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c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

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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.

2024-04-09  Jakub Jelinek  <jakub@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.
This commit is contained in:
Jakub Jelinek 2024-04-09 09:31:42 +02:00
parent 64aa48ced0
commit cfed80b9e4
2 changed files with 31 additions and 7 deletions
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10
gcc/cp/semantics.cc
View File

@ -1126,6 +1126,7 @@ tree
finish_if_stmt_cond (tree orig_cond, tree if_stmt)
{
tree cond = maybe_convert_cond (orig_cond);
maybe_warn_for_constant_evaluated (cond, IF_STMT_CONSTEXPR_P (if_stmt));
if (IF_STMT_CONSTEXPR_P (if_stmt)
&& !type_dependent_expression_p (cond)
&& require_constant_expression (cond)
@ -1134,16 +1135,11 @@ finish_if_stmt_cond (tree orig_cond, tree if_stmt)
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 ();

28
gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C Normal file
View File

@ -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> ();
}