Fixes #2589: type redeclarations cause spurious warnings (#2591)

This addresses the problem by refusing to re-bind the explit type for a name
once one type was already present.  This means it changes the logic from "last
type declaration wins" to "first type declaration wins".  The error about
redeclaring the type is still being reported but the type does not change,
which changes the errors given, producing less surprising results.

Test plan: added a new test case to unit/semanal.  Without the change, there's
spurious errors about names not conforming to the type defined last.  With the
change, only the "Name already defined" errors remain.

Author: ambv

mypy/semanal.py

@@ -1309,10 +1309,11 @@ def analyze_lvalue(self, lval: Lvalue, nested: bool = False,
                 lval.kind = MDEF
                 lval.fullname = lval.name
                 self.type.names[lval.name] = SymbolTableNode(MDEF, v)
+            elif explicit_type:
+                # Don't re-bind types
+                self.name_already_defined(lval.name, lval)
             else:
                 # Bind to an existing name.
-                if explicit_type:
-                    self.name_already_defined(lval.name, lval)
                 lval.accept(self)
                 self.check_lvalue_validity(lval.node, lval)
         elif isinstance(lval, MemberExpr):

test-data/unit/semanal-errors.test

@@ -1311,3 +1311,13 @@ class A:
 [out]
 main:5: error: Name 'x' is not defined
 main:5: error: Name 'y' is not defined
+
+[case testTypeRedeclarationNoSpuriousWarnings]
+from typing import Tuple
+a = 1  # type: int
+a = 's'  # type: str
+a = ('spam', 'spam', 'eggs', 'spam')  # type: Tuple[str]
+
+[out]
+main:3: error: Name 'a' already defined
+main:4: error: Name 'a' already defined