According to the documentation only classes might contain "invariant".

Dmd-0.104 allows "invariant" in structs and checks those contracts.
test case:
svn://svn.kuehne.cn/dstress/nocompile/invariant_04.d

Dmd-0.104 allows "invariant" in unions and checks those contracts.
test case:
svn://svn.kuehne.cn/dstress/nocompile/invariant_06.d

The documentation doesn't forbid the usage of a class function named
"_invariant". The function will not be handled as a "invariant" contract,
that's correct.
The problem is that it's mangled name will conflict with "invariant".
test case:
svn://svn.kuehne.cn/dstress/run/invariant_11.d

Code:
# class Parent{
#        int x;
#        this(){
#                x=3;
#        }
#
#        invariant{
#                assert(x>2);
#        }
# }
#
# class Child : Parent{
#        int y;
#        this(){
#                y=5; // [2]
#        }
#
#        invariant{
#                assert(y>4); // [1]
#        }
# }
#
# int main(){
#        Child gc = new Child();
#        return 0;
# }

The Child's invariant[1] will be called before "y=5" [2] is executed.
It seems that not only inherited the Child it's Parent's invariant
but so did the Parent with the Child's invariant.
I think this is a bug and not a hole in the documentation.
Reason: This way contracts for new class members can't easily be added to a sub-class.

test cases: svn://svn.kuehne.cn/dstress/run/invariant_1[45].d

Thomas

in addition:

the invariant of a class that has got no own constructor is _not_ called after the super
constructor finished.
test case:
svn://svn.kuehne.cn/dstress/run/invariant_18.d

Thomas