coerce_inner: use initial expected_ty

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`.