On Fri, Apr 05, 2024 at 09:40:48AM +0200, Jakub Jelinek wrote:
> Hi!
>
> When looking at maybe_warn_for_constant_evaluated for the trivial
> infinite loops patch, I've noticed that it can emit weird diagnostics
> for if constexpr in templates, first warn that std::is_constant_evaluted()
> always evaluates to false (because the function template is not constexpr)
> and then during instantiation warn that std::is_constant_evaluted()
> always evaluates to true (because it is used in if constexpr condition).
> Now, only the latter is actually true, even when the if constexpr
> is in a non-constexpr function, it will still always evaluate to true.
>
> So, the following patch fixes it to call maybe_warn_for_constant_evaluated
> always with IF_STMT_CONSTEXPR_P (if_stmt) as the second argument rather than
> true if it is if constexpr with non-dependent condition etc.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2024-04-05 Jakub Jelinek <ja...@redhat.com>
>
> PR c++/114580
> * semantics.cc (finish_if_stmt_cond): Call
> maybe_warn_for_constant_evaluated with IF_STMT_CONSTEXPR_P (if_stmt)
> as the second argument, rather than true/false depending on if
> it is if constexpr with non-dependent constant expression with
> bool type.
>
> * g++.dg/cpp2a/is-constant-evaluated15.C: New test.
>
> --- gcc/cp/semantics.cc.jj 2024-04-03 09:58:33.407772541 +0200
> +++ gcc/cp/semantics.cc 2024-04-04 12:11:36.203886572 +0200
> @@ -1126,6 +1126,9 @@ tree
> finish_if_stmt_cond (tree orig_cond, tree if_stmt)
> {
> tree cond = maybe_convert_cond (orig_cond);
> + maybe_warn_for_constant_evaluated (cond,
> + /*constexpr_if=*/
> + IF_STMT_CONSTEXPR_P (if_stmt));
I don't think we need the comment anymore since it's clear what the
argument does, and then the whole call can fit on a single line.
But either way, the patch looks good, thanks.
> if (IF_STMT_CONSTEXPR_P (if_stmt)
> && !type_dependent_expression_p (cond)
> && require_constant_expression (cond)
> @@ -1134,16 +1137,11 @@ finish_if_stmt_cond (tree orig_cond, tre
> converted to bool. */
> && TYPE_MAIN_VARIANT (TREE_TYPE (cond)) == boolean_type_node)
> {
> - maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/true);
> cond = instantiate_non_dependent_expr (cond);
> cond = cxx_constant_value (cond);
> }
> - else
> - {
> - maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/false);
> - if (processing_template_decl)
> - cond = orig_cond;
> - }
> + else if (processing_template_decl)
> + cond = orig_cond;
> finish_cond (&IF_COND (if_stmt), cond);
> add_stmt (if_stmt);
> THEN_CLAUSE (if_stmt) = push_stmt_list ();
> --- gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C.jj 2024-04-04
> 12:23:36.706962932 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C 2024-04-04
> 12:22:29.915882859 +0200
> @@ -0,0 +1,28 @@
> +// PR c++/114580
> +// { dg-do compile { target c++17 } }
> +// { dg-options "-Wtautological-compare" }
> +
> +namespace std {
> + constexpr inline bool
> + is_constant_evaluated () noexcept
> + {
> +#if __cpp_if_consteval >= 202106L
> + if consteval { return true; } else { return false; }
> +#else
> + return __builtin_is_constant_evaluated ();
> +#endif
> + }
> +}
> +
> +template <typename T>
> +void foo ()
> +{
> + if constexpr ((T) std::is_constant_evaluated ()) // { dg-warning
> "'std::is_constant_evaluated' always evaluates to true in 'if constexpr'" }
> + ; // { dg-bogus
> "'std::is_constant_evaluated' always evaluates to false in a non-'constexpr'
> function" }
> +}
> +
> +void
> +bar ()
> +{
> + foo <bool> ();
> +}
>
> Jakub
>
Marek
