Add test for issue-67739

src/test/ui/const-generics/issues/issue-67739.rs

@@ -0,0 +1,18 @@
+// Regression test for #67739
+
+#![allow(incomplete_features)]
+#![feature(const_generics)]
+
+use std::mem;
+
+pub trait Trait {
+    type Associated: Sized;
+
+    fn associated_size(&self) -> usize {
+        [0u8; mem::size_of::<Self::Associated>()];
+        //~^ ERROR: array lengths can't depend on generic parameters
+        0
+    }
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-67739.stderr

@@ -0,0 +1,8 @@
+error: array lengths can't depend on generic parameters
+  --> $DIR/issue-67739.rs:12:15
+   |
+LL |         [0u8; mem::size_of::<Self::Associated>()];
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+