also print the expected type in the error message

src/test/compile-fail/const-eval-overflow-4b.rs

@@ -21,7 +21,9 @@ use std::{u8, u16, u32, u64, usize};
 
 const A_I8_T
     : [u32; (i8::MAX as i8 + 1u8) as usize]
-    //~^ ERROR tried to add two integrals of different types [E0250]
+    //~^ ERROR mismatched types:
+    //~| expected `i8`,
+    //~| found `u8` [E0250]
     = [0; (i8::MAX as usize) + 1];
 
 fn main() {