Add a regression test for issue 62187

Author: varkor

src/test/ui/const-generics/issues/issue-62187-encountered-polymorphic-const.rs

@@ -0,0 +1,16 @@
+// run-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+pub trait BitLen: Sized {
+    const BIT_LEN: usize;
+}
+
+impl<const L: usize> BitLen for [u8; L] {
+    const BIT_LEN: usize = 8 * L;
+}
+
+fn main() {
+    let foo = <[u8; 2]>::BIT_LEN;
+}

src/test/ui/const-generics/issues/issue-62187-encountered-polymorphic-const.stderr

@@ -0,0 +1,16 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/issue-62187-encountered-polymorphic-const.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: unused variable: `foo`
+  --> $DIR/issue-62187-encountered-polymorphic-const.rs:15:9
+   |
+LL |     let foo = <[u8; 2]>::BIT_LEN;
+   |         ^^^ help: consider prefixing with an underscore: `_foo`
+   |
+   = note: `#[warn(unused_variables)]` on by default
+