https://gcc.gnu.org/g:4e13b78bf0e28120d4b1595313c42a1de83fc3ed
commit r16-7405-g4e13b78bf0e28120d4b1595313c42a1de83fc3ed Author: Jakub Jelinek <[email protected]> Date: Mon Feb 9 09:06:47 2026 +0100 c++: Fix up diagnostics of wrong constexpr bodies in C++11 in templates [PR123889] We emit weird diagnostics on the following testcase in C++11. If it is not a template, maybe_save_constexpr_fundef calls first if (!is_valid_constexpr_fn (fun, complain)) return; (which doesn't fail) and then tree massaged = massage_constexpr_body (fun, DECL_SAVED_TREE (fun)); if (massaged == NULL_TREE || massaged == error_mark_node) { if (!DECL_CONSTRUCTOR_P (fun) && complain) error ("body of %<constexpr%> function %qD not a return-statement", fun); return; } which diagnoses it and if even that would succeed, go on with bool potential = potential_rvalue_constant_expression (massaged); if (!potential && complain) require_potential_rvalue_constant_expression_fncheck (massaged); In templates, maybe_save_constexpr_fundef returns early: if (processing_template_decl || cp_function_chain->invalid_constexpr || (DECL_CLONED_FUNCTION_P (fun) && !DECL_DELETING_DESTRUCTOR_P (fun))) return; and then it is called again during instantiation. But in that case DECL_GENERATED_P (fun) is true and so we silently return on errors without diagnosing them: bool complain = !DECL_GENERATED_P (fun) && !implicit; Now, when we actually try to constexpr evaluate those (if at all), we emit an error and then 'constexpr ...' is not usable as a 'constexpr' function because: message and then explain_invalid_constexpr_fn tries to diagnose the errors by calling is_valid_constexpr_fn (fun, true) and require_potential_rvalue_constant_expression (massaged). So it diagnoses those 2 cases, but misses the one where massaged was NULL or error_mark_node for a non-constructor, so after the because: there is no reason emitted. The following patch diagnoses even that. 2026-02-09 Jakub Jelinek <[email protected]> PR c++/123889 * constexpr.cc (explain_invalid_constexpr_fn): Diagnose NULL or error_mark_node massaged on non-constructor. * g++.dg/cpp0x/constexpr-123889.C: New test. Diff: --- gcc/cp/constexpr.cc | 3 +++ gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C | 14 ++++++++++++++ 2 files changed, 17 insertions(+) diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc index a5ccd276a88f..673f4486e42d 100644 --- a/gcc/cp/constexpr.cc +++ b/gcc/cp/constexpr.cc @@ -1124,6 +1124,9 @@ explain_invalid_constexpr_fn (tree fun) /* Also check the body, not just the ctor-initializer. */ require_potential_rvalue_constant_expression (body); } + else if (massaged == NULL_TREE || massaged == error_mark_node) + error ("body of %<constexpr%> function %qD not a return-statement", + fun); } } } diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C new file mode 100644 index 000000000000..c7ca8cdf41d5 --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-123889.C @@ -0,0 +1,14 @@ +// PR c++/123889 +// { dg-do compile { target c++11 } } + +template <class T> +constexpr int func (T) { +// { dg-message "'constexpr int func\\\(T\\\) \\\[with T = int\\\]' is not usable as a 'constexpr' function because:" "" { target c++11_only } .-1 } +// { dg-error "body of 'constexpr' function 'constexpr int func\\\(T\\\) \\\[with T = int\\\]' not a return-statement" "" { target c++11_only } .-2 } + return 1; + return 2; +} + +static_assert (func (1) == 1, ""); +// { dg-error "non-constant condition for static assertion" "" { target c++11_only } .-1 } +// { dg-error "'constexpr int func\\\(T\\\) \\\[with T = int\\\]' called in a constant expression" "" { target c++11_only } .-2 }
