Complain abaout redefinition after only after we decided it is valid (#7093)

Fixes #6957

Using the opportunity, I do a tiny refactoring. The only change in the 
logic is switching the order of things.

Author: ilevkivskyi

mypy/newsemanal/semanal.py

@@ -2686,17 +2686,7 @@ def process_typevar_declaration(self, s: AssignmentStmt) -> bool:
             self.fail("Cannot declare the type of a type variable", s)
             return False
 
-        assert isinstance(s.rvalue, CallExpr)
         name = lvalue.name
-        names = self.current_symbol_table()
-        existing = names.get(name)
-        if existing and not (isinstance(existing.node, PlaceholderNode) or
-                             # Also give error for another type variable with the same name.
-                             (isinstance(existing.node, TypeVarExpr) and
-                              existing.node is s.rvalue.analyzed)):
-            self.fail("Cannot redefine '%s' as a type variable" % name, s)
-            return False
-
         if not self.check_typevar_name(call, name, s):
             return False
 
@@ -2713,6 +2703,14 @@ def process_typevar_declaration(self, s: AssignmentStmt) -> bool:
             return False
         variance, upper_bound = res
 
+        existing = self.current_symbol_table().get(name)
+        if existing and not (isinstance(existing.node, PlaceholderNode) or
+                             # Also give error for another type variable with the same name.
+                             (isinstance(existing.node, TypeVarExpr) and
+                              existing.node is call.analyzed)):
+            self.fail("Cannot redefine '%s' as a type variable" % name, s)
+            return False
+
         if self.options.disallow_any_unimported:
             for idx, constraint in enumerate(values, start=1):
                 if has_any_from_unimported_type(constraint):

test-data/unit/check-newsemanal.test

@@ -2774,3 +2774,14 @@ def g() -> None:
         nonlocal bar
         bar = []  # type: typing.List[int] # E: Name 'bar' already defined on line 11
 [builtins fixtures/list.pyi]
+
+[case testNewAnalyzerMoreInvalidTypeVarArgumentsDeferred]
+from typing import TypeVar, Generic
+
+defer: Yes
+
+S = TypeVar('S', covariant=True, contravariant=True)  # E: TypeVar cannot be both covariant and contravariant \
+                                                      # E: "int" not callable
+
+class Yes: ...
+[builtins fixtures/bool.pyi]