Require that if the new type and LUB can coerce to each other, that they the coerced types must be the same.

Author: jackh726

compiler/rustc_hir_typeck/src/coercion.rs

@@ -454,10 +454,22 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
         let result = self.commit_if_ok(|_| coerce_inner(new_ty, prev_ty));
         let first_error = match result {
-            Ok(_) => {
-                return result.map(|r| InferOk {
-                    obligations: r.obligations,
-                    value: (r.value.0, r.value.1, CoerceDirection::ToPrev),
+            Ok(forwd_result) => {
+                let prev_ty = self.resolve_vars_if_possible(prev_ty);
+                let new_ty = self.resolve_vars_if_possible(new_ty);
+                let rev_result = self.commit_if_ok(|_| coerce_inner(prev_ty, new_ty));
+                if let Ok(rev_result) = rev_result {
+                    let forwd_ty = self.resolve_vars_if_possible(forwd_result.value.1);
+                    let rev_ty = self.resolve_vars_if_possible(rev_result.value.1);
+                    let res = self.unify(forwd_ty, rev_ty, ForceLeakCheck::No);
+                    if let Err(e) = res {
+                        return Err(e);
+                    }
+                }
+
+                return Ok(InferOk {
+                    obligations: forwd_result.obligations,
+                    value: (forwd_result.value.0, forwd_result.value.1, CoerceDirection::ToPrev),
                 });
             }
             Err(e) => e,

tests/ui/coercion/multistep3.rs

@@ -1,4 +1,4 @@
-//@ run-pass
+//@ check-fail
 
 #![feature(unsize, coerce_unsized)]
 #![allow(static_mut_refs)]
@@ -73,6 +73,7 @@ assert_arms(
             0 => &Wrap3(Inner)      as &I,
             1 => &Wrap3(Inner)      as &J,
             2 => &Wrap3(Inner)      as &K,
+            //~^ ERROR `match` arms have incompatible types
             _ => loop {},
         }.complete(),
         &[
@@ -87,6 +88,7 @@ assert_arms(
             0 => &Wrap3(Inner)      as &I,
             1 => &Wrap3(Inner)      as &K,
             2 => &Wrap3(Inner)      as &J,
+            //~^ ERROR `match` arms have incompatible types
             _ => loop {},
         }.complete(),
         &[

tests/ui/coercion/multistep3.stderr

@@ -0,0 +1,41 @@
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/multistep3.rs:75:18
+   |
+LL |           |i| match i {
+   |  _____________-
+LL | |             0 => &Wrap3(Inner)      as &I,
+   | |                  ------------------------ this is found to be of type `&Wrap3<dyn Dynable + Send>`
+LL | |             1 => &Wrap3(Inner)      as &J,
+   | |                  ------------------------ this is found to be of type `&Wrap3<dyn Dynable + Send>`
+LL | |             2 => &Wrap3(Inner)      as &K,
+   | |                  ^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `Dynable`, found trait `Dynable + Send`
+LL | |
+LL | |             _ => loop {},
+LL | |         }.complete(),
+   | |_________- `match` arms have incompatible types
+   |
+   = note: expected reference `&Wrap3<dyn Dynable + Send>`
+              found reference `&Wrap3<(dyn Dynable + 'static)>`
+
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/multistep3.rs:90:18
+   |
+LL |           |i| match i {
+   |  _____________-
+LL | |             0 => &Wrap3(Inner)      as &I,
+   | |                  ------------------------ this is found to be of type `&Wrap3<dyn Dynable>`
+LL | |             1 => &Wrap3(Inner)      as &K,
+   | |                  ------------------------ this is found to be of type `&Wrap3<dyn Dynable>`
+LL | |             2 => &Wrap3(Inner)      as &J,
+   | |                  ^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `Dynable + Send`, found trait `Dynable`
+LL | |
+LL | |             _ => loop {},
+LL | |         }.complete(),
+   | |_________- `match` arms have incompatible types
+   |
+   = note: expected reference `&Wrap3<dyn Dynable>`
+              found reference `&Wrap3<(dyn Dynable + Send + 'static)>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/coercion/multistep5.stderr

@@ -6,6 +6,25 @@ LL |         1 => &P      as &P,
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`multistep5`)
 
-error: aborting due to 1 previous error
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/multistep5.rs:84:14
+   |
+LL |       let _a = match 0 {
+   |  ______________-
+LL | |         0 => &O      as &O,
+   | |              ------------- this is found to be of type `&P`
+LL | |         1 => &P      as &P,
+   | |              ------------- this is found to be of type `&P`
+LL | |         2 => &Q      as &Q,
+   | |              ^^^^^^^^^^^^^ expected `&P`, found `&Q`
+LL | |         _ => loop {},
+LL | |     };
+   | |_____- `match` arms have incompatible types
+   |
+   = note: expected reference `&P`
+              found reference `&Q`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0055`.
+Some errors have detailed explanations: E0055, E0308.
+For more information about an error, try `rustc --explain E0055`.