add testcase for 87076

using
rust-lang#87076 (comment) as
testcase

src/test/ui/const-generics/issues/issue-87076.rs

@@ -0,0 +1,20 @@
+// build-pass
+
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+#[derive(PartialEq, Eq)]
+pub struct UnitDims {
+    pub time: u8,
+    pub length: u8,
+}
+
+pub struct UnitValue<const DIMS: UnitDims>;
+
+impl<const DIMS: UnitDims> UnitValue<DIMS> {
+    fn crash() {}
+}
+
+fn main() {
+    UnitValue::<{ UnitDims { time: 1, length: 2 } }>::crash();
+}