Add test/comment about const patterns with unused params

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -625,6 +625,12 @@ pub fn try_evaluate_const<'tcx>(
                     // So we are free to simply defer evaluation here.
                     //
                     // FIXME: This assumes that `args` are normalized which is not necessarily true
+                    //
+                    // Const patterns are converted to type system constants before being
+                    // evaluated. However, we don't care about them here as pattern evaluation
+                    // logic does not go through type system normalization. If it did this would
+                    // be a backwards compatibility problem as we do not enforce "syntactic" non-
+                    // usage of generic parameters like we do here.
                     if uv.args.has_non_region_param() || uv.args.has_non_region_infer() {
                         return Err(EvaluateConstErr::HasGenericsOrInfers);
                     }

tests/ui/pattern/unused-parameters-const-pattern.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+
+// Tests that const patterns that use generic parameters are
+// allowed if we are still able to evaluate them.
+
+trait Trait { const ASSOC: usize; }
+
+impl<T> Trait for T {
+    const ASSOC: usize = 10;
+}
+
+fn foo<T>(a: usize) {
+    match a {
+        <T as Trait>::ASSOC => (),
+        _ => (),
+    }
+}
+
+fn main() {}