Avoid false possibly-undefined errors due to omitted unrequired else statements. (#20149)

Fixes #14771 

The mentioned issue's `except: assert_never` examples are already fixed
by #15995. This PR only targets the "no unreachable else" examples.

Introducing the special-purpose attribute
`IfStmt.else_irrelevant_for_possibly_undefined` is not super nice, but
it appears more straightforward and less risky than starting to
introduce empty else blocks with `is_unreachable = True` (as discussed
[here](#13926 (comment))).

Author: tyralla

mypy/checker.py

@@ -5070,6 +5070,8 @@ def visit_if_stmt(self, s: IfStmt) -> None:
 
                 if_map, else_map = self.find_isinstance_check(e)
 
+                s.unreachable_else = else_map is None
+
                 # XXX Issue a warning if condition is always False?
                 with self.binder.frame_context(can_skip=True, fall_through=2):
                     self.push_type_map(if_map, from_assignment=False)

mypy/nodes.py

@@ -1832,19 +1832,26 @@ def accept(self, visitor: StatementVisitor[T]) -> T:
 
 
 class IfStmt(Statement):
-    __slots__ = ("expr", "body", "else_body")
+    __slots__ = ("expr", "body", "else_body", "unreachable_else")
 
-    __match_args__ = ("expr", "body", "else_body")
+    __match_args__ = ("expr", "body", "else_body", "unreachable_else")
 
     expr: list[Expression]
     body: list[Block]
     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 current type information.)
 
     def __init__(self, expr: list[Expression], body: list[Block], else_body: Block | None) -> None:
         super().__init__()
         self.expr = expr
         self.body = body
         self.else_body = else_body
+        self.unreachable_else = False
 
     def accept(self, visitor: StatementVisitor[T]) -> T:
         return visitor.visit_if_stmt(self)

mypy/partially_defined.py

@@ -397,11 +397,13 @@ def visit_if_stmt(self, o: IfStmt) -> None:
                 continue
             b.accept(self)
             self.tracker.next_branch()
-        if o.else_body:
-            if not o.else_body.is_unreachable:
-                o.else_body.accept(self)
-            else:
+        if o.unreachable_else:
+            self.tracker.skip_branch()
+        elif o.else_body:
+            if o.else_body.is_unreachable:
                 self.tracker.skip_branch()
+            else:
+                o.else_body.accept(self)
         self.tracker.end_branch_statement()
 
     def visit_match_stmt(self, o: MatchStmt) -> None:

test-data/unit/check-possibly-undefined.test

@@ -1054,3 +1054,27 @@ def foo(x: Union[int, str]) -> None:
         assert_never(x)
     f  # OK
 [builtins fixtures/tuple.pyi]
+
+[case testOmittedUnrequiredElse]
+# flags: --enable-error-code possibly-undefined
+from typing import Literal
+
+a: Literal[True]
+if a:
+    w = 1
+w + 1
+
+b: bool
+if b:
+    x = 1
+elif not b:
+    x = 2
+x + 1
+
+if b:
+    y = 1
+elif not b:
+    if a:
+        z = 2
+    y = z
+y + 1