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  <jakub@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.

4 files changed, 40 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 58bed6f..f5261d0 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9717,7 +9717,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     return true;
 
   if (TREE_THIS_VOLATILE (t) && want_rval
-      && !FUNC_OR_METHOD_TYPE_P (TREE_TYPE (t)))
+      && !FUNC_OR_METHOD_TYPE_P (TREE_TYPE (t))
+      && !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 a11c431..593dcaa 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -4985,7 +4985,8 @@ 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 0000000..5d365f9
--- /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 0000000..5ef0240
--- /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, "");