Elaborate predicates in min_specialization checks

Supertraits of specialization markers could circumvent checks for
min_specialization. Elaborating predicates prevents this.

src/test/ui/specialization/min_specialization/spec-marker-supertraits.rs

@@ -0,0 +1,29 @@
+// Check that supertraits cannot be used to work around min_specialization
+// limitations.
+
+#![feature(min_specialization)]
+#![feature(rustc_attrs)]
+
+trait HasMethod {
+    fn method(&self);
+}
+
+#[rustc_unsafe_specialization_marker]
+trait Marker: HasMethod {}
+
+trait Spec {
+    fn spec_me(&self);
+}
+
+impl<T> Spec for T {
+    default fn spec_me(&self) {}
+}
+
+impl<T: Marker> Spec for T {
+    //~^ ERROR cannot specialize on trait `HasMethod`
+    fn spec_me(&self) {
+        self.method();
+    }
+}
+
+fn main() {}

src/test/ui/specialization/min_specialization/spec-marker-supertraits.stderr

@@ -0,0 +1,13 @@
+error: cannot specialize on trait `HasMethod`
+  --> $DIR/spec-marker-supertraits.rs:22:1
+   |
+LL | / impl<T: Marker> Spec for T {
+LL | |
+LL | |     fn spec_me(&self) {
+LL | |         self.method();
+LL | |     }
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+