GLSL spec section 4.3.2 says "Initializers for const declarations must be constant expressions".

Apparently below 2 examples are not constant expressions, so should not be allowed:
piglit/tests/glslparsertest/shaders/constFunc.frag:
    const vec3 v = vec3(1.0, func()); // user defined functions do not return const value

piglit/tests/glslparsertest/shaders/ParseTest3.frag:
    float f = 1.0;
    const vec4 v2 = vec4(float(vec4(1,2,3,f))); // f is not constant

And below example is not allowed in GLSL v1.10, but allowed starting from GLSL v1.20 (allowing built-in function call whose arguments are all constant expressions):
piglit/tests/glslparsertest/shaders/ParseTest3.frag:
    const vec4 v = vec4(normalize(vec4(1))); // builtIn functions do not return const
    const vec4 v1 = vec4(clamp(1.0, .20, 3.0)); // builtIn functions do not return const

All of above examples are allowed in current mesa.