coerce_inner: use initial expected_ty

Author: Centril

src/librustc_typeck/check/coercion.rs

@@ -1289,8 +1289,20 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                 }
 
                 // Error possibly reported in `check_assign` so avoid emitting error again.
-                err.emit_unless(expression.filter(|e| fcx.is_assign_to_bool(e, expected))
-                    .is_some());
+                let assign_to_bool = expression
+                    // #67273: Use initial expected type as opposed to `expected`.
+                    // Otherwise we end up using prior coercions in e.g. a `match` expression:
+                    // ```
+                    // match i {
+                    //     0 => true, // Because of this...
+                    //     1 => i = 1, // ...`expected == bool` now, but not when checking `i = 1`.
+                    //     _ => (),
+                    // };
+                    // ```
+                    .filter(|e| fcx.is_assign_to_bool(e, self.expected_ty()))
+                    .is_some();
+
+                err.emit_unless(assign_to_bool);
 
                 self.final_ty = Some(fcx.tcx.types.err);
             }

src/test/ui/type/type-check/issue-67273-assignment-match-prior-arm-bool-expected-unit.rs

@@ -0,0 +1,27 @@
+fn main() {
+    let mut i: i64;
+    // Expected type is an inference variable `?T`
+    // because the `match` is used as a statement.
+    // This is the "initial" type of the `coercion`.
+    match i {
+        // Add `bool` to the overall `coercion`.
+        0 => true,
+
+        // Necessary to cause the ICE:
+        1 => true,
+
+        // Suppose that we had `let _: bool = match i { ... }`.
+        // In that case, as the expected type would be `bool`,
+        // we would suggest `i == 1` as a fix.
+        //
+        // However, no type error happens when checking `i = 1` because `expected == ?T`,
+        // which will unify with `typeof(i = 1) == ()`.
+        //
+        // However, in #67273, we would delay the unification of this arm with the above
+        // because we used the hitherto accumulated coercion as opposed to the "initial" type.
+        2 => i = 1,
+        //~^ ERROR match arms have incompatible types
+
+        _ => (),
+    }
+}

src/test/ui/type/type-check/issue-67273-assignment-match-prior-arm-bool-expected-unit.stderr

@@ -0,0 +1,22 @@
+error[E0308]: match arms have incompatible types
+  --> $DIR/issue-67273-assignment-match-prior-arm-bool-expected-unit.rs:22:14
+   |
+LL | /     match i {
+LL | |         // Add `bool` to the overall `coercion`.
+LL | |         0 => true,
+   | |              ---- this is found to be of type `bool`
+LL | |
+LL | |         // Necessary to cause the ICE:
+LL | |         1 => true,
+   | |              ---- this is found to be of type `bool`
+...  |
+LL | |         2 => i = 1,
+   | |              ^^^^^ expected `bool`, found `()`
+...  |
+LL | |         _ => (),
+LL | |     }
+   | |_____- `match` arms have incompatible types
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.