Fix [name-defined] false-positive in class A[X, Y=X]: case (#20021)

This a WIP to see the test result before adding my own tests :)

Closes #20020

Author: sobolevn

mypy/semanal.py

@@ -1791,11 +1791,10 @@ def push_type_args(
                 return None
             tvs.append((p.name, tv))
 
-        for name, tv in tvs:
-            if self.is_defined_type_param(name):
-                self.fail(f'"{name}" already defined as a type parameter', context)
+            if self.is_defined_type_param(p.name):
+                self.fail(f'"{p.name}" already defined as a type parameter', context)
             else:
-                self.add_symbol(name, tv, context, no_progress=True, type_param=True)
+                self.add_symbol(p.name, tv, context, no_progress=True, type_param=True)
 
         return tvs
 

test-data/unit/check-python313.test

@@ -290,3 +290,32 @@ reveal_type(A1().x)  # N: Revealed type is "builtins.int"
 reveal_type(A2().x)  # N: Revealed type is "builtins.int"
 reveal_type(A3().x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/tuple.pyi]
+
+[case testTypeVarDefaultToAnotherTypeVar]
+class A[X, Y = X, Z = Y]:
+    x: X
+    y: Y
+    z: Z
+
+a1: A[int]
+reveal_type(a1.x)  # N: Revealed type is "builtins.int"
+reveal_type(a1.y)  # N: Revealed type is "builtins.int"
+# TODO: this must reveal `int` as well:
+reveal_type(a1.z)  # N: Revealed type is "X`1"
+
+a2: A[int, str]
+reveal_type(a2.x)  # N: Revealed type is "builtins.int"
+reveal_type(a2.y)  # N: Revealed type is "builtins.str"
+reveal_type(a2.z)  # N: Revealed type is "builtins.str"
+
+a3: A[int, str, bool]
+reveal_type(a3.x)  # N: Revealed type is "builtins.int"
+reveal_type(a3.y)  # N: Revealed type is "builtins.str"
+reveal_type(a3.z)  # N: Revealed type is "builtins.bool"
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarDefaultToAnotherTypeVarWrong]
+class A[Y = X, X = int]: ...  # E: Name "X" is not defined
+
+class B[Y = X]: ...  # E: Name "X" is not defined
+[builtins fixtures/tuple.pyi]