Add test self-in-enum-definition

Author: Centril

src/test/ui/type-alias-enum-variants/self-in-enum-definition.rs

@@ -0,0 +1,8 @@
+#[repr(u8)]
+enum Alpha {
+    V1 = 41,
+    V2 = Self::V1 as u8 + 1, // OK -- but why?
+    V3 = Self::V1 {} as u8 + 2, //~ ERROR cycle detected when const-evaluating
+}
+
+fn main() {}

src/test/ui/type-alias-enum-variants/self-in-enum-definition.stderr

@@ -0,0 +1,28 @@
+error[E0391]: cycle detected when const-evaluating + checking `Alpha::V3::{{constant}}#0`
+  --> $DIR/self-in-enum-definition.rs:5:10
+   |
+LL |     V3 = Self::V1 {} as u8 + 2,
+   |          ^^^^^^^^
+   |
+note: ...which requires const-evaluating `Alpha::V3::{{constant}}#0`...
+  --> $DIR/self-in-enum-definition.rs:5:10
+   |
+LL |     V3 = Self::V1 {} as u8 + 2,
+   |          ^^^^^^^^
+   = note: ...which requires computing layout of `Alpha`...
+   = note: ...which again requires const-evaluating + checking `Alpha::V3::{{constant}}#0`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/self-in-enum-definition.rs:1:1
+   |
+LL | / #[repr(u8)]
+LL | | enum Alpha {
+LL | |     V1 = 41,
+LL | |     V2 = Self::V1 as u8 + 1, // OK -- but why?
+...  |
+LL | |
+LL | | fn main() {}
+   | |____________^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.