Fix chrono crash due to empty param_env

tests/run-pass/ice-2760.rs

@@ -0,0 +1,19 @@
+#![allow(unused_variables, blacklisted_name, needless_pass_by_value, dead_code)]
+
+// This should not compile-fail with:
+//
+//      error[E0277]: the trait bound `T: Foo` is not satisfied
+//
+// See https://github.com/rust-lang-nursery/rust-clippy/issues/2760
+
+trait Foo {
+    type Bar;
+}
+
+struct Baz<T: Foo> {
+    bar: T::Bar,
+}
+
+fn take<T: Foo>(baz: Baz<T>) {}
+
+fn main() {}