Explain (the difference between) else_body.is_unreachable and unreachable_else.

Author: tyralla

mypy/nodes.py

@@ -1829,8 +1829,12 @@ class IfStmt(Statement):
 
     expr: list[Expression]
     body: list[Block]
-    else_body: Block | None
-    unreachable_else: bool
+    else_body: Block | None  # If there is actually no else statement, semantic analysis may nevertheless
+    #                          create an empty else block and mark it permanently as unreachable to tell
+    #                          that the control flow must always go through the if block.
+    unreachable_else: bool  # Type checking may modify this flag repeatedly to indicate whether an actually
+    #                         available or unavailable else block is unreachable, considering the currently
+    #                         available type information.
 
     def __init__(self, expr: list[Expression], body: list[Block], else_body: Block | None) -> None:
         super().__init__()