src/test/compile-fail/isssue-38821.rs

// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

pub struct Nullable<T: NotNull>(T);

pub trait NotNull {}

pub trait IntoNullable {
    type Nullable;
}

impl<T: NotNull> IntoNullable for T {
    type Nullable = Nullable<T>;
}

impl<T: NotNull> IntoNullable for Nullable<T> {
    type Nullable = Nullable<T>;
}

pub trait Expression {
    type SqlType;
}

pub trait Column: Expression {}

#[derive(Debug, Copy, Clone)]
//~^ ERROR the trait bound `<Col as Expression>::SqlType: NotNull` is not satisfied
pub enum ColumnInsertValue<Col, Expr> where
    Col: Column,
    Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>,
{
    Expression(Col, Expr),
    Default(Col),
}

fn main() {}