c++ - Implicit type conversion from initialization list compiles in one case but does not in another -

i defined following class:

class {     int a; public:     a(int _a = 0) :a(_a){}     a(initializer_list<int> il) :a(il.size()){}     friend operator+(const a& a1, const a& a2); }; operator+(const a& a1, const a& a2){ return a(); } 

the following client code

a a; operator+(a,{3, 4, 5}); 

can compile, following

a a; + {3, 4, 5}; 

can not compile. error message "error c2059: syntax error : '{'" , "error c2143: syntax error : missing ';' before '{'".

both 2 client codes try implicit type conversion, initialization list {3,4,5} class a, first succeeds while second snippet fails. can not understand why.

can explain it?

i'm using ms visual studio 2013 update 4.

[edit] here follow up, after reading comments , other related materials. think question can reduced following: got brace-init-list not allowed in rhs of binary expression, , allowed in function call. thing is, think binary expression, say, arg1 + arg2 converted internally compiler function call operator+ (arg1, arg2), when arg1 class type. @ point, there no difference between binary expression , function call. result, explanation can figure out there preventing rule applied before such binary-expression-to-function-call conversion, checks particularly whether second argument brace-init-list or not. if is, conversion equivalent function call forbidden , error produced. wonder if these conjectures real , if is, written somewhere in c++ standard? thank participated in question.

the error message generated clang quite clear:

error: initializer list cannot used on right hand side of operator '+'

looking c++11 standard, find (8.5.4/1 [dcl.init.list]):

note: list-initialization can used

• as initializer in variable definition (8.5)
• as initializer in new expression (5.3.4)
• in return statement (6.6.3)
• as function argument (5.2.2)
• as subscript (5.2.1)
• as argument constructor invocation (8.5, 5.2.3)
• as initializer non-static data member (9.2)
• in mem-initializer (12.6.2)
• on right-hand side of assignment (5.17)

so guess braced-init-list can used in cases listed above. operator+(a, {1, 2, 3}) works because of case 4. a = {1, 2, 3} works because of last case. nothing mentioned a + {1, 2, 3}. no.