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  <[email protected]>

	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

@@ -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 ();

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> ();
+}