c++ - brace-or-equal-initializer in mem-initializer -

the c++ standard not allow "= initializer-clause" form of brace-or-equal-initializer (see [dcl.init]) in mem-initializer (see [class.base.init]). example:

struct foo {     int x;      foo(int y): x = y {     } }; 

gives compile-time error.

why that? there syntactic ambiguity if such construct allowed? if not, or there proposals add feature language?

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update. semantics expect of such initializer in context same of in context of usual variable initialization. in opinion, make initialization syntax of language more consistent.

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update 2. pointed out nathanoliver, if initialization of member not depend on selected constructor and/or constructor parameters, member can initialized through default member initializer (see [class.mem]). if does, leaves using direct or list initialization.

like others have said, it's simple not allowed syntax in ctor initialization.

note that, non-primitive classes, ctor() : xyz(...) form of initialization ctor call on type of member xyz. while ctor(): xyz{...} aggregate initialization of either array/list or member initialization of structs/unions have no ctors.

a ctor() : xyz = something seem have 2 things: 1) default ctor call , 2) call operator= type of member xyz of double initialization. why disallowed. of course, non-primitive cases, guess reduced copy-ctor call eliminate double initialization.

now, maybe convince allowing primitive types only ok, introduce other complications language , compilers.