Make new use list initialization if available

This enables uses like `new<std::array<int,5>>(1, 2, 3, 4, 5)` which didn't work before (not even with P0960)

See #740

Note: I chose "use list init if available, else fall back to non-list init" instead of "use paren init if available, else fall back to list init" so that we would get consistent results for initializing a `new<std::vector<int>>` with `(10)`, `(10, 20)`, `(10, 20, 30)`, etc. (i.e., no surprise for `(10, 20)`)

Author: hsutter

include/cpp2util.h

@@ -556,14 +556,26 @@ auto Typeid() -> decltype(auto) {
 struct {
     template<typename T>
     [[nodiscard]] auto cpp2_new(auto&& ...args) const -> std::unique_ptr<T> {
-        return std::make_unique<T>(CPP2_FORWARD(args)...);
+        if constexpr (requires { T{CPP2_FORWARD(args)...}; }) {
+            //  This is because apparently make_unique can't deal with list
+            //  initialization of aggregates, even after P0960
+            return std::unique_ptr<T>( new T{CPP2_FORWARD(args)...} );
+        }
+        else {
+            return std::make_unique<T>(CPP2_FORWARD(args)...);
+        }
     }
 } inline unique;
 
 [[maybe_unused]] struct {
     template<typename T>
     [[nodiscard]] auto cpp2_new(auto&& ...args) const -> std::shared_ptr<T> {
-        return std::make_shared<T>(CPP2_FORWARD(args)...);
+        if constexpr (requires { T{CPP2_FORWARD(args)...}; }) {
+            return unique.cpp2_new<T>(CPP2_FORWARD(args)...);
+        }
+        else {
+            return std::make_shared<T>(CPP2_FORWARD(args)...);
+        }
     }
 } inline shared;
 

regression-tests/test-results/msvc-2022/MSVC-version.output

@@ -1,3 +1,3 @@
-Microsoft (R) C/C++ Optimizing Compiler Version 19.37.32824 for x86
+Microsoft (R) C/C++ Optimizing Compiler Version 19.37.32825 for x86
 Copyright (C) Microsoft Corporation.  All rights reserved.