diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index a4a4f1916..82f3a1341 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -613,16 +613,12 @@ class BuildTarget(Target):
         self.pch = {}
         self.extra_args: T.Dict[str, T.List['FileOrString']] = {}
         self.sources: T.List[File] = []
-        self.generated: T.Sequence[T.Union[GeneratedList, CustomTarget, CustomTargetIndex]] = []
+        self.generated: T.List[T.Union[GeneratedList, CustomTarget, CustomTargetIndex]] = []
         self.d_features = {}
         self.pic = False
         self.pie = False
         # Track build_rpath entries so we can remove them at install time
         self.rpath_dirs_to_remove: T.Set[bytes] = set()
-        # Sources can be:
-        # 1. Pre-existing source files in the source tree
-        # 2. Pre-existing sources generated by configure_file in the build tree
-        # 3. Sources files generated by another target or a Generator
         self.process_sourcelist(sources)
         # Objects can be:
         # 1. Pre-existing objects provided by the user with the `objects:` kwarg
@@ -680,19 +676,23 @@ class BuildTarget(Target):
                 msg = 'Bad object of type {!r} in target {!r}.'.format(type(s).__name__, self.name)
                 raise InvalidArguments(msg)
 
-    def process_sourcelist(self, sources):
-        sources = listify(sources)
-        added_sources = {} # If the same source is defined multiple times, use it only once.
-        for s in unholder(sources):
+    def process_sourcelist(self, sources: T.List['SourceOutputs']) -> None:
+        """Split sources into generated and static sources.
+
+        Sources can be:
+        1. Pre-existing source files in the source tree (static)
+        2. Pre-existing sources generated by configure_file in the build tree.
+           (static as they are only regenerated if meson itself is regenerated)
+        3. Sources files generated by another target or a Generator (generated)
+        """
+        added_sources: T.Set[File] = set() # If the same source is defined multiple times, use it only once.
+        for s in sources:
             if isinstance(s, File):
                 if s not in added_sources:
                     self.sources.append(s)
-                    added_sources[s] = True
-            elif isinstance(s, (GeneratedList, CustomTarget, CustomTargetIndex)):
+                    added_sources.add(s)
+            elif isinstance(s, (CustomTarget, CustomTargetIndex, GeneratedList)):
                 self.generated.append(s)
-            else:
-                msg = 'Bad source of type {!r} in target {!r}.'.format(type(s).__name__, self.name)
-                raise InvalidArguments(msg)
 
     @staticmethod
     def can_compile_remove_sources(compiler, sources):