build: Fix annotations for CustomTarget

and fix a bug in the backend that the correct annotations uncover

mesonbuild/backend/backends.py

@@ -1299,6 +1299,10 @@ class Backend:
             elif isinstance(i, build.ExtractedObjects):
                 outputs = i.get_outputs(self)
                 fname = self.get_extracted_obj_paths(i.target, outputs)
+            elif isinstance(i, programs.ExternalProgram):
+                assert i.found(), "This shouldn't be possible"
+                assert i.path is not None, 'for mypy'
+                fname = [i.path]
             else:
                 fname = [i.rel_to_builddir(self.build_to_src)]
             if target.absolute_paths:

mesonbuild/build.py

@@ -2529,7 +2529,7 @@ class CustomTarget(Target, CommandBase):
     def get_filename(self) -> str:
         return self.outputs[0]
 
-    def get_sources(self) -> T.List[T.Union[str, File, 'CustomTarget', 'CustomTargetIndex', 'GeneratedList', 'ExtractedObjects']]:
+    def get_sources(self) -> T.List[T.Union[str, File, BuildTarget, GeneratedTypes, ExtractedObjects, programs.ExternalProgram]]:
         return self.sources
 
     def get_generated_lists(self) -> T.List[GeneratedList]: