Rollup merge of #65753 - csmoe:derive_fold, r=Centril

Don't assert for different instance on impl trait alias

Closes #65679
r? @Centril @nikomatsakis

Author: Centril

src/librustc_typeck/check/mod.rs

@@ -2767,8 +2767,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let mut opaque_types = self.opaque_types.borrow_mut();
         for (ty, decl) in opaque_type_map {
-            let old_value = opaque_types.insert(ty, decl);
-            assert!(old_value.is_none(), "instantiated twice: {:?}/{:?}", ty, decl);
+            let _ = opaque_types.insert(ty, decl);
         }
 
         value

src/test/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+type T = impl Sized;
+// The concrete type referred by impl-trait-type-alias(`T`) is guaranteed
+// to be the same as where it occurs, whereas `impl Trait`'s instance is location sensitive;
+// so difference assertion should not be declared on impl-trait-type-alias's instances.
+// for details, check RFC-2515:
+// https://github.com/rust-lang/rfcs/blob/master/text/2515-type_alias_impl_trait.md
+
+fn take(_: fn() -> T) {}
+
+fn main() {
+    take(|| {});
+    take(|| {});
+}