Fixes indented block position by PositionProvider (#1380)

libcst/_nodes/statement.py

@@ -712,14 +712,30 @@ def _codegen_impl(self, state: CodegenState) -> None:
         state.increase_indent(state.default_indent if indent is None else indent)
 
         if self.body:
-            with state.record_syntactic_position(
-                self, start_node=self.body[0], end_node=self.body[-1]
+            first_statement = self.body[0]
+            if (
+                isinstance(first_statement, (FunctionDef, ClassDef))
+                and first_statement.decorators
             ):
-                for stmt in self.body:
-                    # IndentedBlock is responsible for adjusting the current indentation level,
-                    # but its children are responsible for actually adding that indentation to
-                    # the token list.
-                    stmt._codegen(state)
+                # If the first statement is a function or class definition, we need to
+                # use the position of the first decorator instead of the function/class definition.
+                with state.record_syntactic_position(
+                    self, start_node=first_statement.decorators[0], end_node=self.body[-1]
+                ):
+                    for stmt in self.body:
+                        # IndentedBlock is responsible for adjusting the current indentation level,
+                        # but its children are responsible for actually adding that indentation to
+                        # the token list.
+                        stmt._codegen(state)
+            else:
+                with state.record_syntactic_position(
+                    self, start_node=first_statement, end_node=self.body[-1]
+                ):
+                    for stmt in self.body:
+                        # IndentedBlock is responsible for adjusting the current indentation level,
+                        # but its children are responsible for actually adding that indentation to
+                        # the token list.
+                        stmt._codegen(state)
         else:
             # Empty indented blocks are not syntactically valid in Python unless
             # they contain a 'pass' statement, so add one here.

libcst/metadata/tests/test_position_provider.py

@@ -83,6 +83,62 @@ def visit_Pass(self, node: cst.Pass) -> None:
         wrapper = MetadataWrapper(parse_module("pass"))
         wrapper.visit_batched([ABatchable()])
 
+    def test_indented_block_starting_with_decorated_function_def(self) -> None:
+        """
+        Tests that the position provider correctly computes positions in an indented block
+        starting with a decorated function definition.
+        """
+        test = self
+
+        class IndentedBlockVisitor(CSTVisitor):
+            METADATA_DEPENDENCIES = (PositionProvider,)
+
+            def visit_IndentedBlock(self, node: cst.IndentedBlock) -> None:
+                test.assertEqual(
+                    self.get_metadata(PositionProvider, node),
+                    CodeRange((3, 4), (5, 15)),
+                )
+
+        wrapper = MetadataWrapper(
+            parse_module(
+                """ # Empty line
+def foo():
+    @decorator
+    def func(): return 42
+    return func
+"""
+            )
+        )
+        wrapper.visit(IndentedBlockVisitor())
+
+    def test_indented_block_starting_with_decorated_class_def(self) -> None:
+        """
+        Tests that the position provider correctly computes positions in an indented block
+        starting with a decorated class definition.
+        """
+        test = self
+
+        class IndentedBlockVisitor(CSTVisitor):
+            METADATA_DEPENDENCIES = (PositionProvider,)
+
+            def visit_IndentedBlock(self, node: cst.IndentedBlock) -> None:
+                test.assertEqual(
+                    self.get_metadata(PositionProvider, node),
+                    CodeRange((3, 4), (5, 18)),
+                )
+
+        wrapper = MetadataWrapper(
+            parse_module(
+                """ # Empty line
+def foo():
+    @decorator
+    class MyClass: pass
+    return MyClass
+"""
+            )
+        )
+        wrapper.visit(IndentedBlockVisitor())
+
     def test_match_statement_position_metadata(self) -> None:
         test = self