Test sample:
class T {
        public P GetService <P> () where P : class
        {
            return null;
        }
}

gmcs il:
.method public hidebysig instance !!P GetService<class P>() cil managed
{
    .maxstack 8
    L_0000: ldnull 
    L_0001: ret 
}

csc il:
.method public hidebysig instance !!P GetService<class P>() cil managed
{
    .maxstack 1
    .locals init (
        [0] !!P local)
    L_0000: ldloca.s local
    L_0002: initobj !!P
    L_0008: ldloc.0 
    L_0009: ret 
}

peverify says (out gmcs):
[IL]: Error: [test.dll : T::GetService[P]][offset 0x00000001][found Nullobjref 'NullReference'][expected (unboxed) 'P'] Unexpected type on the stack.
1 Error(s) Verifying test.dll