Skybox es2 shaders issue

src/extras/shaders/es2/skybox.vert:

gl_Position = vec4(projectionMatrix * mat4(mat3(viewMatrix)) * modelMatrix * vec4(vertexPosition, 1.0)).xyww;

induces QOpenGLShader::compile(Vertex): ERROR: 3:13: constructing matrix from matrix (reserved in GLSL 1.10).

GLSL 1.10 spec section 5.3.2 says:
"It is an error to construct matrices from other matrices. This is reserved for future use."

I tested only with Qt 5.9.5, but as I see in source code it is still relevant to all other versions.