c++: Extend std::is_constant_evaluated in if warning [PR100995]

Jakub pointed me at
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings>
which shows that our existing warning could be extended to handle more
cases.  This patch implements that.

A minor annoyance was handling macros, in libstdc++ we have

  reference operator[](size_type __pos) {
      __glibcxx_assert(__pos <= size());
      ...
  }

wherein __glibcxx_assert expands to

  if (__builtin_is_constant_evaluated() && !bool(__pos <= size())
    ...

but I'm of a mind to not warn on that.

Once consteval if makes it in, we should tweak this warning one more
time.

	PR c++/100995

gcc/cp/ChangeLog:

	* constexpr.c (maybe_constexpr_fn): New.
	* cp-tree.h (maybe_constexpr_fn): Declare.
	* semantics.c (find_std_constant_evaluated_r): New.
	(maybe_warn_for_constant_evaluated): New.
	(finish_if_stmt_cond): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/is-constant-evaluated9.C: Add dg-warning.
	* g++.dg/cpp2a/is-constant-evaluated12.C: New test.

3 files changed, 79 insertions, 14 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 297f207..01b0c42 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5121,6 +5121,16 @@ var_in_constexpr_fn (tree t)
 	  && DECL_DECLARED_CONSTEXPR_P (ctx));
 }
 
+/* True if a function might be constexpr: either a function that was
+   declared constexpr, or a C++17 lambda op().  */
+
+bool
+maybe_constexpr_fn (tree t)
+{
+  return (DECL_DECLARED_CONSTEXPR_P (t)
+	  || (cxx_dialect >= cxx17 && LAMBDA_FUNCTION_P (t)));
+}
+
 /* True if T was declared in a function that might be constexpr: either a
    function that was declared constexpr, or a C++17 lambda op().  */
 
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b1b7e61..9ac8b52 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8244,6 +8244,7 @@ extern bool reduced_constant_expression_p       (tree);
 extern bool is_instantiation_of_constexpr       (tree);
 extern bool var_in_constexpr_fn                 (tree);
 extern bool var_in_maybe_constexpr_fn           (tree);
+extern bool maybe_constexpr_fn			(tree);
 extern void explain_invalid_constexpr_fn        (tree);
 extern vec<tree> cx_error_context               (void);
 extern tree fold_sizeof_expr			(tree);
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index f506a23..384c54b 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -927,6 +927,71 @@ is_std_constant_evaluated_p (tree fn)
   return name && id_equal (name, "is_constant_evaluated");
 }
 
+/* Callback function for maybe_warn_for_constant_evaluated that looks
+   for calls to std::is_constant_evaluated in TP.  */
+
+static tree
+find_std_constant_evaluated_r (tree *tp, int *walk_subtrees, void *)
+{
+  tree t = *tp;
+
+  if (TYPE_P (t) || TREE_CONSTANT (t))
+    {
+      *walk_subtrees = false;
+      return NULL_TREE;
+    }
+
+  switch (TREE_CODE (t))
+    {
+    case CALL_EXPR:
+      if (is_std_constant_evaluated_p (t))
+	return t;
+      break;
+    case EXPR_STMT:
+      /* Don't warn in statement expressions.  */
+      *walk_subtrees = false;
+      return NULL_TREE;
+    default:
+      break;
+    }
+
+  return NULL_TREE;
+}
+
+/* In certain contexts, std::is_constant_evaluated() is always true (for
+   instance, in a consteval function or in a constexpr if), or always false
+   (e.g., in a non-constexpr non-consteval function) so give the user a clue.  */
+
+static void
+maybe_warn_for_constant_evaluated (tree cond, bool constexpr_if)
+{
+  if (!warn_tautological_compare)
+    return;
+
+  /* Suppress warning for std::is_constant_evaluated if the conditional
+     comes from a macro.  */
+  if (from_macro_expansion_at (EXPR_LOCATION (cond)))
+    return;
+
+  cond = cp_walk_tree_without_duplicates (&cond, find_std_constant_evaluated_r,
+					  NULL);
+  if (cond)
+    {
+      if (constexpr_if)
+	warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
+		    "%<std::is_constant_evaluated%> always evaluates to "
+		    "true in %<if constexpr%>");
+      else if (!maybe_constexpr_fn (current_function_decl))
+	warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
+		    "%<std::is_constant_evaluated%> always evaluates to "
+		    "false in a non-%<constexpr%> function");
+      else if (DECL_IMMEDIATE_FUNCTION_P (current_function_decl))
+	warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
+		    "%<std::is_constant_evaluated%> always evaluates to "
+		    "true in a %<consteval%> function");
+    }
+}
+
 /* Process the COND of an if-statement, which may be given by
    IF_STMT.  */
 
@@ -942,23 +1007,12 @@ finish_if_stmt_cond (tree cond, tree if_stmt)
 	 converted to bool.  */
       && TYPE_MAIN_VARIANT (TREE_TYPE (cond)) == boolean_type_node)
     {
-      /* if constexpr (std::is_constant_evaluated()) is always true,
-	 so give the user a clue.  */
-      if (warn_tautological_compare)
-	{
-	  tree t = cond;
-	  if (TREE_CODE (t) == CLEANUP_POINT_EXPR)
-	    t = TREE_OPERAND (t, 0);
-	  if (TREE_CODE (t) == CALL_EXPR
-	      && is_std_constant_evaluated_p (t))
-	    warning_at (EXPR_LOCATION (cond), OPT_Wtautological_compare,
-			"%qs always evaluates to true in %<if constexpr%>",
-			"std::is_constant_evaluated");
-	}
-
+      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 ();