[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 Andrew Pinski changed: What|Removed |Added Blocks||55004 Resolution|--- |FIXED Status|NEW |RESOLVED Target Milestone|--- |11.0 --- Comment #8 from Andrew Pinski --- Fixed so closing. Referenced Bugs: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55004 [Bug 55004] [meta-bug] constexpr issues
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 --- Comment #7 from CVS Commits --- The master branch has been updated by Jakub Jelinek : https://gcc.gnu.org/g:5f9669d9e23a1116e040c80e0f3d4f43639bda52 commit r11-2473-g5f9669d9e23a1116e040c80e0f3d4f43639bda52 Author: Jakub Jelinek Date: Fri Jul 31 23:08:00 2020 +0200 c++: Use error_at rather than warning_at for missing return in constexpr functions [PR96182] For C++11 we already emit an error if a constexpr function doesn't contain a return statement, because in C++11 that is the only thing it needs to contain, but for C++14 we would normally issue a -Wreturn-type warning. As mentioned by Jonathan, such constexpr functions are invalid, no diagnostics required, because there doesn't exist any arguments for which it would result in valid constant expression. This raises it to an error in such cases. The !LAMBDA_TYPE_P case is to avoid error on g++.dg/pr81194.C where the user didn't write constexpr anywhere and the operator() is compiler generated. 2020-07-31 Jakub Jelinek PR c++/96182 * decl.c (finish_function): In constexpr functions use for C++14 and later error instead of warning if no return statement is present and diagnose it regardless of warn_return_type. Move the warn_return_type diagnostics earlier in the function. * g++.dg/cpp1y/constexpr-96182.C: New test. * g++.dg/other/error35.C (S::g()): Add return statement. * g++.dg/cpp1y/pr63996.C (foo): Likewise. * g++.dg/cpp1y/constexpr-return2.C (f): Likewise. * g++.dg/cpp1y/var-templ44.C (make_array): Add throw 1.
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 --- Comment #6 from Jakub Jelinek --- So we could do something like: --- gcc/cp/decl.c.jj2020-07-09 11:27:51.088908783 +0200 +++ gcc/cp/decl.c 2020-07-13 14:34:59.887259561 +0200 @@ -17164,7 +17164,9 @@ finish_function (bool inline_p) BLOCK_SUPERCONTEXT (DECL_INITIAL (fndecl)) = fndecl; /* Complain if there's just no return statement. */ - if (warn_return_type + if ((warn_return_type + || (cxx_dialect >= cxx14 + && DECL_DECLARED_CONSTEXPR_P (fndecl))) && !VOID_TYPE_P (TREE_TYPE (fntype)) && !dependent_type_p (TREE_TYPE (fntype)) && !current_function_returns_value && !current_function_returns_null @@ -17196,8 +17198,12 @@ finish_function (bool inline_p) global_dc->option_state)) add_return_star_this_fixit (&richloc, fndecl); } - if (warning_at (&richloc, OPT_Wreturn_type, - "no return statement in function returning non-void")) + if (cxx_dialect >= cxx14 && DECL_DECLARED_CONSTEXPR_P (fndecl)) + error_at (&richloc, "no return statement in % function " + "returning non-void"); + else if (warning_at (&richloc, OPT_Wreturn_type, + "no return statement in function returning " + "non-void")) TREE_NO_WARNING (fndecl) = 1; } and look for what breaks.
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 Jonathan Wakely changed: What|Removed |Added Status|UNCONFIRMED |NEW Ever confirmed|0 |1 Keywords|accepts-invalid |diagnostic Last reconfirmed||2020-07-13 Severity|normal |enhancement --- Comment #5 from Jonathan Wakely --- [decl.constexpr] p6 in the C++ standard says: "if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression [...] the program is ill-formed, no diagnostic required." So the foo() function is ill-formed, but implementations are not required to diagnose it unless the function is actually evaluated. So this is not accepts-invalid, but I'll confirm it as a request for a diagnostic enhancement.
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 --- Comment #4 from Jakub Jelinek --- Or another possible wording would be that a constexpr function which is not a valid constant expression for all possible parameter values is invalid. I believe such wording is there for templates and instead of parameter values all template arguments.
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182
Jakub Jelinek changed:
What|Removed |Added
CC||jason at gcc dot gnu.org
--- Comment #3 from Jakub Jelinek ---
E.g.
constexpr int foo (int x) { if (x == 5) return 3; } constexpr int a = foo (5);
is accepted just with warning by both compilers and I think that is right, it
is only an error if one does constexpr int b = foo (4); or similar.
But for some reason clang treats your case differently (when there are no
return statements at all), but only in constexpr functions. Not sure how is it
backed up, unless the standard says that a constexpr function must have a
return statement or something similar, not sure if that is ok. It is true that
in these cases all invocations of such function in constant expression contexts
will result in an error, but if you don't invoke them...
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 --- Comment #2 from Haoxin Tu --- (In reply to Jakub Jelinek from comment #1) > The difference is that in C++11 the standard requires that the body of a > constexpr function is return expression, that is not the case of C++14 > anymore. > And, you'd get an error if you tried constexpr int a = foo (); i.e. when it > is evaluated in constant expression, but when it is only evaluated e.g. in > int b = foo (); it is a problem only at runtime. Thank you, Jakub. At runtime this must be a error. But I guess should it be rejected at compile time? Maybe reject this in compile time will help users to fix this issue early, like other mainstream compilers do. Just for a suggestion, please understand if anything I stated is unsuitable. Thanks.
[Bug c++/96182] GCC accepts constexpr function with no return-statement
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96182 Jakub Jelinek changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #1 from Jakub Jelinek --- The difference is that in C++11 the standard requires that the body of a constexpr function is return expression, that is not the case of C++14 anymore. And, you'd get an error if you tried constexpr int a = foo (); i.e. when it is evaluated in constant expression, but when it is only evaluated e.g. in int b = foo (); it is a problem only at runtime.
