New semantic analyzer: support --allow-redefinition (#6466)

Fixes #6317.

Author: JukkaL

mypy/build.py

@@ -59,6 +59,7 @@
 from mypy.fscache import FileSystemCache
 from mypy.metastore import MetadataStore, FilesystemMetadataStore, SqliteMetadataStore
 from mypy.typestate import TypeState, reset_global_state
+from mypy.renaming import VariableRenameVisitor
 
 from mypy.mypyc_hacks import BuildManagerBase
 
@@ -1916,6 +1917,9 @@ def semantic_analysis_pass1(self) -> None:
                 analyzer.visit_file(self.tree, self.xpath, self.id, options)
             # TODO: Do this while contructing the AST?
             self.tree.names = SymbolTable()
+            if options.allow_redefinition:
+                # Perform renaming across the AST to allow variable redefinitions
+                self.tree.accept(VariableRenameVisitor())
         else:
             # Do the first pass of semantic analysis: add top-level
             # definitions in the file to the symbol table.  We must do

mypy/newsemanal/semanal.py

@@ -4180,14 +4180,18 @@ def name_already_defined(self, name: str, ctx: Context,
             # Therefore its line number is always 1, which is not useful for this
             # error message.
             extra_msg = ' (by an import)'
-        elif node and node.line != -1 and self.cur_mod_id in split_module_names(node.fullname()):
+        elif node and node.line != -1 and self.is_local_name(node.fullname()):
             # TODO: Using previous symbol node may give wrong line. We should use
             #       the line number where the binding was established instead.
             extra_msg = ' on line {}'.format(node.line)
         else:
             extra_msg = ' (possibly by an import)'
         self.fail("Name '{}' already defined{}".format(unmangle(name), extra_msg), ctx)
 
+    def is_local_name(self, name: str) -> bool:
+        """Does name look like reference to a definition in the current module?"""
+        return self.cur_mod_id in split_module_names(name) or '.' not in name
+
     def fail(self, msg: str, ctx: Context, serious: bool = False, *,
              blocker: bool = False) -> None:
         if (not serious and

mypy/test/hacks.py

@@ -30,7 +30,6 @@
     'check-overloading.test',
     'check-protocols.test',
     'check-python2.test',
-    'check-redefine.test',
     'check-semanal-error.test',
     'check-serialize.test',
     'check-statements.test',