Do not show unused-ignore errors in unreachable code, and make it a real error code

docs/source/error_code_list2.rst

@@ -347,3 +347,47 @@ silence the error:
 
     async def g() -> None:
         _ = asyncio.create_task(f())  # No error
+
+Check that ``# type: ignore`` comment is used [unused-ignore]
+-------------------------------------------------------------
+
+If you use :option:`--enable-error-code unused-ignore <mypy --enable-error-code>`,
+or :option:`--warn-unused-ignores <mypy --warn-unused-ignores>`
+mypy generates an error if you don't use a ``# type: ignore`` comment, i.e. if
+there is a comment, but there would be no error generated by mypy on this line
+anyway.
+
+Example:
+
+.. code-block:: python
+
+    # Use "mypy --warn-unused-ignores ..."
+
+    def add(a: int, b: int) -> int:
+        # Error: unused "type: ignore" comment
+        return a + b  # type: ignore
+
+Note that due to a specific nature of this comment, the only way to selectively
+silence it, is to include the error code explicitly. Also note that this error is
+not shown if the the ``# type: ignore`` is not used due to code being statically
+unreachable (e.g. due to platform or version check).
+
+Example:
+
+.. code-block:: python
+
+    # Use "mypy --warn-unused-ignores ..."
+
+    import sys
+
+    try:
+        # The "[unused-ignore]" is needed to get clean mypy run
+        # on both Python 3.8, and 3.9 where this module was added
+        import graphlib  # type: ignore[import,unused-ignore]
+    except ImportError:
+        pass
+
+    if sys.version_info >= (3, 9):
+        # The following will not generate an error on either
+        # Python 3.8, or Python 3.9
+        42 + "testing..."  # type: ignore

mypy/build.py

@@ -2238,6 +2238,7 @@ def semantic_analysis_pass1(self) -> None:
         analyzer = SemanticAnalyzerPreAnalysis()
         with self.wrap_context():
             analyzer.visit_file(self.tree, self.xpath, self.id, options)
+        self.manager.errors.set_unreachable_lines(self.xpath, self.tree.unreachable_lines)
         # TODO: Do this while constructing the AST?
         self.tree.names = SymbolTable()
         if not self.tree.is_stub:
@@ -2572,7 +2573,10 @@ def dependency_lines(self) -> list[int]:
         return [self.dep_line_map.get(dep, 1) for dep in self.dependencies + self.suppressed]
 
     def generate_unused_ignore_notes(self) -> None:
-        if self.options.warn_unused_ignores:
+        if (
+            self.options.warn_unused_ignores
+            or codes.UNUSED_IGNORE in self.options.enabled_error_codes
+        ) and codes.UNUSED_IGNORE not in self.options.disabled_error_codes:
             # If this file was initially loaded from the cache, it may have suppressed
             # dependencies due to imports with ignores on them. We need to generate
             # those errors to avoid spuriously flagging them as unused ignores.

mypy/errorcodes.py

@@ -221,6 +221,9 @@ def __str__(self) -> str:
 USED_BEFORE_DEF: Final[ErrorCode] = ErrorCode(
     "used-before-def", "Warn about variables that are used before they are defined", "General"
 )
+UNUSED_IGNORE: Final = ErrorCode(
+    "unused-ignore", "Ensure that all type ignores are used", "General", default_enabled=False
+)
 
 
 # Syntax errors are often blocking.

mypy/errors.py

@@ -222,6 +222,9 @@ class Errors:
     # (path -> line -> error-codes)
     ignored_lines: dict[str, dict[int, list[str]]]
 
+    # Lines that are statically unreachable (e.g. due to platform/version check).
+    unreachable_lines: dict[str, set[int]]
+
     # Lines on which an error was actually ignored.
     used_ignored_lines: dict[str, dict[int, list[str]]]
 
@@ -277,6 +280,7 @@ def initialize(self) -> None:
         self.import_ctx = []
         self.function_or_member = [None]
         self.ignored_lines = {}
+        self.unreachable_lines = {}
         self.used_ignored_lines = defaultdict(lambda: defaultdict(list))
         self.ignored_files = set()
         self.only_once_messages = set()
@@ -325,6 +329,9 @@ def set_file_ignored_lines(
         if ignore_all:
             self.ignored_files.add(file)
 
+    def set_unreachable_lines(self, file: str, unreachable_lines: set[int]) -> None:
+        self.unreachable_lines[file] = unreachable_lines
+
     def current_target(self) -> str | None:
         """Retrieves the current target from the associated scope.
 
