[PEP 695] Fix multiple nested classes don't work (#17820)

This PR modifies the `lookup_fully_qualified_or_none` method to support
multiple nested classes.

Fixes #17780

mypy/semanal.py

@@ -6461,18 +6461,46 @@ def lookup_fully_qualified_or_none(self, fullname: str) -> SymbolTableNode | Non
         Note that this can't be used for names nested in class namespaces.
         """
         # TODO: unify/clean-up/simplify lookup methods, see #4157.
-        # TODO: support nested classes (but consider performance impact,
-        #       we might keep the module level only lookup for thing like 'builtins.int').
-        assert "." in fullname
         module, name = fullname.rsplit(".", maxsplit=1)
-        if module not in self.modules:
-            return None
-        filenode = self.modules[module]
-        result = filenode.names.get(name)
-        if result is None and self.is_incomplete_namespace(module):
-            # TODO: More explicit handling of incomplete refs?
-            self.record_incomplete_ref()
-        return result
+
+        if module in self.modules:
+            # If the module exists, look up the name in the module.
+            # This is the common case.
+            filenode = self.modules[module]
+            result = filenode.names.get(name)
+            if result is None and self.is_incomplete_namespace(module):
+                # TODO: More explicit handling of incomplete refs?
+                self.record_incomplete_ref()
+            return result
+        else:
+            # Else, try to find the longest prefix of the module name that is in the modules dictionary.
+            splitted_modules = fullname.split(".")
+            names = []
+
+            while splitted_modules and ".".join(splitted_modules) not in self.modules:
+                names.append(splitted_modules.pop())
+
+            if not splitted_modules or not names:
+                # If no module or name is found, return None.
+                return None
+
+            # Reverse the names list to get the correct order of names.
+            names.reverse()
+
+            module = ".".join(splitted_modules)
+            filenode = self.modules[module]
+            result = filenode.names.get(names[0])
+
+            if result is None and self.is_incomplete_namespace(module):
+                # TODO: More explicit handling of incomplete refs?
+                self.record_incomplete_ref()
+
+            for part in names[1:]:
+                if result is not None and isinstance(result.node, TypeInfo):
+                    result = result.node.names.get(part)
+                else:
+                    return None
+            return result
 
     def object_type(self) -> Instance:
         return self.named_type("builtins.object")

test-data/unit/check-python312.test

@@ -1873,3 +1873,79 @@ d1: Multi[int, str] = Multi[float, str]()  # E: Incompatible types in assignment
 d2: Multi[float, str] = Multi[int, str]()  # E: Incompatible types in assignment (expression has type "Multi[int, str]", variable has type "Multi[float, str]")
 d3: Multi[str, int] = Multi[str, float]()
 d4: Multi[str, float] = Multi[str, int]()  # E: Incompatible types in assignment (expression has type "Multi[str, int]", variable has type "Multi[str, float]")
+
+[case testPEP695MultipleNestedGenericClass1]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+class A:
+    class B:
+        class C:
+            class D[Q]:
+                def g(self, x: Q): ...
+            d: D[str]
+
+x: A.B.C.D[int]
+x.g('a') # E: Argument 1 to "g" of "D" has incompatible type "str"; expected "int"
+reveal_type(x)  # N: Revealed type is "__main__.A.B.C.D[builtins.int]"
+reveal_type(A.B.C.d)  # N: Revealed type is "__main__.A.B.C.D[builtins.str]"
+
+[case testPEP695MultipleNestedGenericClass2]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+class A:
+    class B:
+        def m(self) -> None:
+            class C[T]:
+                def f(self) -> T: ...
+            x: C[int]
+            reveal_type(x.f())  # N: Revealed type is "builtins.int"
+            self.a = C[str]()
+
+reveal_type(A().B().a) # N: Revealed type is "__main__.C@5[builtins.str]"
+
+[case testPEP695MultipleNestedGenericClass3]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+class A:
+    class C[T]:
+        def f(self) -> T: ...
+        class D[S]:
+            x: T  # E: Name "T" is not defined
+            def g(self) -> S: ...
+
+a: A.C[int]
+reveal_type(a.f())  # N: Revealed type is "builtins.int"
+b: A.C.D[str]
+reveal_type(b.g())  # N: Revealed type is "builtins.str"
+
+class B:
+    class E[T]:
+        class F[T]:  # E: "T" already defined as a type parameter
+            x: T
+
+c: B.E.F[int]
+
+[case testPEP695MultipleNestedGenericClass4]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+class Z:
+    class A:
+        class B[T]:
+            def __get__(self, instance: Z.A, owner: type[Z.A]) -> T:
+                return None  # E: Incompatible return value type (got "None", expected "T")
+        f = B[int]()
+
+a = Z.A()
+v = a.f
+
+[case testPEP695MultipleNestedGenericClass5]
+# flags: --enable-incomplete-feature=NewGenericSyntax
+from a.b.c import d
+x: d.D.E.F.G[int]
+x.g('a') # E: Argument 1 to "g" of "G" has incompatible type "str"; expected "int"
+reveal_type(x)  # N: Revealed type is "a.b.c.d.D.E.F.G[builtins.int]"
+reveal_type(d.D.E.F.d)  # N: Revealed type is "a.b.c.d.D.E.F.G[builtins.str]"
+
+[file a/b/c/d.py]
+class D:
+    class E:
+        class F:
+            class G[Q]:
+                def g(self, x: Q): ...
+            d: G[str]

test-data/unit/fine-grained-python312.test

@@ -95,3 +95,23 @@ def f(x: int) -> None: pass
 [out]
 ==
 main:7: error: Missing positional argument "x" in call to "f"
+
+[case testPEP695MultipleNestedGenericClassMethodUpdated]
+from a import f
+
+class A:
+    class C:
+        class D[T]:
+            x: T
+            def m(self) -> T:
+                f()
+                return self.x
+
+[file a.py]
+def f() -> None: pass
+
+[file a.py.2]
+def f(x: int) -> None: pass
+[out]
+==
+main:8: error: Missing positional argument "x" in call to "f"