c++: Fix up recent regression in convert_nontype_argument for C++1[14] [PR124173]

The following testcase is rejected since my recent convert_nontype_argument
change, but only in C++11/14 modes.
The problem is that C++17 says that all NTTPs should be converted constant
expressions, but C++11 only said that for integral and enumeration ones and
something that applied to the decltype(nullptr) case was
- a constant expression that evaluates to a null pointer value
The problem is that for NULLPTR_TYPE_P for C++11/14, we just never called
maybe_constant_value, so obviously just making sure it is integer_zerop and
without tf_error returning NULL_TREE otherwise changes behavior on valid
programs, we really need to constant evaluate it first.

The following patch fixes that.

I'm afraid I actually don't know what exactly C++14 specifies, whether I've
grabbed a pre-C++14 or post-C++14 draft, which looked like the C++17
wording.   So, if C++14 needs something different, we'll need a further
change, but this at least fixes the regression.

2026-02-22  Jakub Jelinek  <jakub@redhat.com>

	PR c++/124173
	* pt.cc (convert_nontype_argument): For C++11/C++14 handle
	NULLPTR_TYPE_P non-type arguments like TYPE_PTR_P.

	* g++.dg/cpp0x/pr124173.C: New test.

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 11711ee3543e5aa0e106e58ecce2ba62b5b0feea..bcc1525ff8b6911041980962ce825a0051c31f81 100644 (file)
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7690,10 +7690,12 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
          /* EXPR may have become value-dependent.  */
          val_dep_p = value_dependent_expression_p (expr);
        }
-      else if (TYPE_PTR_OR_PTRMEM_P (type))
+      else if (TYPE_PTR_OR_PTRMEM_P (type)
+              || NULLPTR_TYPE_P (type))
        {
          tree folded = maybe_constant_value (expr, NULL_TREE, mce_true);
-         if (TYPE_PTR_P (type) ? integer_zerop (folded)
+         if ((TYPE_PTR_P (type) || NULLPTR_TYPE_P (type))
+             ? integer_zerop (folded)
              : null_member_pointer_value_p (folded))
            expr = folded;
        }

diff --git a/gcc/testsuite/g++.dg/cpp0x/pr124173.C b/gcc/testsuite/g++.dg/cpp0x/pr124173.C
new file mode 100644 (file)
index 0000000..a8e9ae7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/pr124173.C
@@ -0,0 +1,15 @@
+// PR c++/124173
+// { dg-do compile { target c++11 } }
+
+template <decltype (nullptr)>
+void
+foo ()
+{
+}
+
+int
+main ()
+{
+  constexpr decltype (nullptr) t = nullptr;
+  foo <t> ();
+}