@@ -623,6 +630,10 @@ def generate_unused_ignore_errors(self, file: str) -> None:
         ignored_lines = self.ignored_lines[file]
         used_ignored_lines = self.used_ignored_lines[file]
         for line, ignored_codes in ignored_lines.items():
+            if line in self.unreachable_lines[file]:
+                continue
+            if codes.UNUSED_IGNORE.code in ignored_codes:
+                continue
             used_ignored_codes = used_ignored_lines[line]
             unused_ignored_codes = set(ignored_codes) - set(used_ignored_codes)
             # `ignore` is used
@@ -639,7 +650,7 @@ def generate_unused_ignore_errors(self, file: str) -> None:
             for unused in unused_ignored_codes:
                 narrower = set(used_ignored_codes) & codes.sub_code_map[unused]
                 if narrower:
-                    message += f", use narrower [{', '.join(narrower)}] instead of [{unused}]"
+                    message += f", use narrower [{', '.join(narrower)}] instead of [{unused}] code"
             # Don't use report since add_error_info will ignore the error!
             info = ErrorInfo(
                 self.import_context(),
@@ -653,7 +664,7 @@ def generate_unused_ignore_errors(self, file: str) -> None:
                 -1,
                 "error",
                 message,
-                None,
+                codes.UNUSED_IGNORE,
                 False,
                 False,
                 False,

mypy/fastparse.py

@@ -516,6 +516,7 @@ def translate_stmt_list(
                 codes.FILE.code
             )
             block = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
+            self.set_block_lines(block, stmts)
             mark_block_unreachable(block)
             return [block]
 
@@ -611,30 +612,38 @@ def from_comp_operator(self, op: ast3.cmpop) -> str:
         else:
             return op_name
 
-    def as_block(self, stmts: list[ast3.stmt], lineno: int) -> Block | None:
+    def set_block_lines(self, b: Block, stmts: Sequence[ast3.stmt]) -> None:
+        first, last = stmts[0], stmts[-1]
+        b.line = first.lineno
+        b.column = first.col_offset
+        b.end_line = getattr(last, "end_lineno", None)
+        b.end_column = getattr(last, "end_col_offset", None)
+        if not b.body:
+            return
+        new_first = b.body[0]
+        if isinstance(new_first, (Decorator, OverloadedFuncDef)):
+            # Decorated function lines are different between Python versions.
+            # copy the normalization we do for them to block first lines.
+            b.line = new_first.line
+            b.column = new_first.column
+
+    def as_block(self, stmts: list[ast3.stmt]) -> Block | None:
         b = None
         if stmts:
             b = Block(self.fix_function_overloads(self.translate_stmt_list(stmts)))
-            b.set_line(lineno)
+            self.set_block_lines(b, stmts)
         return b
 
     def as_required_block(
-        self,
-        stmts: list[ast3.stmt],
-        lineno: int,
-        *,
-        can_strip: bool = False,
-        is_coroutine: bool = False,
+        self, stmts: list[ast3.stmt], *, can_strip: bool = False, is_coroutine: bool = False
     ) -> Block:
         assert stmts  # must be non-empty
         b = Block(
             self.fix_function_overloads(
                 self.translate_stmt_list(stmts, can_strip=can_strip, is_coroutine=is_coroutine)
             )
         )
-        # TODO: in most call sites line is wrong (includes first line of enclosing statement)
-        # TODO: also we need to set the column, and the end position here.
-        b.set_line(lineno)
+        self.set_block_lines(b, stmts)
         return b
 
