Created attachment 27266 [details] A (hopefully) minimal test case g++ 4.6.2 gives an undefined reference error in the attached code. It works fine in clang++. Set the macro WORKAROUND to 1 to workaround the issue (also highlighting the fact that the template function can be instantiated).
Looks as though we need to call mark_used for function templates used as non-type template parameters
*** Bug 82657 has been marked as a duplicate of this bug. ***
(In reply to Jonathan Wakely from comment #1) > Looks as though we need to call mark_used for function templates used as > non-type template parameters Yep :)
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:733a792a2b2e1662e738fa358b45a2720a8618a7 commit r13-995-g733a792a2b2e1662e738fa358b45a2720a8618a7 Author: Patrick Palka <ppalka@redhat.com> Date: Mon Jun 6 14:29:12 2022 -0400 c++: function NTTP argument considered unused [PR53164, PR105848] Here at parse time the template argument f (an OVERLOAD) in A<f> gets resolved ahead of time to the FUNCTION_DECL f<int>, and we defer marking f<int> as used until instantiation (of g) as usual. Later when instantiating g the type A<f> (where f has already been resolved) is non-dependent, so tsubst_aggr_type avoids re-processing its template arguments, and we end up never actually marking f<int> as used (which means we never instantiate it) even though A<f>::h() later calls it, leading to a link error. This patch works around this issue by looking through ADDR_EXPR when calling mark_used on the substituted callee of a CALL_EXPR. PR c++/53164 PR c++/105848 gcc/cp/ChangeLog: * pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Look through an ADDR_EXPR callee when calling mark_used. gcc/testsuite/ChangeLog: * g++.dg/template/fn-ptr3.C: New test.
The releases/gcc-12 branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:670ef5b108d0acfbde96f44b064079f2fa0c92d4 commit r12-8604-g670ef5b108d0acfbde96f44b064079f2fa0c92d4 Author: Patrick Palka <ppalka@redhat.com> Date: Mon Jun 6 14:29:12 2022 -0400 c++: function NTTP argument considered unused [PR53164, PR105848] Here at parse time the template argument f (an OVERLOAD) in A<f> gets resolved ahead of time to the FUNCTION_DECL f<int>, and we defer marking f<int> as used until instantiation (of g) as usual. Later when instantiating g the type A<f> (where f has already been resolved) is non-dependent, so tsubst_aggr_type avoids re-processing its template arguments, and we end up never actually marking f<int> as used (which means we never instantiate it) even though A<f>::h() later calls it, leading to a link error. This patch works around this issue by looking through ADDR_EXPR when calling mark_used on the substituted callee of a CALL_EXPR. PR c++/53164 PR c++/105848 gcc/cp/ChangeLog: * pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Look through an ADDR_EXPR callee when calling mark_used. gcc/testsuite/ChangeLog: * g++.dg/template/fn-ptr3.C: New test. (cherry picked from commit 733a792a2b2e1662e738fa358b45a2720a8618a7)
Fixed for GCC 12.2/13
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>: https://gcc.gnu.org/g:b5e38b1c166357e2a63d38ae6da7ae5d68fc115b commit r13-6970-gb5e38b1c166357e2a63d38ae6da7ae5d68fc115b Author: Patrick Palka <ppalka@redhat.com> Date: Sat Apr 1 10:19:08 2023 -0400 c++: improve "NTTP argument considered unused" fix [PR53164, PR105848] r13-995-g733a792a2b2e16 worked around the problem of (pointer to) function NTTP arguments not always getting marked as odr-used, by redundantly calling mark_used on the substituted ADDR_EXPR callee of a CALL_EXPR. That is just a narrow workaround however, since it assumes the function is later called, but the use as a template argument alone should constitute an odr-use of the function (since template arguments are an evaluated context, and we're really passing its address); we shouldn't need to subsequently call or otherwise use the function NTTP argument. This patch fixes this in a more general way by walking the template arguments of each specialization that's about to be instantiated and redundantly calling mark_used on all entities used within. As before, the call to mark_used as it worst a no-op, but it compensates for the situation where the specialization was first formed in a template context in which mark_used is inhibited. Another approach would be to call mark_used whenever we substitute a TEMPLATE_PARM_INDEX, but that would result in many more redundant calls to mark_used compared to this approach. And as the second testcase below illustrates, we also need to walk C++20 class NTTP arguments which can be large and thus expensive to walk repeatedly. The change to invalid_tparm_referent_p is needed to avoid incorrectly rejecting class NTTP arguments containing function pointers as in the testcase. (The third testcase is unrelated to this fix, but it helped rule out an earlier approach I was considering and it seems we don't have existing test coverage for this situation.) PR c++/53164 PR c++/105848 gcc/cp/ChangeLog: * pt.cc (invalid_tparm_referent_p): Accept ADDR_EXPR of FUNCTION_DECL. (instantiate_class_template): Call mark_template_arguments_used. (tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995 change. (mark_template_arguments_used): Define. (instantiate_body): Call mark_template_arguments_used. gcc/testsuite/ChangeLog: * g++.dg/template/fn-ptr3a.C: New test. * g++.dg/template/fn-ptr3b.C: New test. * g++.dg/template/fn-ptr4.C: New test.