Hi! The following testcase is rejected since my recent convert_nontype_argument change, but only in C++11/14 modes. The problem is that C++17 says that all NTTPs should be converted constant expressions, but C++11 only said that for integral and enumeration ones and something that applied to the decltype(nullptr) case was - a constant expression that evaluates to a null pointer value The problem is that for NULLPTR_TYPE_P for C++11/14, we just never called maybe_constant_value, so obviously just making sure it is integer_zerop and without tf_error returning NULL_TREE otherwise changes behavior on valid programs, we really need to constant evaluate it first.
The following patch fixes that. I'm afraid I actually don't know what exactly C++14 specifies, whether I've grabbed a pre-C++14 or post-C++14 draft, which looked like the C++17 wording. So, if C++14 needs something different, we'll need a further change, but this at least fixes the regression. Bootstrapped/regtested on x86_64-linux, ok for trunk> 2026-02-21 Jakub Jelinek <[email protected]> PR c++/124173 * pt.cc (convert_nontype_argument): For C++11/C++14 handle NULLPTR_TYPE_P non-type arguments like TYPE_PTR_P. * g++.dg/cpp0x/pr124173.C: New test. --- gcc/cp/pt.cc.jj 2026-02-20 13:29:35.824439071 +0100 +++ gcc/cp/pt.cc 2026-02-20 20:13:05.074572910 +0100 @@ -7690,10 +7690,12 @@ convert_nontype_argument (tree type, tre /* EXPR may have become value-dependent. */ val_dep_p = value_dependent_expression_p (expr); } - else if (TYPE_PTR_OR_PTRMEM_P (type)) + else if (TYPE_PTR_OR_PTRMEM_P (type) + || NULLPTR_TYPE_P (type)) { tree folded = maybe_constant_value (expr, NULL_TREE, mce_true); - if (TYPE_PTR_P (type) ? integer_zerop (folded) + if ((TYPE_PTR_P (type) || NULLPTR_TYPE_P (type)) + ? integer_zerop (folded) : null_member_pointer_value_p (folded)) expr = folded; } --- gcc/testsuite/g++.dg/cpp0x/pr124173.C.jj 2026-02-20 20:29:26.948958968 +0100 +++ gcc/testsuite/g++.dg/cpp0x/pr124173.C 2026-02-20 20:28:30.846049284 +0100 @@ -0,0 +1,15 @@ +// PR c++/124173 +// { dg-do compile { target c++11 } } + +template <decltype (nullptr)> +void +foo () +{ +} + +int +main () +{ + constexpr decltype (nullptr) t = nullptr; + foo <t> (); +} Jakub
