Add test demonstrating existing behaviour.

This commit adds a test that demonstrates the compiler's current
behaviour when a function attempts to return a value that was unwrapped
by a `?` operator when the omission of `?` would have made the code
compile.

Author: davidtwco

src/test/ui/issue-59756.rs

@@ -0,0 +1,15 @@
+#![allow(warnings)]
+
+struct A;
+struct B;
+
+fn foo() -> Result<A, B> {
+    Ok(A)
+}
+
+fn bar() -> Result<A, B> {
+    foo()?
+    //~^ ERROR try expression alternatives have incompatible types [E0308]
+}
+
+fn main() {}

src/test/ui/issue-59756.stderr

@@ -0,0 +1,12 @@
+error[E0308]: try expression alternatives have incompatible types
+  --> $DIR/issue-59756.rs:11:5
+   |
+LL |     foo()?
+   |     ^^^^^^ expected enum `std::result::Result`, found struct `A`
+   |
+   = note: expected type `std::result::Result<A, B>`
+              found type `A`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.