Fix a crash on walrus in comprehension at class scope (#14556)

Fixes #14201

The fix is trivial, turn an assert condition into a blocker error (with
message matching Python syntax error). I also add a test case for a
crash from the same issue that looks already fixed.

mypy/semanal.py

@@ -2658,8 +2658,33 @@ def visit_import_all(self, i: ImportAll) -> None:
 
     def visit_assignment_expr(self, s: AssignmentExpr) -> None:
         s.value.accept(self)
+        if self.is_func_scope():
+            if not self.check_valid_comprehension(s):
+                return
         self.analyze_lvalue(s.target, escape_comprehensions=True, has_explicit_value=True)
 
+    def check_valid_comprehension(self, s: AssignmentExpr) -> bool:
+        """Check that assignment expression is not nested within comprehension at class scope.
+
+        class C:
+            [(j := i) for i in [1, 2, 3]]
+        is a syntax error that is not enforced by Python parser, but at later steps.
+        """
+        for i, is_comprehension in enumerate(reversed(self.is_comprehension_stack)):
+            if not is_comprehension and i < len(self.locals) - 1:
+                if self.locals[-1 - i] is None:
+                    self.fail(
+                        "Assignment expression within a comprehension"
+                        " cannot be used in a class body",
+                        s,
+                        code=codes.SYNTAX,
+                        serious=True,
+                        blocker=True,
+                    )
+                    return False
+                break
+        return True
+
     def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         self.statement = s
 

test-data/unit/check-python38.test

@@ -734,3 +734,8 @@ class C(Generic[T]):
 [out]
 main:10: note: Revealed type is "builtins.int"
 main:10: note: Revealed type is "builtins.str"
+
+[case testNoCrashOnAssignmentExprClass]
+class C:
+    [(j := i) for i in [1, 2, 3]]  # E: Assignment expression within a comprehension cannot be used in a class body
+[builtins fixtures/list.pyi]

test-data/unit/check-statements.test

@@ -2194,3 +2194,7 @@ class B: pass
 
 def foo(x: int) -> Union[Generator[A, None, None], Generator[B, None, None]]:
     yield x  # E: Incompatible types in "yield" (actual type "int", expected type "Union[A, B]")
+
+[case testNoCrashOnStarRightHandSide]
+x = *(1, 2, 3)  # E: Can use starred expression only as assignment target
+[builtins fixtures/tuple.pyi]