Fix type inference, isinstance and typevar values

If a if block, that is skipped due to an isinstance test that is
always False, contains the primamry initialization of a variable, the
second initializer in the else block is used to infer the type of
variable instead.

Author: JukkaL

mypy/checker.py

@@ -502,11 +502,19 @@ def check_assignments(self, lvalues: List[Node],
 
     def is_definition(self, s: Node) -> bool:
         if isinstance(s, NameExpr):
-            return s.is_def
+            if s.is_def:
+                return True
+            # If the node type is not defined, this must the first assignment
+            # that we process => this is a definition, even though the semantic
+            # analyzer did not recognize this as such. This can arise in code
+            # that uses isinstance checks, if type checking of the primary
+            # definition is skipped due to an always False type check.
+            node = s.node
+            if isinstance(node, Var):
+                return node.type is None
         elif isinstance(s, MemberExpr):
             return s.is_def
-        else:
-            return False
+        return False
 
     def expand_lvalues(self, n: Node) -> List[Node]:
         if isinstance(n, TupleExpr):

mypy/test/data/check-typevar-values.test

@@ -148,6 +148,43 @@ def f(x: T) -> T:
 [out]
 main: In function "f":
 
+[case testIsinstanceAndTypeVarValues3]
+from typing import typevar, Undefined
+T = typevar('T', values=(int, str))
+def f(x: T) -> T:
+    if isinstance(x, int):
+        y = 1
+    else:
+        y = ''
+    return y
+[builtins fixtures/isinstance.py]
+
+[case testIsinstanceAndTypeVarValues4]
+from typing import typevar, Undefined
+T = typevar('T', values=(int, str))
+def f(x: T) -> T:
+    if isinstance(x, int):
+        y = 1
+    else:
+        y = object()
+    return y # E: Incompatible return value type
+[builtins fixtures/isinstance.py]
+[out]
+main: In function "f":
+
+[case testIsinstanceAndTypeVarValues5]
+from typing import typevar, Undefined
+T = typevar('T', values=(int, str))
+def f(x: T) -> T:
+    if isinstance(x, int):
+        y = object()
+    else:
+        y = ''
+    return y # E: Incompatible return value type
+[builtins fixtures/isinstance.py]
+[out]
+main: In function "f":
+
 [case testTypeVarValuesAndNestedCalls]
 from typing import typevar
 T = typevar('T', values=(int, str))