diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index f142dd32bc8..f8f9a9df1a2 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2909,6 +2909,13 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	  gcc_assert (arg0);
 	  if (new_op_p)
 	    {
+	      if (!tree_fits_uhwi_p (arg0))
+		{
+		  if (!ctx->quiet)
+		    error_at (loc, "cannot allocate array: size too large");
+		  *non_constant_p = true;
+		  return t;
+		}
 	      tree type = build_array_type_nelts (char_type_node,
 						  tree_to_uhwi (arg0));
 	      tree var = build_decl (loc, VAR_DECL,
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new24.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new24.C
new file mode 100644
index 00000000000..debb7f0f5c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new24.C
@@ -0,0 +1,25 @@
+// PR c++/118775
+// { dg-do compile { target c++20 } }
+
+int a;
+
+constexpr char *
+f1 ()
+{
+  constexpr auto p = new char[(long int) &a]; // { dg-error "size too large" }
+  return p;
+}
+
+constexpr char *
+f2 ()
+{
+  auto p = new char[(long int) &a];  // { dg-error "size too large" }
+  return p;
+}
+
+void
+g ()
+{
+  auto r1 = f2 ();
+  constexpr auto r2 = f2 (); // { dg-message "in .constexpr. expansion" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new25.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-new25.C
new file mode 100644
index 00000000000..91c0318abd8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new25.C
@@ -0,0 +1,27 @@
+// PR c++/118775
+// { dg-do compile { target c++20 } }
+
+namespace std {
+struct __uniq_ptr_impl {
+  constexpr __uniq_ptr_impl(char *) {}
+};
+template <typename> struct unique_ptr {
+  __uniq_ptr_impl _M_t;
+  constexpr ~unique_ptr() {}
+};
+template <typename> struct _MakeUniq;
+template <typename _Tp> struct _MakeUniq<_Tp[]> {
+  typedef unique_ptr<_Tp[]> __array;
+};
+template <typename _Tp> using __unique_ptr_array_t = _MakeUniq<_Tp>::__array;
+constexpr __unique_ptr_array_t<char[]> make_unique(long __num) {
+  return unique_ptr<char[]>(new char[__num]);
+}
+} // namespace std
+int a;
+int
+main ()
+{
+  std::unique_ptr p = std::make_unique((long)&a);
+  constexpr std::unique_ptr p2 = std::make_unique((long)&a); // { dg-error "conversion" }
+}