The following valid test case was reduced from the invalid one submitted with bug 70377.  GCC rejects the constexpr function simple_find because it contains a throw statement even though the statement is not evaluated in the (constexpr) contexts the function is called.  Clang 5.0 and EDG eccp 4.13 both accept the program.

$ cat t.C && gcc -Wall -Wextra -Wpedantic t.C
struct array
{
  int a [3];
  constexpr const int& operator[](int i) const { return a[i]; }
};

constexpr int
simple_find (const array &a, int x)
{
  for (int i = 0; i != 3; ++i)
    if (a[i] == x)
      return i;

  throw;
}

static constexpr array some_ints { { 10, 11 } };

static_assert (simple_find (some_ints, 10) == 0, "#1");
static_assert (simple_find (some_ints, 11) == 1, "#2");
t.C: In function ‘constexpr int simple_find(const array&, int)’:
t.C:14:3: error: expression ‘<throw-expression>’ is not a constant expression
   throw;
   ^~~~~
t.C: At global scope:
t.C:19:1: error: non-constant condition for static assertion
 static_assert (simple_find (some_ints, 10) == 0, "#1");
 ^~~~~~~~~~~~~
t.C:19:28: error: ‘constexpr int simple_find(const array&, int)’ called in a constant expression
 static_assert (simple_find (some_ints, 10) == 0, "#1");
                ~~~~~~~~~~~~^~~~~~~~~~~~~~~
t.C:20:1: error: non-constant condition for static assertion
 static_assert (simple_find (some_ints, 11) == 1, "#2");
 ^~~~~~~~~~~~~
t.C:20:28: error: ‘constexpr int simple_find(const array&, int)’ called in a constant expression
 static_assert (simple_find (some_ints, 11) == 1, "#2");
                ~~~~~~~~~~~~^~~~~~~~~~~~~~~