Added checks for invalid usage of continue/break/return in except* block (#18132)

Fixes #18123 

This PR addresses an issue where mypy incorrectly allows
break/continue/return statements in the except* block. (see
https://peps.python.org/pep-0654/#forbidden-combinations)

Author: coldwolverine

Diff for: mypy/semanal.py

@@ -484,6 +484,12 @@ def __init__(
         # Used to pass information about current overload index to visit_func_def().
         self.current_overload_item: int | None = None
 
+        # Used to track whether currently inside an except* block. This helps
+        # to invoke errors when continue/break/return is used inside except* block.
+        self.inside_except_star_block: bool = False
+        # Used to track edge case when return is still inside except* if it enters a loop
+        self.return_stmt_inside_except_star_block: bool = False
+
     # mypyc doesn't properly handle implementing an abstractproperty
     # with a regular attribute so we make them properties
     @property
@@ -511,6 +517,25 @@ def allow_unbound_tvars_set(self) -> Iterator[None]:
         finally:
             self.allow_unbound_tvars = old
 
+    @contextmanager
+    def inside_except_star_block_set(
+        self, value: bool, entering_loop: bool = False
+    ) -> Iterator[None]:
+        old = self.inside_except_star_block
+        self.inside_except_star_block = value
+
+        # Return statement would still be in except* scope if entering loops
+        if not entering_loop:
+            old_return_stmt_flag = self.return_stmt_inside_except_star_block
+            self.return_stmt_inside_except_star_block = value
+
+        try:
+            yield
+        finally:
+            self.inside_except_star_block = old
+            if not entering_loop:
+                self.return_stmt_inside_except_star_block = old_return_stmt_flag
+
     #
     # Preparing module (performed before semantic analysis)
     #
@@ -877,7 +902,8 @@ def visit_func_def(self, defn: FuncDef) -> None:
             return
 
         with self.scope.function_scope(defn):
-            self.analyze_func_def(defn)
+            with self.inside_except_star_block_set(value=False):
+                self.analyze_func_def(defn)
 
     def function_fullname(self, fullname: str) -> str:
         if self.current_overload_item is None:
@@ -5264,6 +5290,8 @@ def visit_return_stmt(self, s: ReturnStmt) -> None:
         self.statement = s
         if not self.is_func_scope():
             self.fail('"return" outside function', s)
+        if self.return_stmt_inside_except_star_block:
+            self.fail('"return" not allowed in except* block', s, serious=True)
         if s.expr:
             s.expr.accept(self)
 
@@ -5297,7 +5325,8 @@ def visit_while_stmt(self, s: WhileStmt) -> None:
         self.statement = s
         s.expr.accept(self)
         self.loop_depth[-1] += 1
-        s.body.accept(self)
+        with self.inside_except_star_block_set(value=False, entering_loop=True):
+            s.body.accept(self)
         self.loop_depth[-1] -= 1
         self.visit_block_maybe(s.else_body)
 
@@ -5321,20 +5350,24 @@ def visit_for_stmt(self, s: ForStmt) -> None:
                 s.index_type = analyzed
 
         self.loop_depth[-1] += 1
-        self.visit_block(s.body)
+        with self.inside_except_star_block_set(value=False, entering_loop=True):
+            self.visit_block(s.body)
         self.loop_depth[-1] -= 1
-
         self.visit_block_maybe(s.else_body)
 
     def visit_break_stmt(self, s: BreakStmt) -> None:
         self.statement = s
         if self.loop_depth[-1] == 0:
             self.fail('"break" outside loop', s, serious=True, blocker=True)
+        if self.inside_except_star_block:
+            self.fail('"break" not allowed in except* block', s, serious=True)
 
     def visit_continue_stmt(self, s: ContinueStmt) -> None:
         self.statement = s
         if self.loop_depth[-1] == 0:
             self.fail('"continue" outside loop', s, serious=True, blocker=True)
+        if self.inside_except_star_block:
+            self.fail('"continue" not allowed in except* block', s, serious=True)
 
     def visit_if_stmt(self, s: IfStmt) -> None:
         self.statement = s
@@ -5355,7 +5388,8 @@ def analyze_try_stmt(self, s: TryStmt, visitor: NodeVisitor[None]) -> None:
                 type.accept(visitor)
             if var:
                 self.analyze_lvalue(var)
-            handler.accept(visitor)
+            with self.inside_except_star_block_set(self.inside_except_star_block or s.is_star):
+                handler.accept(visitor)
         if s.else_body:
             s.else_body.accept(visitor)
         if s.finally_body:

Diff for: test-data/unit/check-python311.test

@@ -173,3 +173,89 @@ Alias4 = Callable[[*IntList], int]  # E: "List[int]" cannot be unpacked (must be
 x4: Alias4[int]  # E: Bad number of arguments for type alias, expected 0, given 1
 reveal_type(x4)  # N: Revealed type is "def (*Any) -> builtins.int"
 [builtins fixtures/tuple.pyi]
+
+[case testReturnInExceptStarBlock1]
+# flags: --python-version 3.11
+def foo() -> None:
+    try:
+        pass
+    except* Exception:
+        return   # E: "return" not allowed in except* block
+    finally:
+        return
+[builtins fixtures/exception.pyi]
+
+[case testReturnInExceptStarBlock2]
+# flags: --python-version 3.11
+def foo():
+    while True:
+        try:
+            pass
+        except* Exception:
+            while True:
+                return  # E: "return" not allowed in except* block
+[builtins fixtures/exception.pyi]
+
+[case testContinueInExceptBlockNestedInExceptStarBlock]
+# flags: --python-version 3.11
+while True:
+    try:
+        ...
+    except* Exception:
+        try:
+            ...
+        except Exception:
+            continue  # E: "continue" not allowed in except* block
+        continue  # E: "continue" not allowed in except* block
+[builtins fixtures/exception.pyi]
+
+[case testReturnInExceptBlockNestedInExceptStarBlock]
+# flags: --python-version 3.11
+def foo():
+    try:
+        ...
+    except* Exception:
+        try:
+            ...
+        except Exception:
+            return  # E: "return" not allowed in except* block
+        return  # E: "return" not allowed in except* block
+[builtins fixtures/exception.pyi]
+
+[case testBreakContinueReturnInExceptStarBlock1]
+# flags: --python-version 3.11
+from typing import Iterable
+def foo(x: Iterable[int]) -> None:
+    for _ in x:
+        try:
+            pass
+        except* Exception:
+            continue   # E: "continue" not allowed in except* block
+        except* Exception:
+            for _ in x:
+                continue
+            break  # E: "break" not allowed in except* block
+        except* Exception:
+            return  # E: "return" not allowed in except* block
+[builtins fixtures/exception.pyi]
+
+[case testBreakContinueReturnInExceptStarBlock2]
+# flags: --python-version 3.11
+def foo():
+    while True:
+        try:
+            pass
+        except* Exception:
+            def inner():
+                while True:
+                    if 1 < 1:
+                        continue
+                    else:
+                        break
+                return
+            if 1 < 2:
+                break  # E: "break" not allowed in except* block
+            if 1 < 2:
+                continue  # E: "continue" not allowed in except* block
+            return  # E: "return" not allowed in except* block
+[builtins fixtures/exception.pyi]