GLSL spec section 6.1 says:
"If functions' names and argument types match, then their return type and parameter qualifiers must also match."

But in piglit/tests/glslparsertest/shaders/function9.frag, the parameter qualifier in function declaration and definition doesn't match (note "const"), but it's compiled successfully: 

void function(const in int i);  

void main()
{
    int i;
    function(i);  
}

// function definition has different parameter qualifiers than function declaration
void function(in int i)  
{  
   i = 3;
}