     def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
@@ -1023,7 +1032,7 @@ def do_func_def(
 
         self.class_and_function_stack.pop()
         self.class_and_function_stack.append("F")
-        body = self.as_required_block(n.body, lineno, can_strip=True, is_coroutine=is_coroutine)
+        body = self.as_required_block(n.body, can_strip=True, is_coroutine=is_coroutine)
         func_def = FuncDef(n.name, args, body, func_type)
         if isinstance(func_def.type, CallableType):
             # semanal.py does some in-place modifications we want to avoid
@@ -1052,9 +1061,6 @@ def do_func_def(
             func_def.is_decorated = True
             func_def.deco_line = deco_line
             func_def.set_line(lineno, n.col_offset, end_line, end_column)
-            # Set the line again after we updated it (to make value same in Python 3.7/3.8)
-            # Note that TODOs in as_required_block() apply here as well.
-            func_def.body.set_line(lineno)
 
             deco = Decorator(func_def, self.translate_expr_list(n.decorator_list), var)
             first = n.decorator_list[0]
@@ -1165,7 +1171,7 @@ def visit_ClassDef(self, n: ast3.ClassDef) -> ClassDef:
 
         cdef = ClassDef(
             n.name,
-            self.as_required_block(n.body, n.lineno),
+            self.as_required_block(n.body),
             None,
             self.translate_expr_list(n.bases),
             metaclass=dict(keywords).get("metaclass"),
@@ -1237,8 +1243,8 @@ def visit_For(self, n: ast3.For) -> ForStmt:
         node = ForStmt(
             self.visit(n.target),
             self.visit(n.iter),
-            self.as_required_block(n.body, n.lineno),
-            self.as_block(n.orelse, n.lineno),
+            self.as_required_block(n.body),
+            self.as_block(n.orelse),
             target_type,
         )
         return self.set_line(node, n)
@@ -1249,8 +1255,8 @@ def visit_AsyncFor(self, n: ast3.AsyncFor) -> ForStmt:
         node = ForStmt(
             self.visit(n.target),
             self.visit(n.iter),
-            self.as_required_block(n.body, n.lineno),
-            self.as_block(n.orelse, n.lineno),
+            self.as_required_block(n.body),
+            self.as_block(n.orelse),
             target_type,
         )
         node.is_async = True
@@ -1259,19 +1265,14 @@ def visit_AsyncFor(self, n: ast3.AsyncFor) -> ForStmt:
     # While(expr test, stmt* body, stmt* orelse)
     def visit_While(self, n: ast3.While) -> WhileStmt:
         node = WhileStmt(
-            self.visit(n.test),
-            self.as_required_block(n.body, n.lineno),
-            self.as_block(n.orelse, n.lineno),
+            self.visit(n.test), self.as_required_block(n.body), self.as_block(n.orelse)
         )
         return self.set_line(node, n)
 
     # If(expr test, stmt* body, stmt* orelse)
     def visit_If(self, n: ast3.If) -> IfStmt:
-        lineno = n.lineno
         node = IfStmt(
-            [self.visit(n.test)],
-            [self.as_required_block(n.body, lineno)],
-            self.as_block(n.orelse, lineno),
+            [self.visit(n.test)], [self.as_required_block(n.body)], self.as_block(n.orelse)
         )
         return self.set_line(node, n)
 
