Passing swizzles as inout/out arguments doesn't compile.

[gwiazdorrr]

void foo(inout vec2 v) {}

vec3 position = vec3(0.0);
foo(position.yz); // error

Swizzles (aka proxies) are convertible to vectors, which are returned by value. In the above example, the resulting rvalue can't be converted to a reference (inout).

[gwiazdorrr]

Solved.