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);
// function definition has different parameter qualifiers than function declaration
void function(in int i)
i = 3;
Fixed with the glsl2 merge.
verified with mesa master d442a01ac14382d83cdaac87d2832315ceb3e963.