https://gcc.gnu.org/g:eb3e7de323a67e5a92f1c2224f3251cb47b49c0b
commit r13-9588-geb3e7de323a67e5a92f1c2224f3251cb47b49c0b
Author: Jakub Jelinek <ja...@redhat.com>
Date: Fri Feb 7 14:27:18 2025 +0100
c++: Allow constexpr reads from volatile std::nullptr_t objects [PR118661]
As mentioned in the PR, https://eel.is/c++draft/conv.lval#note-1
says that even volatile reads from std::nullptr_t typed objects actually
don't read anything and https://eel.is/c++draft/expr.const#10.9
says that even those are ok in constant expressions.
So, the following patch adjusts the r9-4793 changes to have an exception
for NULLPTR_TYPE.
As [conv.lval]/3 also talks about accessing to inactive member, I've added
testcase to cover that as well.
2025-02-07 Jakub Jelinek <ja...@redhat.com>
PR c++/118661
* constexpr.cc (potential_constant_expression_1): Don't diagnose
lvalue-to-rvalue conversion of volatile lvalue if it has
NULLPTR_TYPE.
* decl2.cc (decl_maybe_constant_var_p): Return true for constexpr
decls with NULLPTR_TYPE even if they are volatile.
* g++.dg/cpp0x/constexpr-volatile4.C: New test.
* g++.dg/cpp0x/constexpr-union9.C: New test.
(cherry picked from commit 6c8e6d6febaed3c167ca9534935c2cb18045528e)
Diff:
---
gcc/cp/constexpr.cc | 3 ++-
gcc/cp/decl2.cc | 3 ++-
gcc/testsuite/g++.dg/cpp0x/constexpr-union9.C | 16 ++++++++++++++++
gcc/testsuite/g++.dg/cpp0x/constexpr-volatile4.C | 20 ++++++++++++++++++++
4 files changed, 40 insertions(+), 2 deletions(-)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 8a66528d5caf..503af26fc929 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9075,7 +9075,8 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
available, so we don't bother with switch tracking. */
return true;
- if (TREE_THIS_VOLATILE (t) && want_rval)
+ if (TREE_THIS_VOLATILE (t) && want_rval
+ && !NULLPTR_TYPE_P (TREE_TYPE (t)))
{
if (flags & tf_error)
constexpr_error (loc, fundef_p, "lvalue-to-rvalue conversion of "
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 0ab7e502dc86..89c64277b5d6 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -4521,7 +4521,8 @@ decl_maybe_constant_var_p (tree decl)
tree type = TREE_TYPE (decl);
if (!VAR_P (decl))
return false;
- if (DECL_DECLARED_CONSTEXPR_P (decl) && !TREE_THIS_VOLATILE (decl))
+ if (DECL_DECLARED_CONSTEXPR_P (decl)
+ && (!TREE_THIS_VOLATILE (decl) || NULLPTR_TYPE_P (type)))
return true;
if (DECL_HAS_VALUE_EXPR_P (decl))
/* A proxy isn't constant. */
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union9.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-union9.C
new file mode 100644
index 000000000000..5d365f9cca62
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union9.C
@@ -0,0 +1,16 @@
+// PR c++/118661
+// { dg-do compile { target c++11 } }
+
+using nullptr_t = decltype (nullptr);
+union U { int i; nullptr_t n; };
+constexpr U u = { 42 };
+static_assert (u.n == nullptr, "");
+
+#if __cplusplus >= 201402L
+constexpr nullptr_t
+foo ()
+{
+ union U { int i; nullptr_t n; } u = { 42 };
+ return u.n;
+}
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile4.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile4.C
new file mode 100644
index 000000000000..5ef024009b50
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-volatile4.C
@@ -0,0 +1,20 @@
+// PR c++/118661
+// { dg-do compile { target c++11 } }
+
+using nullptr_t = decltype (nullptr);
+constexpr volatile nullptr_t a = {};
+constexpr nullptr_t b = a;
+
+constexpr nullptr_t
+foo ()
+{
+#if __cplusplus >= 201402L
+ volatile nullptr_t c = {};
+ return c;
+#else
+ return nullptr;
+#endif
+}
+
+static_assert (b == nullptr, "");
+static_assert (foo () == nullptr, "");
