c++ - Weird behavior with operator defined as friend inside class -

i don't understand going on in following piece of code:

struct { };  struct b {   b() { }   b(const a&) { }    friend b operator*(const b&, const b&)   {     return b();   } };  int main() {   b x = a() * a();    return 0; } 

when compile (with both clang , gcc 4.9.2) error message on "b x = a() * a()" line; clang says "invalid operands binary expression".

if take operator* definition inside class, 100% ok!

struct { };  struct b {   b() { }   b(const a&) { }    friend b operator*(const b&, const b&); };  b operator*(const b&, const b&) {   return b(); }  int main() {   b x = a() * a();    return 0; } 

what going on?

since operator*() defined inside function friend, can found adl, not normal unqualified lookup. type of lookup requires arguments exact types, not types implicitly convertible. means operator cannot found if a can converted b.

when declare function outside class, can found normal unqualified lookup rules.