Fix ClassDef.fromlineno for Python < 3.8

ChangeLog

@@ -42,6 +42,9 @@ Release date: TBA
 
   Closes #1085
 
+* Fix ``ClassDef.fromlineno``. For Python < 3.8 the ``lineno`` attribute includes decorators.
+  ``fromlineno`` should return the line of the ``class`` statement itself.
+
 What's New in astroid 2.9.4?
 ============================
 Release date: TBA

astroid/nodes/scoped_nodes/scoped_nodes.py

@@ -55,7 +55,7 @@
 from astroid import bases
 from astroid import decorators as decorators_mod
 from astroid import mixins, util
-from astroid.const import PY39_PLUS
+from astroid.const import PY38_PLUS, PY39_PLUS
 from astroid.context import (
     CallContext,
     InferenceContext,
@@ -1706,13 +1706,10 @@ def type(
         return type_name
 
     @decorators_mod.cachedproperty
-    def fromlineno(self):
-        """The first line that this node appears on in the source code.
-
-        :type: int or None
-        """
+    def fromlineno(self) -> Optional[int]:
+        """The first line that this node appears on in the source code."""
         # lineno is the line number of the first decorator, we want the def
-        # statement lineno
+        # statement lineno. Similar to 'ClassDef.fromlineno'
         lineno = self.lineno
         if self.decorators is not None:
             lineno += sum(
@@ -2297,6 +2294,21 @@ def _newstyle_impl(self, context=None):
         doc=("Whether this is a new style class or not\n\n" ":type: bool or None"),
     )
 
+    @decorators_mod.cachedproperty
+    def fromlineno(self) -> Optional[int]:
+        """The first line that this node appears on in the source code."""
+        if not PY38_PLUS:
+            # For Python < 3.8 the lineno is the line number of the first decorator.
+            # We want the class statement lineno. Similar to 'FunctionDef.fromlineno'
+            lineno = self.lineno
+            if self.decorators is not None:
+                lineno += sum(
+                    node.tolineno - node.lineno + 1 for node in self.decorators.nodes
+                )
+
+            return lineno
+        return super().fromlineno
+
     @decorators_mod.cachedproperty
     def blockstart_tolineno(self):
         """The line on which the beginning of this block ends.

tests/unittest_builder.py

@@ -29,6 +29,7 @@
 import os
 import socket
 import sys
+import textwrap
 import unittest
 
 import pytest
@@ -132,12 +133,54 @@ def function(
             __name__,
         )
         function = astroid["function"]
-        # XXX discussable, but that's what is expected by pylint right now
+        # XXX discussable, but that's what is expected by pylint right now, similar to ClassDef
         self.assertEqual(function.fromlineno, 3)
         self.assertEqual(function.tolineno, 5)
         self.assertEqual(function.decorators.fromlineno, 2)
         self.assertEqual(function.decorators.tolineno, 2)
 
+    @staticmethod
+    def test_decorated_class_lineno() -> None:
+        code = textwrap.dedent(
+            """
+        class A:
+            ...
+
+        @decorator
+        class B:
+            ...
+
+        @deco1
+        @deco2(
+            var=42
+        )
+        class C:
+            ...
+        """
+        )
+
+        ast_module: nodes.Module = builder.parse(code)  # type: ignore[assignment]
+
+        a = ast_module.body[0]
+        assert isinstance(a, nodes.ClassDef)
+        assert a.fromlineno == 2
+        assert a.tolineno == 3
+
+        b = ast_module.body[1]
+        assert isinstance(b, nodes.ClassDef)
+        assert b.fromlineno == 6
+        assert b.tolineno == 7
+
+        c = ast_module.body[2]
+        assert isinstance(c, nodes.ClassDef)
+        if not PY38_PLUS:
+            # Not perfect, but best we can do for Python 3.7
+            # Can't detect closing bracket on new line.
+            assert c.fromlineno == 12
+        else:
+            assert c.fromlineno == 13
+        assert c.tolineno == 14
+
     def test_class_lineno(self) -> None:
         stmts = self.astroid.body
         # on line 20: