regression test for #38821

Closes #38821

r? @Mark-Simulacrum

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

@@ -0,0 +1,43 @@
+// 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() {}