From 26dbe85a3781af913639b17bc966f4a0b8209f3b Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Wed, 9 Jun 2021 15:18:39 -0400
Subject: [PATCH] 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.
---
gcc/cp/constexpr.c | 10 +++
gcc/cp/cp-tree.h | 1 +
gcc/cp/semantics.c | 82 +++++++++++++++----
.../g++.dg/cpp2a/is-constant-evaluated12.C | 79 ++++++++++++++++++
.../g++.dg/cpp2a/is-constant-evaluated9.C | 4 +-
5 files changed, 160 insertions(+), 16 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 297f2072de80..01b0c42471d9 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 b1b7e615bcc3..9ac8b527749a 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 f506a239864d..384c54bd025f 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 ();
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
new file mode 100644
index 000000000000..0de6bf74f4a8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated12.C
@@ -0,0 +1,79 @@
+// PR c++/100995
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wtautological-compare" }
+
+namespace std {
+ constexpr inline bool
+ is_constant_evaluated () noexcept
+ {
+ return __builtin_is_constant_evaluated ();
+ }
+}
+
+template <int I = []() constexpr { if (std::is_constant_evaluated()) return 1; return 0; }()>
+struct X { };
+
+template <int I = [](){ if (std::is_constant_evaluated()) return 1; return 0; }()>
+struct Y { };
+
+template <int I = [](){ if constexpr (std::is_constant_evaluated()) return 1; return 0; }()> // { dg-warning "always evaluates to true" }
+struct Z { };
+
+constexpr bool b = true;
+
+#define __glibcxx_assert(cond) \
+ if (__builtin_is_constant_evaluated() && !bool(cond)) \
+__builtin_unreachable()
+#define CHECK __builtin_is_constant_evaluated() // { dg-warning "always evaluates to false" }
+#define CHECK2 __builtin_is_constant_evaluated()
+
+int
+foo ()
+{
+ if (std::is_constant_evaluated ()) // { dg-warning "always evaluates to false" }
+ return 1;
+ __glibcxx_assert(b);
+ if (CHECK && b)
+ return 2;
+ if (CHECK2)
+ return 3;
+ return 0;
+}
+
+constexpr int
+bar ()
+{
+ if (std::is_constant_evaluated ())
+ return 1;
+ if constexpr (std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
+ return 2;
+ if constexpr (std::is_constant_evaluated () && b) // { dg-warning "always evaluates to true" }
+ return 3;
+ if constexpr (!std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
+ return 4;
+ return 0;
+}
+
+consteval int
+baz ()
+{
+ if (std::is_constant_evaluated ()) // { dg-warning "always evaluates to true" }
+ return 1;
+ return 0;
+}
+
+int
+qux ()
+{
+ if (({ static bool a = std::is_constant_evaluated (); a; }))
+ return 1;
+ if (({ bool a = std::is_constant_evaluated (); a; }))
+ return 2;
+ if (static bool a = std::is_constant_evaluated (); a)
+ return 3;
+ if (bool a = std::is_constant_evaluated (); a)
+ return 4;
+ if constexpr (constexpr bool a = std::is_constant_evaluated (); a)
+ return 5;
+ return 0;
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
index 0e96a1a37291..5e405e71cc0c 100644
--- a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
+++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated9.C
@@ -32,7 +32,7 @@ constexpr int
foo3(int i)
{
// I is not a constant expression but we short-circuit it.
- if constexpr (__builtin_is_constant_evaluated () || i)
+ if constexpr (__builtin_is_constant_evaluated () || i) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
return 42;
else
return i;
@@ -42,7 +42,7 @@ constexpr int
foo4(int i)
{
const int j = 0;
- if constexpr (j && __builtin_is_constant_evaluated ())
+ if constexpr (j && __builtin_is_constant_evaluated ()) // { dg-warning ".std::is_constant_evaluated. always evaluates to true in .if constexpr." }
return 42;
else
return i;
--
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