3 files changed, 7 insertions, 4 deletions
diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 4eeaec7..361f5d6 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -1023,6 +1023,7 @@ class Compiler(metaclass=abc.ABCMeta):
         raise EnvironmentException('This compiler does not have a preprocessor')
 
     def get_default_include_dirs(self) -> T.List[str]:
+        # TODO: This is a candidate for returning an immutable list
         return []
 
     def get_largefile_args(self) -> T.List[str]:
diff --git a/mesonbuild/compilers/mixins/clike.py b/mesonbuild/compilers/mixins/clike.py
index 0637439..0c0ade3 100644
--- a/mesonbuild/compilers/mixins/clike.py
+++ b/mesonbuild/compilers/mixins/clike.py
@@ -42,6 +42,7 @@ from .visualstudio import VisualStudioLikeCompiler
 
 if T.TYPE_CHECKING:
     from ...dependencies import Dependency
+    from ..._typing import ImmutableListProtocol
     from ...environment import Environment
     from ...compilers.compilers import Compiler
     from ...programs import ExternalProgram
@@ -207,7 +208,7 @@ class CLikeCompiler(Compiler):
 
     @functools.lru_cache()
     def _get_library_dirs(self, env: 'Environment',
-                          elf_class: T.Optional[int] = None) -> T.List[str]:
+                          elf_class: T.Optional[int] = None) -> 'ImmutableListProtocol[str]':
         # TODO: replace elf_class with enum
         dirs = self.get_compiler_dirs(env, 'libraries')
         if elf_class is None or elf_class == 0:
@@ -253,7 +254,7 @@ class CLikeCompiler(Compiler):
         return self._get_library_dirs(env, elf_class).copy()
 
     @functools.lru_cache()
-    def _get_program_dirs(self, env: 'Environment') -> T.List[str]:
+    def _get_program_dirs(self, env: 'Environment') -> 'ImmutableListProtocol[str]':
         '''
         Programs used by the compiler. Also where toolchain DLLs such as
         libstdc++-6.dll are found with MinGW.
diff --git a/mesonbuild/compilers/mixins/gnu.py b/mesonbuild/compilers/mixins/gnu.py
index b007ff0..9a94703 100644
--- a/mesonbuild/compilers/mixins/gnu.py
+++ b/mesonbuild/compilers/mixins/gnu.py
@@ -28,6 +28,7 @@ from ... import mlog
 from ...mesonlib import OptionKey
 
 if T.TYPE_CHECKING:
+    from ..._typing import ImmutableListProtocol
     from ...environment import Environment
     from ..compilers import Compiler
 else:
@@ -92,7 +93,7 @@ gnu_color_args = {
 
 
 @functools.lru_cache(maxsize=None)
-def gnulike_default_include_dirs(compiler: T.Tuple[str], lang: str) -> T.List[str]:
+def gnulike_default_include_dirs(compiler: T.Tuple[str, ...], lang: str) -> 'ImmutableListProtocol[str]':
     lang_map = {
         'c': 'c',
         'cpp': 'c++',
@@ -189,7 +190,7 @@ class GnuLikeCompiler(Compiler, metaclass=abc.ABCMeta):
         return gnulike_instruction_set_args.get(instruction_set, None)
 
     def get_default_include_dirs(self) -> T.List[str]:
-        return gnulike_default_include_dirs(tuple(self.exelist), self.language)
+        return gnulike_default_include_dirs(tuple(self.exelist), self.language).copy()
 
     @abc.abstractmethod
     def openmp_flags(self) -> T.List[str]: