Add test for issue-47486

src/test/ui/issues/issue-47486.rs

@@ -0,0 +1,4 @@
+fn main() {
+    () < std::mem::size_of::<_>(); //~ ERROR: mismatched types
+    [0u8; std::mem::size_of::<_>()]; //~ ERROR: type annotations needed
+}

src/test/ui/issues/issue-47486.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-47486.rs:2:10
+   |
+LL |     () < std::mem::size_of::<_>();
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found usize
+   |
+   = note: expected type `()`
+              found type `usize`
+
+error[E0282]: type annotations needed
+  --> $DIR/issue-47486.rs:3:11
+   |
+LL |     [0u8; std::mem::size_of::<_>()];
+   |           ^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0282, E0308.
+For more information about an error, try `rustc --explain E0282`.