[Backport maintenance/4.0.x] Handle FunctionDef blockstart_tolineno edge cases (#2881)

Handle FunctionDef blockstart_tolineno edge cases (#2880)

Getting the lineno of the start of the block for function definition(`FunctionDef.blockstart_tolineno`) can be quite tricky. Take below example:

```python
# Case A
def foo(bar: str) -> None:
    pass
# should returns line=1

# Case B
def foo(
        bar:str):
    pass
# should returns line=2

# Case C
def foo(
    bar:str
) -> None:
    pass
# should returns line=3

# Case D
def foo(
    bar:str
):
# should returns line=3
    pass
```

Currently we only handled Case A, B. With this commit we can cover case C.

But for Case D, we will need a better solution

(cherry picked from commit 8fa18c7)

Co-authored-by: Low, Zhi Hao <[email protected]>

Author: pylint-backport[bot]

astroid/nodes/scoped_nodes/scoped_nodes.py

@@ -1401,6 +1401,8 @@ def blockstart_tolineno(self):
 
         :type: int
         """
+        if self.returns:
+            return self.returns.tolineno
         return self.args.tolineno
 
     def implicit_parameters(self) -> Literal[0, 1]:

tests/test_scoped_nodes.py

@@ -982,6 +982,46 @@ def foo():
         with pytest.raises(AttributeInferenceError):
             func.getattr("")
 
+    @staticmethod
+    def test_blockstart_tolineno() -> None:
+        code = textwrap.dedent(
+            """\
+        def f1(bar: str) -> None:  #@
+            pass
+
+        def f2(  #@
+               bar: str) -> None:
+            pass
+
+        def f3(  #@
+            bar: str
+        ) -> None:
+            pass
+
+        def f4(  #@
+            bar: str
+        ):
+            pass
+
+        def f5(  #@
+               bar: str):
+            pass
+        """
+        )
+        ast_nodes: list[nodes.FunctionDef] = builder.extract_node(code)  # type: ignore[assignment]
+        assert len(ast_nodes) == 5
+
+        assert ast_nodes[0].blockstart_tolineno == 1
+
+        assert ast_nodes[1].blockstart_tolineno == 5
+
+        assert ast_nodes[2].blockstart_tolineno == 10
+
+        # Unimplemented, will return line 14 for now.
+        # assert ast_nodes[3].blockstart_tolineno == 15
+
+        assert ast_nodes[4].blockstart_tolineno == 19
+
 
 class ClassNodeTest(ModuleLoader, unittest.TestCase):
     def test_dict_interface(self) -> None: