g++-10.0.0-alpha20190915 snapshot (r275736) ICEs when compiling the following testcase reduced from test/SemaCXX/constant-expression-cxx2a.cpp from the clang 9.0.0 test suite:

struct B {
  virtual constexpr char f() const { return 'B'; }
  char b = f();
};

struct C {
  virtual constexpr char f() const { return 'C'; }
  B *pba;
  char c = ((B*)this)->f();
  char ba = pba->f();
};

struct D : B, C {
  char d = ' ';
};

template<typename T>
struct Covariant2 {
  virtual const T *f() const;
};

template<typename T>
struct Covariant3 : Covariant2<T> {
  constexpr virtual const D *f() const { return nullptr; }
};

constexpr Covariant3<C> cc;

static_assert(cc.f()->d == 'D');

% g++-10.0.0-alpha20190915 -c wkzgbhd7.cpp
wkzgbhd7.cpp: In instantiation of 'constexpr const D* Covariant3<T>::f() const [with T = C]':
wkzgbhd7.cpp:29:19:   required from here
wkzgbhd7.cpp:24:58: internal compiler error: in use_thunk, at cp/method.c:316
   24 |   constexpr virtual const D *f() const { return nullptr; }
      |                                                          ^
0x616688 use_thunk(tree_node*, bool)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/method.c:316
0xa38119 emit_associated_thunks(tree_node*)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/semantics.c:4341
0xa3845b expand_or_defer_fn(tree_node*)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/semantics.c:4443
0x9ee958 instantiate_decl(tree_node*, bool, bool)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/pt.c:24802
0x8d46bc instantiate_cx_fn_r
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/constexpr.c:5495
0x123b246 walk_tree_1(tree_node**, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*, tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/tree.c:12259
0x123bab0 walk_tree_1(tree_node**, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*, tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/tree.c:12589
0x123bab0 walk_tree_1(tree_node**, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*, tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/tree.c:12589
0x123e343 walk_tree_without_duplicates_1(tree_node**, tree_node* (*)(tree_node**, int*, void*), void*, tree_node* (*)(tree_node**, int*, tree_node* (*)(tree_node**, int*, void*), void*, hash_set<tree_node*, false, default_hash_traits<tree_node*> >*))
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/tree.c:12615
0x8dfa3f instantiate_constexpr_fns
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/constexpr.c:5514
0x8dfa3f cxx_eval_outermost_constant_expr
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/constexpr.c:5576
0x8e3cdd maybe_constant_value(tree_node*, tree_node*, bool)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/constexpr.c:5790
0x8f8c85 cp_fully_fold(tree_node*)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/cp-gimplify.c:2197
0xa6db26 cp_build_binary_op(op_location_t const&, tree_code, tree_node*, tree_node*, int)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/typeck.c:5561
0x8b654e build_new_op_1
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/call.c:6231
0x8b6faa build_new_op(op_location_t const&, tree_code, int, tree_node*, tree_node*, tree_node*, tree_node**, int)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/call.c:6275
0xa62611 build_x_binary_op(op_location_t const&, tree_code, tree_node*, tree_code, tree_node*, tree_code, tree_node**, int)
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/typeck.c:4185
0x99e924 cp_parser_binary_expression
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/parser.c:9645
0x99f689 cp_parser_assignment_expression
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/parser.c:9780
0x99f0b9 cp_parser_constant_expression
	/var/tmp/portage/sys-devel/gcc-10.0.0_alpha20190915/work/gcc-10-20190915/gcc/cp/parser.c:10074