Add regression test for #102206

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

src/test/ui/async-await/issues/issue-102206.rs

@@ -0,0 +1,8 @@
+// edition:2021
+
+async fn foo() {}
+
+fn main() {
+    std::mem::size_of_val(foo());
+    //~^ ERROR: mismatched types
+}

src/test/ui/async-await/issues/issue-102206.stderr

@@ -0,0 +1,23 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-102206.rs:6:27
+   |
+LL |     std::mem::size_of_val(foo());
+   |     --------------------- ^^^^^
+   |     |                     |
+   |     |                     expected reference, found opaque type
+   |     |                     help: consider borrowing here: `&foo()`
+   |     arguments to this function are incorrect
+   |
+note: while checking the return type of the `async fn`
+  --> $DIR/issue-102206.rs:3:16
+   |
+LL | async fn foo() {}
+   |                ^ checked the `Output` of this `async fn`, found opaque type
+   = note: expected reference `&_`
+            found opaque type `impl Future<Output = ()>`
+note: function defined here
+  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.