stubgen: use PEP 604 unions everywhere (#16519)

Fixes #12920

mypy/stubgen.py

@@ -22,7 +22,7 @@
   => Generate out/urllib/parse.pyi.
 
   $ stubgen -p urllib
-  => Generate stubs for whole urlib package (recursively).
+  => Generate stubs for whole urllib package (recursively).
 
 For C modules, you can get more precise function signatures by parsing .rst (Sphinx)
 documentation for extra information. For this, use the --doc-dir option:
@@ -306,6 +306,13 @@ def visit_str_expr(self, node: StrExpr) -> str:
         return repr(node.value)
 
     def visit_index_expr(self, node: IndexExpr) -> str:
+        base_fullname = self.stubgen.get_fullname(node.base)
+        if base_fullname == "typing.Union":
+            if isinstance(node.index, TupleExpr):
+                return " | ".join([item.accept(self) for item in node.index.items])
+            return node.index.accept(self)
+        if base_fullname == "typing.Optional":
+            return f"{node.index.accept(self)} | None"
         base = node.base.accept(self)
         index = node.index.accept(self)
         if len(index) > 2 and index.startswith("(") and index.endswith(")"):
@@ -682,7 +689,7 @@ def process_decorator(self, o: Decorator) -> None:
                 self.add_decorator(qualname, require_name=False)
 
     def get_fullname(self, expr: Expression) -> str:
-        """Return the full name resolving imports and import aliases."""
+        """Return the expression's full name."""
         if (
             self.analyzed
             and isinstance(expr, (NameExpr, MemberExpr))
@@ -691,16 +698,7 @@ def get_fullname(self, expr: Expression) -> str:
         ):
             return expr.fullname
         name = get_qualified_name(expr)
-        if "." not in name:
-            real_module = self.import_tracker.module_for.get(name)
-            real_short = self.import_tracker.reverse_alias.get(name, name)
-            if real_module is None and real_short not in self.defined_names:
-                real_module = "builtins"  # not imported and not defined, must be a builtin
-        else:
-            name_module, real_short = name.split(".", 1)
-            real_module = self.import_tracker.reverse_alias.get(name_module, name_module)
-        resolved_name = real_short if real_module is None else f"{real_module}.{real_short}"
-        return resolved_name
+        return self.resolve_name(name)
 
     def visit_class_def(self, o: ClassDef) -> None:
         self._current_class = o

mypy/stubutil.py

@@ -226,6 +226,11 @@ def visit_any(self, t: AnyType) -> str:
 
     def visit_unbound_type(self, t: UnboundType) -> str:
         s = t.name
+        fullname = self.stubgen.resolve_name(s)
+        if fullname == "typing.Union":
+            return " | ".join([item.accept(self) for item in t.args])
+        if fullname == "typing.Optional":
+            return f"{t.args[0].accept(self)} | None"
         if self.known_modules is not None and "." in s:
             # see if this object is from any of the modules that we're currently processing.
             # reverse sort so that subpackages come before parents: e.g. "foo.bar" before "foo".
@@ -588,14 +593,18 @@ def __init__(
     def get_sig_generators(self) -> list[SignatureGenerator]:
         return []
 
-    def refers_to_fullname(self, name: str, fullname: str | tuple[str, ...]) -> bool:
-        """Return True if the variable name identifies the same object as the given fullname(s)."""
-        if isinstance(fullname, tuple):
-            return any(self.refers_to_fullname(name, fname) for fname in fullname)
-        module, short = fullname.rsplit(".", 1)
-        return self.import_tracker.module_for.get(name) == module and (
-            name == short or self.import_tracker.reverse_alias.get(name) == short
-        )
+    def resolve_name(self, name: str) -> str:
+        """Return the full name resolving imports and import aliases."""
+        if "." not in name:
+            real_module = self.import_tracker.module_for.get(name)
+            real_short = self.import_tracker.reverse_alias.get(name, name)
+            if real_module is None and real_short not in self.defined_names:
+                real_module = "builtins"  # not imported and not defined, must be a builtin
+        else:
+            name_module, real_short = name.split(".", 1)
+            real_module = self.import_tracker.reverse_alias.get(name_module, name_module)
+        resolved_name = real_short if real_module is None else f"{real_module}.{real_short}"
+        return resolved_name
 
     def add_name(self, fullname: str, require: bool = True) -> str:
         """Add a name to be imported and return the name reference.

test-data/unit/stubgen.test

@@ -4139,3 +4139,33 @@ from dataclasses import dataclass
 class X(missing.Base):
     a: int
     def __init__(self, *selfa_, a, **selfa__) -> None: ...
+
+[case testAlwaysUsePEP604Union]
+import typing
+import typing as t
+from typing import Optional, Union, Optional as O, Union as U
+import x
+
+union = Union[int, str]
+bad_union = Union[int]
+nested_union = Optional[Union[int, str]]
+not_union = x.Union[int, str]
+u = U[int, str]
+o = O[int]
+
+def f1(a: Union["int", Optional[tuple[int, t.Optional[int]]]]) -> int: ...
+def f2(a: typing.Union[int | x.Union[int, int], O[float]]) -> int: ...
+
+[out]
+import x
+from _typeshed import Incomplete
+
+union = int | str
+bad_union = int
+nested_union = int | str | None
+not_union: Incomplete
+u = int | str
+o = int | None
+
+def f1(a: int | tuple[int, int | None] | None) -> int: ...
+def f2(a: int | x.Union[int, int] | float | None) -> int: ...