Avoid crash in edge case of multiple assignment. Fix #974, # 1420, #1438.

Author: Guido van Rossum

mypy/checker.py

@@ -1143,6 +1143,8 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> Type:
         if len(s.lvalues) > 1:
             # Chained assignment (e.g. x = y = ...).
             # Make sure that rvalue type will not be reinferred.
+            if s.rvalue not in self.type_map:
+                self.accept(s.rvalue)
             rvalue = self.temp_node(self.type_map[s.rvalue], s)
             for lv in s.lvalues[:-1]:
                 self.check_assignment(lv, rvalue, s.type is None)

mypy/test/data/check-tuples.test

@@ -364,6 +364,12 @@ b = ''
 a = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
 b = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "str")
 
+[case testMultipleAssignmentWithMixedVariables]
+a = b, c = 1, 1
+x, y = p, q = 1, 1
+u, v, w = r, s = 1, 1 # E: Need more than 2 values to unpack (3 expected)
+d, e = f, g, h = 1, 1 # E: Need more than 2 values to unpack (3 expected)
+
 
 -- Assignment to starred expressions
 -- ---------------------------------