[gcc r12-10521] c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]

[Jakub Jelinek]

https://gcc.gnu.org/g:b3b7176d5857f116a4a42d885df70f8847e4cd2a

commit r12-10521-gb3b7176d5857f116a4a42d885df70f8847e4cd2a
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Apr 9 09:31:42 2024 +0200

    c++: Fix up maybe_warn_for_constant_evaluated calls [PR114580]
    
    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.
    
    (cherry picked from commit cfed80b9e4f562c99679739548df9369117dd791)

Diff:
---
 gcc/cp/semantics.cc                                |  4 +---
 .../g++.dg/cpp2a/is-constant-evaluated15.C         | 28 ++++++++++++++++++++++
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 0672d6c5b68..d4d1b01b91e 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -1026,6 +1026,7 @@ tree
 finish_if_stmt_cond (tree cond, tree if_stmt)
 {
   cond = maybe_convert_cond (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)
@@ -1034,12 +1035,9 @@ finish_if_stmt_cond (tree 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, NULL_TREE);
     }
-  else
-    maybe_warn_for_constant_evaluated (cond, /*constexpr_if=*/false);
   finish_cond (&IF_COND (if_stmt), cond);
   add_stmt (if_stmt);
   THEN_CLAUSE (if_stmt) = push_stmt_list ();
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C
new file mode 100644
index 00000000000..50a3cac6e07
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated15.C
@@ -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> ();
+}