>>42 n3690から抜粋すると 8.5.17:The semantics of initializers are as follows. The destination type is the type of the object or reference being initialized and the source type is the type of the initializer expression. If the initializer is not a single (possibly parenthesized) expression, the source type is not defined. — If the initializer is a (non-parenthesized) braced-init-list, the object or reference is list-initialized 8.5.4.3: List-initialization of an object or reference of type T is defined as follows: if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized. 8.5.8: To value-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9) with either no default constructor (12.1) or a default constructor that is user-provided or deleted, then the object is default-initialized 8.5.7: To default-initialize an object of type T means: — if T is a (possibly cv-qualified) class type (Clause 9), the default constructor (12.1) for T is called
