[Bug c++/85125] New: constant expression with const_cast UB does not emit error

[john at mcfarlane dot name]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85125

            Bug ID: 85125
           Summary: constant expression with const_cast UB does not emit
                    error
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: john at mcfarlane dot name
  Target Milestone: ---

Tested in godbolt.org ([example](https://godbolt.org/g/dhtXmH)) with x86-64
versions from 5.1 to 7.3 and trunk. Options are `-Wall -Wextra` but can be
reproduced without.

The following code compiles:

    constexpr int f() {
      int const i = 77;
      const_cast<int&>(i) = 88;
      return i;
    }
    static_assert(f()==77, "");

>From reading of http://eel.is/c++draft/dcl.type.cv#4 the `const_cast` line
above results in UB and from http://eel.is/c++draft/expr.const#2.6 an
expression causing UB cannot be a constant expression.

Expected: an error is emitted on the `const_cast` line explaining that a
constant value is being written to.
Actual: code compiles without warnings or errors and the `const_cast` line
appears to be ignored. (When `constexpr` is removed, produces code for a
function that returns 77.)