Subtype when normalization returns an infer var

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -68,7 +68,37 @@ impl<'tcx> InferCtxt<'tcx> {
             let normalized_alias = relation.try_eagerly_normalize_alias(*alias);
 
             if normalized_alias.is_ty_var() {
-                normalized_alias
+                // If normalization returns an infer var, we shouldn't directly output it.
+                // This can cause us to wrongly assume that another type is *exactly equal*
+                // to the alias that this infer var represents, while it may actually be a subtype.
+                // Unless the variance is invariant, we therefore generate a new infer var,
+                // with a subtyping relation to the infer var returned by normalization.
+                match instantiation_variance {
+                    ty::Invariant => normalized_alias,
+                    ty::Covariant => {
+                        let new_tyvar = self.next_ty_var(relation.span());
+                        relation.register_predicates([ty::PredicateKind::Subtype(
+                            ty::SubtypePredicate {
+                                a_is_expected: true,
+                                a: new_tyvar,
+                                b: normalized_alias,
+                            },
+                        )]);
+                        new_tyvar
+                    }
+                    ty::Contravariant => {
+                        let new_tyvar = self.next_ty_var(relation.span());
+                        relation.register_predicates([ty::PredicateKind::Subtype(
+                            ty::SubtypePredicate {
+                                a_is_expected: true,
+                                a: normalized_alias,
+                                b: new_tyvar,
+                            },
+                        )]);
+                        new_tyvar
+                    }
+                    ty::Bivariant => unreachable!("bivariant generalization"),
+                }
             } else {
                 let Generalization { value_may_be_infer: generalized_ty } = self.generalize(
                     relation.span(),

tests/ui/impl-trait/unsized_coercion.next.stderr

@@ -0,0 +1,35 @@
+error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
+  --> $DIR/unsized_coercion.rs:15:17
+   |
+LL |         let x = hello();
+   |                 ^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Trait`
+
+error[E0308]: mismatched types
+  --> $DIR/unsized_coercion.rs:19:5
+   |
+LL | fn hello() -> Box<impl Trait> {
+   |               ---------------
+   |               |   |
+   |               |   the expected opaque type
+   |               expected `Box<impl Trait>` because of return type
+...
+LL |     Box::new(1u32)
+   |     ^^^^^^^^^^^^^^ types differ
+   |
+   = note: expected struct `Box<impl Trait>`
+              found struct `Box<u32>`
+
+error[E0277]: the size for values of type `dyn Trait` cannot be known at compilation time
+  --> $DIR/unsized_coercion.rs:12:1
+   |
+LL | fn hello() -> Box<impl Trait> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `dyn Trait`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion.rs

@@ -3,18 +3,21 @@
 
 //@ revisions: next old
 //@[next] compile-flags: -Znext-solver
-//@ check-pass
+//@[old] check-pass
 
 trait Trait {}
 
 impl Trait for u32 {}
 
 fn hello() -> Box<impl Trait> {
+    //[next]~^ ERROR: the size for values of type `dyn Trait` cannot be known at compilation time
     if true {
         let x = hello();
+        //[next]~^ ERROR: the size for values of type `dyn Trait` cannot be known at compilation time
         let y: Box<dyn Trait> = x;
     }
     Box::new(1u32)
+    //[next]~^ ERROR: mismatched types
 }
 
 fn main() {}

tests/ui/impl-trait/unsized_coercion3.next.stderr

@@ -9,8 +9,6 @@ help: the trait `Trait` is implemented for `u32`
    |
 LL | impl Trait for u32 {}
    | ^^^^^^^^^^^^^^^^^^
-note: required by a bound in `Box`
-  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
 
 error[E0308]: mismatched types
   --> $DIR/unsized_coercion3.rs:19:5