; ``` See https://godbolt.org/z/3E7WE6zKo
Partially fixed by r13-6970-gb5e38b1c166357
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:c3afdb8ba8f1839544c414f57e41a58c8fda5349 commit r14-708-gc3afdb8ba8f1839544c414f57e41a58c8fda5349 Author: Patrick Palka <ppalka@redhat.com> Date: Thu May 11 10:04:25 2023 -0400 c++: converted lambda as template argument [PR83258, ...] r8-1253-g3d2e25a240c711 removed the template argument linkage requirement in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later factored out into invalid_tparm_referent_p), but we need to also remove the one in convert_nontype_argument_function for benefit of the first and third testcase which we currently reject even in C++17/20 mode. And in invalid_tparm_referent_p we're inadvertendly returning false for the address of a lambda's static op() since it's DECL_ARTIFICIAL, which currently causes us to reject the second (C++20) testcase. But this DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact this code path was originally reachable only for VAR_DECL until recently (r13-6970-gb5e38b1c166357). So this patch restricts the check to VAR_DECL. Co-authored-by: Jonathan Wakely <jwakely@redhat.com> PR c++/83258 PR c++/80488 PR c++/97700 gcc/cp/ChangeLog: * pt.cc (convert_nontype_argument_function): Remove linkage requirement for C++17 and later. (invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict DECL_ARTIFICIAL rejection test to VAR_DECL. gcc/testsuite/ChangeLog: * g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage" error for the template argument &B2:fn in C++17 mode. * g++.dg/cpp0x/lambda/lambda-conv15.C: New test. * g++.dg/cpp2a/nontype-class56.C: New test. * g++.dg/template/function2.C: New test.
Fixed on trunk so far.
The releases/gcc-13 branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:42f9b481be3527b336b800128247d053fd18d121 commit r13-7324-g42f9b481be3527b336b800128247d053fd18d121 Author: Patrick Palka <ppalka@redhat.com> Date: Thu May 11 10:04:25 2023 -0400 c++: converted lambda as template argument [PR83258, ...] r8-1253-g3d2e25a240c711 removed the template argument linkage requirement in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later factored out into invalid_tparm_referent_p), but we need to also remove the one in convert_nontype_argument_function for benefit of the first and third testcase which we currently reject even in C++17/20 mode. And in invalid_tparm_referent_p we're inadvertendly returning false for the address of a lambda's static op() since it's DECL_ARTIFICIAL, which currently causes us to reject the second (C++20) testcase. But this DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact this code path was originally reachable only for VAR_DECL until recently (r13-6970-gb5e38b1c166357). So this patch restricts the check to VAR_DECL. Co-authored-by: Jonathan Wakely <jwakely@redhat.com> PR c++/83258 PR c++/80488 PR c++/97700 gcc/cp/ChangeLog: * pt.cc (convert_nontype_argument_function): Remove linkage requirement for C++17 and later. (invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict DECL_ARTIFICIAL rejection test to VAR_DECL. gcc/testsuite/ChangeLog: * g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage" error for the template argument &B2:fn in C++17 mode. * g++.dg/cpp0x/lambda/lambda-conv15.C: New test. * g++.dg/cpp2a/nontype-class56.C: New test. * g++.dg/template/function2.C: New test. (cherry picked from commit c3afdb8ba8f1839544c414f57e41a58c8fda5349)
Should be fully fixed for GCC 13.2