normalize opaque alias type

compiler/rustc_const_eval/src/transform/validate.rs

@@ -677,7 +677,14 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
 
                 let kind = match parent_ty.ty.kind() {
                     &ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
-                        self.tcx.type_of(def_id).instantiate(self.tcx, args).kind()
+                        let ty = self.tcx.type_of(def_id).instantiate(self.tcx, args);
+                        // If the type we get is opaque, we want to normalize it.
+                        if ty.is_impl_trait() {
+                            let inner_ty = self.tcx.expand_opaque_types(ty).kind();
+                            inner_ty
+                        } else {
+                            ty.kind()
+                        }
                     }
                     kind => kind,
                 };

tests/ui/type-alias-impl-trait/normalize-alias-type.rs

@@ -0,0 +1,32 @@
+// check-pass
+// compile-flags: -Z mir-opt-level=3
+#![feature(type_alias_impl_trait)]
+#![crate_type = "lib"]
+pub trait Tr {
+    fn get(&self) -> u32;
+}
+
+impl Tr for (u32,) {
+    #[inline]
+    fn get(&self) -> u32 { self.0 }
+}
+
+pub fn tr1() -> impl Tr {
+    (32,)
+}
+
+pub fn tr2() -> impl Tr {
+    struct Inner {
+        x: X,
+    }
+    type X = impl Tr;
+    impl Tr for Inner {
+        fn get(&self) -> u32 {
+            self.x.get()
+        }
+    }
+
+    Inner {
+        x: tr1(),
+    }
+}