[PEP 695] Don't crash when redefining something as a type alias (#17335)

Generate an error instead.

Work on #15238.

Author: JukkaL

mypy/semanal.py

@@ -5323,6 +5323,14 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
         all_type_params_names = [p.name for p in s.type_args]
 
         try:
+            existing = self.current_symbol_table().get(s.name.name)
+            if existing and not (
+                isinstance(existing.node, TypeAlias)
+                or (isinstance(existing.node, PlaceholderNode) and existing.node.line == s.line)
+            ):
+                self.already_defined(s.name.name, s, existing, "Name")
+                return
+
             tag = self.track_incomplete_refs()
             res, alias_tvars, depends_on, qualified_tvars, empty_tuple_index = self.analyze_alias(
                 s.name.name,
@@ -5378,7 +5386,6 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
                 python_3_12_type_alias=True,
             )
 
-            existing = self.current_symbol_table().get(s.name.name)
             if (
                 existing
                 and isinstance(existing.node, (PlaceholderNode, TypeAlias))

test-data/unit/check-python312.test

@@ -1453,3 +1453,39 @@ class E[T]:
 reveal_type(E[str]().a)  # N: Revealed type is "builtins.list[Any]"
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testPEP695RedefineAsTypeAlias1]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+class C: pass
+type C = int  # E: Name "C" already defined on line 2
+
+A = 0
+type A = str  # E: Name "A" already defined on line 5
+reveal_type(A)  # N: Revealed type is "builtins.int"
+
+[case testPEP695RedefineAsTypeAlias2]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from m import D
+type D = int  # E: Name "D" already defined (possibly by an import)
+a: D
+reveal_type(a)  # N: Revealed type is "m.D"
+[file m.py]
+class D: pass
+
+[case testPEP695RedefineAsTypeAlias3]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+D = list["Forward"]
+type D = int  # E: Name "D" already defined on line 2
+Forward = str
+x: D
+reveal_type(x)  # N: Revealed type is "builtins.list[builtins.str]"
+
+[case testPEP695MultiDefinitionsForTypeAlias]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+if int():
+    type A[T] = list[T]
+else:
+    type A[T] = str  # E: Name "A" already defined on line 3
+x: T  # E: Name "T" is not defined
+a: A[int]
+reveal_type(a)  # N: Revealed type is "builtins.list[builtins.int]"