Fix include_directories test for relative path

- On Windows, it was not detected if include directory was an absolute
  path to source directory, because of the mis of path separators.

- In the edgecase the include directory begins with the exact same
  string as the source directory, but is a different directory, it was
  falsely reported as an error.

Fixes #12217.

mesonbuild/interpreter/interpreter.py

@@ -25,7 +25,7 @@ from ..wrap import wrap, WrapMode
 from .. import mesonlib
 from ..mesonlib import (EnvironmentVariables, ExecutableSerialisation, MesonBugException, MesonException, HoldableObject,
                         FileMode, MachineChoice, OptionKey, listify,
-                        extract_as_list, has_path_sep, PerMachine)
+                        extract_as_list, has_path_sep, path_is_in_root, PerMachine)
 from ..programs import ExternalProgram, NonExistingExternalProgram
 from ..dependencies import Dependency
 from ..depfile import DepFile
@@ -551,7 +551,7 @@ class Interpreter(InterpreterBase, HoldableObject):
             if f.is_built:
                 return
             f = os.path.normpath(f.relative_name())
-        elif os.path.isfile(f) and not f.startswith('/dev'):
+        elif os.path.isfile(f) and not f.startswith('/dev/'):
             srcdir = Path(self.environment.get_source_dir())
             builddir = Path(self.environment.get_build_dir())
             try:
@@ -2773,7 +2773,7 @@ class Interpreter(InterpreterBase, HoldableObject):
         absbase_build = os.path.join(build_root, self.subdir)
 
         for a in incdir_strings:
-            if a.startswith(src_root):
+            if path_is_in_root(Path(a), Path(src_root)):
                 raise InvalidArguments(textwrap.dedent('''\
                     Tried to form an absolute path to a dir in the source tree.
                     You should not do that but use relative paths instead, for

run_project_tests.py

@@ -942,6 +942,8 @@ def gather_tests(testdir: Path, stdout_mandatory: bool, only: T.List[str], skip_
         # Filter non-tests files (dot files, etc)
         if not t.is_dir() or t.name.startswith('.'):
             continue
+        if t.name in {'18 includedirxyz'}:
+            continue
         if only and not any(t.name.startswith(prefix) for prefix in only):
             continue
         test_def = TestDef(t, None, [], skip_category=skip_category)

test cases/common/18 includedir/meson.build

@@ -2,3 +2,31 @@ project('include dir test', 'c')
 
 inc = include_directories('include')
 subdir('src')
+
+errormsg = '''Tried to form an absolute path to a dir in the source tree.
+You should not do that but use relative paths instead, for
+directories that are part of your project.
+
+To get include path to any directory relative to the current dir do
+
+incdir = include_directories(dirname)
+
+After this incdir will contain both the current source dir as well as the
+corresponding build dir. It can then be used in any subdirectory and
+Meson will take care of all the busywork to make paths work.
+
+Dirname can even be '.' to mark the current directory. Though you should
+remember that the current source and build directories are always
+put in the include directories by default so you only need to do
+include_directories('.') if you intend to use the result in a
+different subdirectory.
+
+Note that this error message can also be triggered by
+external dependencies being installed within your source
+tree - it's not recommended to do this.
+'''
+testcase expect_error(errormsg)
+  include_directories(meson.current_source_dir() / 'include')
+endtestcase
+# Test for issue #12217
+include_directories(meson.current_source_dir() + 'xyz')

test cases/common/18 includedirxyz/do_not_delete

@@ -0,0 +1 @@
+This file is to ensure this directory exists