regression test for #39974

closes #39974

r? @Mark-Simulacrum

src/test/compile-fail/issue-39974.rs

@@ -0,0 +1,21 @@
+// 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.
+
+const LENGTH: f64 = 2;
+
+struct Thing {
+    f: [[f64; 2]; LENGTH],
+    //~^ ERROR mismatched types
+    //~| expected usize, found f64
+}
+
+fn main() {
+    let _t = Thing { f: [[0.0, 0.0], [0.0, 0.0], [0.0, 0.0], [0.0, 0.0]] };
+}