Is this legal? Should it be?

  struct E {
    union {
      struct { int x; } s;
    } v;
  };

One compiler faults a type definition (i.e. of the anonymous struct) since it is in an anonymous union [11.5 [class.union] paragraph 2: "The member-specification of an anonymous union shall only define non-static data members."].

I would suggest that compiler B is correctly interpreting the standard but that this is a defect in the standard. There is no reason to disallow definition of anonymous structs.

Furthermore, is it really necessary to disallow definition of named types in anonymous unions in general, as long as the types do not need fully qualified names for external linkage? Why should this be illegal?

  struct E {
    union {
      typedef int Int;
      struct X { X *next; Int n; } list;
    } v;
  };