mypy: enable allow-redefinition-new and fix fallout

Reduces 3 errors that show up in newer mypy versions than pinned in CI.

It is new since 1.16 and a likely future default for mypy 2.0. It allows
things like:

```
for i in ['one', 'two', 'three']:
    frob_a(i)

for i in [1, 2, 3]:
    frob_b(i)
```

since "i" is obviously used as a loop holder and its type can be freely
reinvented. Note: allow-redefinition-new isn't actually about this at
all, it has greater scope than loop holders and allows redefining
"unannotated variables" of all kinds. No granularity in what to accept
redefinition of. :P

To enable this, we must also opt in to local-partial-types, which has
some overlap with None-safety. Specifically:

> the most common cases for partial types are variables initialized
> using None, but without explicit X | None annotations. By default, mypy
> won’t check partial types spanning module top level or class top level.
> This flag changes the behavior to only allow partial types at local
> level, therefore it disallows inferring variable type for None from two
> assignments in different scopes.

So with this, we also fix a couple of actual type errors this revealed.
Where possible, stop None-initializing at all -- it's not strictly
needed for global variables, anyway, and it's a coding error if it is
possible to hit these variables without defining them first.

Bug: https://github.com/python/mypy/issues/19280

7 files changed, 17 insertions, 11 deletions

diff --git a/.mypy.ini b/.mypy.ini
index d00944bb2..936b1ac2c 100644
--- a/.mypy.ini
+++ b/.mypy.ini
@@ -22,3 +22,7 @@ check_untyped_defs            = True
 # disallow_any_explicit       = True
 # disallow_any_generics       = True
 # disallow_subclassing_any    = True
+
+# future default behaviors
+allow_redefinition_new        = True
+local_partial_types           = True
diff --git a/mesonbuild/ast/interpreter.py b/mesonbuild/ast/interpreter.py
index 524f33193..2f82a4a84 100644
--- a/mesonbuild/ast/interpreter.py
+++ b/mesonbuild/ast/interpreter.py
@@ -731,7 +731,8 @@ class AstInterpreter(InterpreterBase):
         else:
             args = [args_raw]
 
-        flattened_args: T.List[TYPE_var] = []
+        # BaseNode resolves to Any. :/
+        flattened_args: T.List[T.Union[TYPE_var, T.Any]] = []
 
         # Resolve the contents of args
         for i in args:
diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 732bae544..72dbfdf2d 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -631,7 +631,8 @@ def process_method_kw(possible: T.Iterable[DependencyMethods], kwargs: T.Dict[st
     # Set the detection method. If the method is set to auto, use any available method.
     # If method is set to a specific string, allow only that detection method.
     if method == DependencyMethods.AUTO:
-        methods = list(possible)
+        # annotated for https://github.com/python/mypy/issues/19894
+        methods: T.List[DependencyMethods] = list(possible)
     elif method in possible:
         methods = [method]
     else:
diff --git a/mesonbuild/modules/cmake.py b/mesonbuild/modules/cmake.py
index f12cc51a6..fc300fab2 100644
--- a/mesonbuild/modules/cmake.py
+++ b/mesonbuild/modules/cmake.py
@@ -243,7 +243,7 @@ class CMakeSubprojectOptions(ModuleObject):
 
 class CmakeModule(ExtensionModule):
     cmake_detected = False
-    cmake_root = None
+    cmake_root: str
 
     INFO = ModuleInfo('cmake', '0.50.0')
 
diff --git a/mesonbuild/modules/external_project.py b/mesonbuild/modules/external_project.py
index ba7b30075..0881df94e 100644
--- a/mesonbuild/modules/external_project.py
+++ b/mesonbuild/modules/external_project.py
@@ -169,7 +169,7 @@ class ExternalProject(NewExtensionModule):
     def _quote_and_join(self, array: T.List[str]) -> str:
         return ' '.join([shlex.quote(i) for i in array])
 
-    def _validate_configure_options(self, variables: T.List[T.Tuple[str, str, str]], state: 'ModuleState') -> None:
+    def _validate_configure_options(self, variables: T.Sequence[T.Tuple[str, T.Optional[str], str]], state: 'ModuleState') -> None:
         # Ensure the user at least try to pass basic info to the build system,
         # like the prefix, libdir, etc.
         for key, default, val in variables:
@@ -183,7 +183,7 @@ class ExternalProject(NewExtensionModule):
                 FeatureNew('Default configure_option', '0.57.0').use(self.subproject, state.current_node)
                 self.configure_options.append(default)
 
-    def _format_options(self, options: T.List[str], variables: T.List[T.Tuple[str, str, str]]) -> T.List[str]:
+    def _format_options(self, options: T.List[str], variables: T.Sequence[T.Tuple[str, T.Optional[str], str]]) -> T.List[str]:
         out: T.List[str] = []
         missing = set()
         regex = get_variable_regex('meson')
diff --git a/mesonbuild/scripts/symbolextractor.py b/mesonbuild/scripts/symbolextractor.py
index b0a07d906..c0aa6502b 100644
--- a/mesonbuild/scripts/symbolextractor.py
+++ b/mesonbuild/scripts/symbolextractor.py
@@ -23,7 +23,7 @@ parser.add_argument('--cross-host', default=None, dest='cross_host',
                     help='cross compilation host platform')
 parser.add_argument('args', nargs='+')
 
-TOOL_WARNING_FILE = None
+TOOL_WARNING_FILE: str
 RELINKING_WARNING = 'Relinking will always happen on source changes.'
 
 def dummy_syms(outfilename: str) -> None:
diff --git a/run_project_tests.py b/run_project_tests.py
index b2442703e..7067bfa0b 100755
--- a/run_project_tests.py
+++ b/run_project_tests.py
@@ -1577,11 +1577,11 @@ def detect_tools(report: bool = True) -> None:
         print('{0:<{2}}: {1}'.format(tool.tool, get_version(tool), max_width))
     print()
 
-symlink_test_dir1 = None
-symlink_test_dir2 = None
-symlink_file1 = None
-symlink_file2 = None
-symlink_file3 = None
+symlink_test_dir1: T.Optional[Path] = None
+symlink_test_dir2: T.Optional[Path] = None
+symlink_file1: T.Optional[Path] = None
+symlink_file2: T.Optional[Path] = None
+symlink_file3: T.Optional[Path] = None
 
 def scan_test_data_symlinks() -> None:
     global symlink_test_dir1, symlink_test_dir2, symlink_file1, symlink_file2, symlink_file3