Fix regression introduced by 3117036b80075390dbc46f60aa0d595e5a44661b
:
Matrix<Scalar,1,1>(int) did not compile if Scalar is not constructible from int. Now this falls back to the (Index size) constructor.

Eigen/src/Core/PlainObjectBase.h

diff --git a/Eigen/src/Core/PlainObjectBase.h b/Eigen/src/Core/PlainObjectBase.h
index b5b25ef..ae5b342 100644
--- a/Eigen/src/Core/PlainObjectBase.h
+++ b/Eigen/src/Core/PlainObjectBase.h
@@ -692,7 +692,7 @@
 
     template<typename T>
     EIGEN_DEVICE_FUNC
-    EIGEN_STRONG_INLINE void _init1(Index size, typename internal::enable_if<Base::SizeAtCompileTime!=1,T>::type* = 0)
+    EIGEN_STRONG_INLINE void _init1(Index size, typename internal::enable_if<Base::SizeAtCompileTime!=1 || !internal::is_convertible<T, Scalar>::value,T>::type* = 0)
     {
       EIGEN_STATIC_ASSERT(bool(NumTraits<T>::IsInteger),
                           FLOATING_POINT_ARGUMENT_PASSED__INTEGER_WAS_EXPECTED)
@@ -700,7 +700,7 @@
     }
     template<typename T>
     EIGEN_DEVICE_FUNC
-    EIGEN_STRONG_INLINE void _init1(const Scalar& val0, typename internal::enable_if<Base::SizeAtCompileTime==1,T>::type* = 0)
+    EIGEN_STRONG_INLINE void _init1(const Scalar& val0, typename internal::enable_if<Base::SizeAtCompileTime==1 && internal::is_convertible<T, Scalar>::value,T>::type* = 0)
     {
       EIGEN_STATIC_ASSERT_VECTOR_SPECIFIC_SIZE(PlainObjectBase, 1)
       m_storage.data()[0] = val0;

Eigen/src/Core/util/Meta.h

diff --git a/Eigen/src/Core/util/Meta.h b/Eigen/src/Core/util/Meta.h
index e4e4d4a..795197f 100644
--- a/Eigen/src/Core/util/Meta.h
+++ b/Eigen/src/Core/util/Meta.h
@@ -80,6 +80,25 @@
 template<typename T> struct add_const_on_value_type<T* const>  { typedef T const* const type; };
 template<typename T> struct add_const_on_value_type<T const* const>  { typedef T const* const type; };
 
+
+template<typename From, typename To>
+struct is_convertible
+{
+private:
+  struct yes {int a[1];};
+  struct no  {int a[2];};
+  
+  template<typename T>
+  static yes test (const T&) {}
+  
+  template<typename> static no test (...) {}
+
+public:
+  static From ms_from;
+  enum { value = sizeof(test<To>(ms_from))==sizeof(yes) };
+};
+
+
 /** \internal Allows to enable/disable an overload
   * according to a compile time condition.
   */