Passing too many arguments works sometimes, why? - function

Question

I was using this site for testing: http://glslsandbox.com/
This shows the color red:
#ifdef GL_ES
precision mediump float;
#endif
void main( void ) {
vec4 c = vec4(1.0, 0.0, 0.0, 1.0);
gl_FragColor = c;
}
I can change the color line in different ways, sometimes it compiles and sometimes not:
vec4 c = vec4(1.0, vec2(0.0), vec4(1.0)); // works
vec4 c = vec4(vec2(1.0), vec2(0.0), 0.0); // doesn't compile
vec4 c = vec4(1.0, vec2(0.0), vec2(1.0)); // works
vec4 c = vec4(1.0, vec4(0.0), 0.0); // doesn't compile
vec4 c = vec4(vec4(1.0), vec4(0.0)); // doesn't compile
Why does passing too many arguments work sometimes and sometimes not?

Answer 1

See OpenGL Shading Language 4.60 Specification (HTML) - 5.4.2. Vector and Matrix Constructors:
[...] The arguments will be consumed left to right, and each argument will have all its components consumed, in order, before any components from the next argument are consumed. [...]
In these cases, there must be enough components provided in the arguments to provide an initializer for every component in the constructed value. It is a compile-time error to provide extra arguments beyond this last used argument.
Hence, the following is allowed:
vec4 c = vec4(1.0, 2.0, 3.0, vec4(4.0));
vec3 c = vec3(vec4(4.0));
However, the following is not allowed because the last element in the constructor (vec4(4.0)) causes a compile time error (It is a compile-time error to provide extra arguments beyond this last used argument.):
vec3 c = vec3(1.0, 2.0, 3.0, vec4(4.0));
The reason for this is that it should be allowed to construct a smaller vector (or matrix) from a larger vector (or matrix). For instance:
vec4 v4;
vec3 v3 = vec3(v4);