clang shocks on this.

According to people on #llvm, this is indeed not allowed by c++ standard:

[01:33] <coppro> what good would mutable do on a reference?
[01:33] <dgregor> orzel: gcc is wrong to allow "mutable" on references
[01:33] <coppro> just remove mutable; it won't damage the code at all
[01:34] <dgregor> "The mutable specifier can be applied only to names of
class data members (9.2) and cannot be applied to
[01:34] <dgregor> names declared const or static, and cannot be applied to
reference members."
[01:34] <coppro> constness is not passed from an object to the referents of
its members anyways

Eigen/src/Core/products/GeneralMatrixMatrix.h

@@ -245,7 +245,7 @@ struct ei_gemm_functor
   protected:
     const Lhs& m_lhs;
     const Rhs& m_rhs;
-    mutable Dest& m_dest;
+    Dest& m_dest;
     Scalar m_actualAlpha;
 };