Even though this PR is very close to PR117101, it's not addressed by the
fix I made through r15-4958-g5821f5c8c89a05 because cxx_placement_new_fn
has the very same issue as std_placement_new_fn_p used to have.

This patch fixes the issue exactly the same, by checking the first
parameter against NULL_TREE. I considered somehow sharing code between
the two *_placement_new_fn* static functions, but it looked pointless
since the shared part ("the second parameter is the last one, and a
pointer") is unlikely to be useful elsewhere.

Successfully tested on x86_64-pc-linux-gnu.

        PR c++/117463

gcc/cp/ChangeLog:

        * constexpr.cc (cxx_placement_new_fn): Check first_arg against
        NULL_TREE.

gcc/testsuite/ChangeLog:

        * g++.dg/init/new54.C: New test.

---
 gcc/cp/constexpr.cc               |  3 ++-
 gcc/testsuite/g++.dg/init/new54.C | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/init/new54.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 71e6dc4ef32..379700ca2c1 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2334,7 +2334,8 @@ cxx_placement_new_fn (tree fndecl)
       && TREE_CODE (TREE_TYPE (fndecl)) == FUNCTION_TYPE)
     {
       tree first_arg = TREE_CHAIN (TYPE_ARG_TYPES (TREE_TYPE (fndecl)));
-      if (TREE_VALUE (first_arg) == ptr_type_node
+      if (first_arg
+         && TREE_VALUE (first_arg) == ptr_type_node
          && TREE_CHAIN (first_arg) == void_list_node)
        return true;
     }
diff --git a/gcc/testsuite/g++.dg/init/new54.C 
b/gcc/testsuite/g++.dg/init/new54.C
new file mode 100644
index 00000000000..fdff1b55f0d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/new54.C
@@ -0,0 +1,14 @@
+// PR c++/117463
+// { dg-do "compile" { target c++20 } }
+
+struct S {};
+void *operator new[] (unsigned long, // { dg-bogus "first parameter" "" { 
xfail *-*-* } }
+                     void void *volatile p); // { dg-error "two or more" }
+S *fun(void *p) {
+  return new(p) S[10];
+}
+
+void *operator new (decltype(sizeof(0)), // { dg-bogus "first parameter" "" { 
xfail *-*-* } }
+                   void void * p); // { dg-error "two or more" }
+void *p;
+auto t = new(p) int;
-- 
2.44.0

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