[stubgen] Add required ... rhs to NamedTuple fields with default values (#15680)

Closes #15638

Author: sobolevn

mypy/stubgen.py

@@ -102,6 +102,7 @@
     OverloadedFuncDef,
     Statement,
     StrExpr,
+    TempNode,
     TupleExpr,
     TypeInfo,
     UnaryExpr,
@@ -637,6 +638,7 @@ def __init__(
         self._state = EMPTY
         self._toplevel_names: list[str] = []
         self._include_private = include_private
+        self._current_class: ClassDef | None = None
         self.import_tracker = ImportTracker()
         # Was the tree semantically analysed before?
         self.analyzed = analyzed
@@ -886,6 +888,7 @@ def get_fullname(self, expr: Expression) -> str:
         return resolved_name
 
     def visit_class_def(self, o: ClassDef) -> None:
+        self._current_class = o
         self.method_names = find_method_names(o.defs.body)
         sep: int | None = None
         if not self._indent and self._state != EMPTY:
@@ -922,6 +925,7 @@ def visit_class_def(self, o: ClassDef) -> None:
         else:
             self._state = CLASS
         self.method_names = set()
+        self._current_class = None
 
     def get_base_types(self, cdef: ClassDef) -> list[str]:
         """Get list of base classes for a class."""
@@ -1330,7 +1334,20 @@ def get_init(
                 typename += f"[{final_arg}]"
         else:
             typename = self.get_str_type_of_node(rvalue)
-        return f"{self._indent}{lvalue}: {typename}\n"
+        initializer = self.get_assign_initializer(rvalue)
+        return f"{self._indent}{lvalue}: {typename}{initializer}\n"
+
+    def get_assign_initializer(self, rvalue: Expression) -> str:
+        """Does this rvalue need some special initializer value?"""
+        if self._current_class and self._current_class.info:
+            # Current rules
+            # 1. Return `...` if we are dealing with `NamedTuple` and it has an existing default value
+            if self._current_class.info.is_named_tuple and not isinstance(rvalue, TempNode):
+                return " = ..."
+            # TODO: support other possible cases, where initializer is important
+
+        # By default, no initializer is required:
+        return ""
 
     def add(self, string: str) -> None:
         """Add text to generated stub."""

test-data/unit/stubgen.test

@@ -698,6 +698,62 @@ class Y(NamedTuple):
     a: int
     b: str
 
+[case testNamedTupleClassSyntax_semanal]
+from typing import NamedTuple
+
+class A(NamedTuple):
+    x: int
+    y: str = 'a'
+
+class B(A):
+    z1: str
+    z2 = 1
+    z3: str = 'b'
+
+class RegularClass:
+    x: int
+    y: str = 'a'
+    class NestedNamedTuple(NamedTuple):
+        x: int
+        y: str = 'a'
+    z: str = 'b'
+[out]
+from typing import NamedTuple
+
+class A(NamedTuple):
+    x: int
+    y: str = ...
+
+class B(A):
+    z1: str
+    z2: int
+    z3: str
+
+class RegularClass:
+    x: int
+    y: str
+    class NestedNamedTuple(NamedTuple):
+        x: int
+        y: str = ...
+    z: str
+
+
+[case testNestedClassInNamedTuple_semanal-xfail]
+from typing import NamedTuple
+
+# TODO: make sure that nested classes in `NamedTuple` are supported:
+class NamedTupleWithNestedClass(NamedTuple):
+    class Nested:
+        x: int
+        y: str = 'a'
+[out]
+from typing import NamedTuple
+
+class NamedTupleWithNestedClass(NamedTuple):
+    class Nested:
+        x: int
+        y: str
+
 [case testEmptyNamedtuple]
 import collections, typing
 X = collections.namedtuple('X', [])