https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82877
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Clang issues the expected errors, as does ICC:
$ clang -S pr82877.C
pr82877.C:1:15: error: constexpr function never produces a constant expression
[-Winvalid-constexpr]
constexpr int f ()
^
pr82877.C:6:21: note: cannot refer to element -1 of array of 1 element in a
constant expression
return &s.a[0] - &s.a[-1]; // undefined, should be rejected
^
pr82877.C:6:21: warning: array index -1 is before the beginning of the array
[-Warray-bounds]
return &s.a[0] - &s.a[-1]; // undefined, should be rejected
^ ~~
pr82877.C:3:14: note: array 'a' declared here
struct S { int a[1]; };
^
pr82877.C:9:15: error: constexpr variable 'i' must be initialized by a constant
expression
constexpr int i = f ();
^ ~~~~
pr82877.C:6:21: note: cannot refer to element -1 of array of 1 element in a
constant expression
return &s.a[0] - &s.a[-1]; // undefined, should be rejected
^
pr82877.C:9:19: note: in call to 'f()'
constexpr int i = f ();
^
1 warning and 2 errors generated.