@@ -1281,7 +1282,7 @@ def visit_With(self, n: ast3.With) -> WithStmt:
         node = WithStmt(
             [self.visit(i.context_expr) for i in n.items],
             [self.visit(i.optional_vars) for i in n.items],
-            self.as_required_block(n.body, n.lineno),
+            self.as_required_block(n.body),
             target_type,
         )
         return self.set_line(node, n)
@@ -1292,7 +1293,7 @@ def visit_AsyncWith(self, n: ast3.AsyncWith) -> WithStmt:
         s = WithStmt(
             [self.visit(i.context_expr) for i in n.items],
             [self.visit(i.optional_vars) for i in n.items],
-            self.as_required_block(n.body, n.lineno),
+            self.as_required_block(n.body),
             target_type,
         )
         s.is_async = True
@@ -1309,15 +1310,15 @@ def visit_Try(self, n: ast3.Try) -> TryStmt:
             self.set_line(NameExpr(h.name), h) if h.name is not None else None for h in n.handlers
         ]
         types = [self.visit(h.type) for h in n.handlers]
-        handlers = [self.as_required_block(h.body, h.lineno) for h in n.handlers]
+        handlers = [self.as_required_block(h.body) for h in n.handlers]
 
         node = TryStmt(
-            self.as_required_block(n.body, n.lineno),
+            self.as_required_block(n.body),
             vs,
             types,
             handlers,
-            self.as_block(n.orelse, n.lineno),
-            self.as_block(n.finalbody, n.lineno),
+            self.as_block(n.orelse),
+            self.as_block(n.finalbody),
         )
         return self.set_line(node, n)
 
@@ -1326,15 +1327,15 @@ def visit_TryStar(self, n: TryStar) -> TryStmt:
             self.set_line(NameExpr(h.name), h) if h.name is not None else None for h in n.handlers
         ]
         types = [self.visit(h.type) for h in n.handlers]
-        handlers = [self.as_required_block(h.body, h.lineno) for h in n.handlers]
+        handlers = [self.as_required_block(h.body) for h in n.handlers]
 
         node = TryStmt(
-            self.as_required_block(n.body, n.lineno),
+            self.as_required_block(n.body),
             vs,
             types,
             handlers,
-            self.as_block(n.orelse, n.lineno),
-            self.as_block(n.finalbody, n.lineno),
+            self.as_block(n.orelse),
+            self.as_block(n.finalbody),
         )
         node.is_star = True
         return self.set_line(node, n)
@@ -1469,9 +1470,7 @@ def visit_Lambda(self, n: ast3.Lambda) -> LambdaExpr:
         body.col_offset = n.body.col_offset
 
         self.class_and_function_stack.append("L")
-        e = LambdaExpr(
-            self.transform_args(n.args, n.lineno), self.as_required_block([body], n.lineno)
-        )
+        e = LambdaExpr(self.transform_args(n.args, n.lineno), self.as_required_block([body]))
         self.class_and_function_stack.pop()
         e.set_line(n.lineno, n.col_offset)  # Overrides set_line -- can't use self.set_line
         return e
@@ -1743,7 +1742,7 @@ def visit_Match(self, n: Match) -> MatchStmt:
             self.visit(n.subject),
             [self.visit(c.pattern) for c in n.cases],
             [self.visit(c.guard) for c in n.cases],
-            [self.as_required_block(c.body, n.lineno) for c in n.cases],
+            [self.as_required_block(c.body) for c in n.cases],
         )
         return self.set_line(node, n)
 

mypy/nodes.py

@@ -287,6 +287,7 @@ class MypyFile(SymbolNode):
         "names",
         "imports",
         "ignored_lines",
+        "unreachable_lines",
         "is_stub",
         "is_cache_skeleton",
         "is_partial_stub_package",
@@ -313,6 +314,8 @@ class MypyFile(SymbolNode):
     # If the value is empty, ignore all errors; otherwise, the list contains all
     # error codes to ignore.
     ignored_lines: dict[int, list[str]]
+    # Lines that are statically unreachable (e.g. due to platform/version check).
+    unreachable_lines: set[int]
     # Is this file represented by a stub file (.pyi)?
     is_stub: bool
     # Is this loaded from the cache and thus missing the actual body of the file?
@@ -345,6 +348,7 @@ def __init__(
             self.ignored_lines = ignored_lines
         else:
             self.ignored_lines = {}
+        self.unreachable_lines = set()
 
         self.path = ""
         self.is_stub = False