fix wrong suggestions for boxed trait objects instead of impl trait

src/test/ui/mismatched_types/do-not-suggest-boxed-trait-objects-instead-of-impl-trait.rs

@@ -0,0 +1,23 @@
+struct S;
+struct Y;
+
+trait Trait {}
+
+impl Trait for Y {}
+
+fn foo() -> impl Trait {
+    if true {
+        S
+    } else {
+        Y //~ ERROR `if` and `else` have incompatible types
+    }
+}
+
+fn bar() -> impl Trait {
+    match true {
+        true => S,
+        false => Y, //~ ERROR `match` arms have incompatible types
+    }
+}
+
+fn main() {}

src/test/ui/mismatched_types/do-not-suggest-boxed-trait-objects-instead-of-impl-trait.stderr

@@ -0,0 +1,26 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/do-not-suggest-boxed-trait-objects-instead-of-impl-trait.rs:12:9
+   |
+LL | /     if true {
+LL | |         S
+   | |         - expected because of this
+LL | |     } else {
+LL | |         Y
+   | |         ^ expected struct `S`, found struct `Y`
+LL | |     }
+   | |_____- `if` and `else` have incompatible types
+
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/do-not-suggest-boxed-trait-objects-instead-of-impl-trait.rs:19:18
+   |
+LL | /     match true {
+LL | |         true => S,
+   | |                 - this is found to be of type `S`
+LL | |         false => Y,
+   | |                  ^ expected struct `S`, found struct `Y`
+LL | |     }
+   | |_____- `match` arms have incompatible types
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/mismatched_types/suggest-boxed-trait-objects-instead-of-impl-trait.fixed

@@ -10,7 +10,7 @@ trait Trait {}
 impl Trait for S {}
 impl Trait for Y {}
 
-fn baz() -> Box<dyn Trait> {
+fn foo() -> Box<dyn Trait> {
     if true {
         Box::new(S)
     } else {
@@ -18,4 +18,11 @@ fn baz() -> Box<dyn Trait> {
     }
 }
 
+fn bar() -> Box<dyn Trait> {
+    match true {
+        true => Box::new(S),
+        false => Box::new(Y), //~ ERROR `match` arms have incompatible types
+    }
+}
+
 fn main() {}

src/test/ui/mismatched_types/suggest-boxed-trait-objects-instead-of-impl-trait.rs

@@ -10,7 +10,7 @@ trait Trait {}
 impl Trait for S {}
 impl Trait for Y {}
 
-fn baz() -> impl Trait {
+fn foo() -> impl Trait {
     if true {
         S
     } else {
@@ -18,4 +18,11 @@ fn baz() -> impl Trait {
     }
 }
 
+fn bar() -> impl Trait {
+    match true {
+        true => S,
+        false => Y, //~ ERROR `match` arms have incompatible types
+    }
+}
+
 fn main() {}

src/test/ui/mismatched_types/suggest-boxed-trait-objects-instead-of-impl-trait.stderr

@@ -12,7 +12,7 @@ LL | |     }
    |
 help: you could change the return type to be a boxed trait object
    |
-LL | fn baz() -> Box<dyn Trait> {
+LL | fn foo() -> Box<dyn Trait> {
    |             ~~~~~~~      +
 help: if you change the return type to expect trait objects, box the returned expressions
    |
@@ -21,6 +21,27 @@ LL |     } else {
 LL ~         Box::new(Y)
    |
 
-error: aborting due to previous error
+error[E0308]: `match` arms have incompatible types
+  --> $DIR/suggest-boxed-trait-objects-instead-of-impl-trait.rs:24:18
+   |
+LL | /     match true {
+LL | |         true => S,
+   | |                 - this is found to be of type `S`
+LL | |         false => Y,
+   | |                  ^ expected struct `S`, found struct `Y`
+LL | |     }
+   | |_____- `match` arms have incompatible types
+   |
+help: you could change the return type to be a boxed trait object
+   |
+LL | fn bar() -> Box<dyn Trait> {
+   |             ~~~~~~~      +
+help: if you change the return type to expect trait objects, box the returned expressions
+   |
+LL ~         true => Box::new(S),
+LL ~         false => Box::new(Y),
+   |
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0308`.