Fix scoping of list comprehensions within class bodies

Fixes #1000.

mypy/semanal.py

@@ -1780,26 +1780,41 @@ def visit_dictionary_comprehension(self, expr: DictionaryComprehension) -> None:
         expr.key.accept(self)
         expr.value.accept(self)
         self.leave()
+        self.analyze_comp_for_2(expr)
 
     def visit_generator_expr(self, expr: GeneratorExpr) -> None:
         self.enter()
         self.analyze_comp_for(expr)
         expr.left_expr.accept(self)
         self.leave()
+        self.analyze_comp_for_2(expr)
 
     def analyze_comp_for(self, expr: Union[GeneratorExpr,
                                            DictionaryComprehension]) -> None:
-        """Analyses the 'comp_for' part of comprehensions.
-        That is the part after 'for' in (x for x in l if p)
+        """Analyses the 'comp_for' part of comprehensions (part 1).
+
+        That is the part after 'for' in (x for x in l if p). This analyzes
+        variables and conditions which are analyzed in a local scope.
         """
-        for index, sequence, conditions in zip(expr.indices, expr.sequences,
-                                               expr.condlists):
-            sequence.accept(self)
+        for i, (index, sequence, conditions) in enumerate(zip(expr.indices,
+                                                              expr.sequences,
+                                                              expr.condlists)):
+            if i > 0:
+                sequence.accept(self)
             # Bind index variables.
             self.analyze_lvalue(index)
             for cond in conditions:
                 cond.accept(self)
 
+    def analyze_comp_for_2(self, expr: Union[GeneratorExpr,
+                                             DictionaryComprehension]) -> None:
+        """Analyses the 'comp_for' part of comprehensions (part 2).
+
+        That is the part after 'for' in (x for x in l if p). This analyzes
+        the 'l' part which is analyzed in the surrounding scope.
+        """
+        expr.sequences[0].accept(self)
+
     def visit_func_expr(self, expr: FuncExpr) -> None:
         self.analyze_function(expr)
 

mypy/test/data/check-expressions.test

@@ -1104,6 +1104,17 @@ o = [x for x in a] # type: List[object]
 class A: pass
 [builtins fixtures/for.py]
 
+[case testSimpleListComprehensionInClassBody]
+from typing import List
+class A:
+    a = None # type: List[A]
+    a = [x for x in a]
+    b = [x for x in a] # type: List[B] # E: List comprehension has incompatible type List[A]
+class B: pass
+[builtins fixtures/for.py]
+[out]
+main: note: In class "A":
+
 
 -- Set comprehension
 -- -----------------

mypy/test/data/semanal-errors.test

@@ -1410,3 +1410,13 @@ import tkinter
 main:1: error: No library stub file for standard library module 'tabnanny'
 main:1: note: (Stub files are from https://github.com/python/typeshed)
 main:2: error: No library stub file for standard library module 'tkinter'
+
+[case testListComprehensionSpecialScoping]
+class A:
+    x = 1
+    y = 1
+    z = 1
+    [x for i in z if y]
+[out]
+main:5: error: Name 'y' is not defined
+main:5: error: Name 'x' is not defined