Use stricter brace initialization

This updates the `py::init` constructors to only use brace
initialization for aggregate initiailization if there is no constructor
with the given arguments.

This, in particular, fixes the regression in #1247 where the presence of
a `std::initializer_list<T>` constructor started being invoked for
constructor invocations in 2.2 even when there was a specific
constructor of the desired type.

The added test case demonstrates: without this change, it fails to
compile because the `.def(py::init<std::vector<int>>())` constructor
tries to invoke the `T(std::initializer_list<std::vector<int>>)`
constructor rather than the `T(std::vector<int>)` constructor.

By only using `new T{...}`-style construction when a `T(...)`
constructor doesn't exist, we should bypass this by while still allowing
`py::init<...>` to be used for aggregate type initialization (since such
types, by definition, don't have a user-declared constructor).

include/pybind11/detail/init.h

@@ -52,6 +52,16 @@ bool is_alias(Cpp<Class> *ptr) {
 template <typename /*Class*/>
 constexpr bool is_alias(void *) { return false; }
 
+// Constructs and returns a new object; if the given arguments don't map to a constructor, we fall
+// back to brace aggregate initiailization so that for aggregate initialization can be used with
+// py::init, e.g.  `py::init<int, int>` to initialize a `struct T { int a; int b; }`.  For
+// non-aggregate types, we need to use an ordinary T(...) constructor (invoking as `T{...}` usually
+// works, but will not do the expected thing when `T` has an `initializer_list<T>` constructor).
+template <typename Class, typename... Args, detail::enable_if_t<std::is_constructible<Class, Args...>::value, int> = 0>
+inline Class *construct_or_initialize(Args &&...args) { return new Class(std::forward<Args>(args)...); }
+template <typename Class, typename... Args, detail::enable_if_t<!std::is_constructible<Class, Args...>::value, int> = 0>
+inline Class *construct_or_initialize(Args &&...args) { return new Class{std::forward<Args>(args)...}; }
+
 // Attempts to constructs an alias using a `Alias(Cpp &&)` constructor.  This allows types with
 // an alias to provide only a single Cpp factory function as long as the Alias can be
 // constructed from an rvalue reference of the base Cpp type.  This means that Alias classes
@@ -161,7 +171,7 @@ struct constructor {
     template <typename Class, typename... Extra, enable_if_t<!Class::has_alias, int> = 0>
     static void execute(Class &cl, const Extra&... extra) {
         cl.def("__init__", [](value_and_holder &v_h, Args... args) {
-            v_h.value_ptr() = new Cpp<Class>{std::forward<Args>(args)...};
+            v_h.value_ptr() = construct_or_initialize<Cpp<Class>>(std::forward<Args>(args)...);
         }, is_new_style_constructor(), extra...);
     }
 
@@ -171,9 +181,9 @@ struct constructor {
     static void execute(Class &cl, const Extra&... extra) {
         cl.def("__init__", [](value_and_holder &v_h, Args... args) {
             if (Py_TYPE(v_h.inst) == v_h.type->type)
-                v_h.value_ptr() = new Cpp<Class>{std::forward<Args>(args)...};
+                v_h.value_ptr() = construct_or_initialize<Cpp<Class>>(std::forward<Args>(args)...);
             else
-                v_h.value_ptr() = new Alias<Class>{std::forward<Args>(args)...};
+                v_h.value_ptr() = construct_or_initialize<Alias<Class>>(std::forward<Args>(args)...);
         }, is_new_style_constructor(), extra...);
     }
 
@@ -182,7 +192,7 @@ struct constructor {
                           !std::is_constructible<Cpp<Class>, Args...>::value, int> = 0>
     static void execute(Class &cl, const Extra&... extra) {
         cl.def("__init__", [](value_and_holder &v_h, Args... args) {
-            v_h.value_ptr() = new Alias<Class>{std::forward<Args>(args)...};
+            v_h.value_ptr() = construct_or_initialize<Alias<Class>>(std::forward<Args>(args)...);
         }, is_new_style_constructor(), extra...);
     }
 };
@@ -193,7 +203,7 @@ template <typename... Args> struct alias_constructor {
               enable_if_t<Class::has_alias && std::is_constructible<Alias<Class>, Args...>::value, int> = 0>
     static void execute(Class &cl, const Extra&... extra) {
         cl.def("__init__", [](value_and_holder &v_h, Args... args) {
-            v_h.value_ptr() = new Alias<Class>{std::forward<Args>(args)...};
+            v_h.value_ptr() = construct_or_initialize<Alias<Class>>(std::forward<Args>(args)...);
         }, is_new_style_constructor(), extra...);
     }
 };