Fix [override] error with no line number when argument node has no line number (python#18122)

Refs python#18115

When a parameter type in a method override is incompatible with the
parameter type in the supertype definition, mypy emits an error using
the `Argument` node as the context.

However, sometimes the the `Argument` node doesn't have a line number
set, causing the error message to have no associated line number. This
happens with the `__replace__` methods created in the dataclass plugin,
which have line numbers set on the `FuncDef` nodes, but no line numbers
set on the individual argument nodes.

This PR fixes the missing line number in the error by falling-back to
the FuncDef line number when a line number isn't set on the `Argument`
node.

(As an alternative fix, we could add line numbers to the `Argument`
nodes in the dataclass plugin, but that looks like a more complicated
change since multiple methods would be affected).

Author: brianschubert

mypy/checker.py

@@ -2367,10 +2367,11 @@ def erase_override(t: Type) -> Type:
                     else:
                         continue
                     if not is_subtype(original_arg_type, erase_override(override_arg_type)):
+                        context: Context = node
                         if isinstance(node, FuncDef) and not node.is_property:
-                            context: Context = node.arguments[i + len(override.bound_args)]
-                        else:
-                            context = node
+                            arg_node = node.arguments[i + len(override.bound_args)]
+                            if arg_node.line != -1:
+                                context = arg_node
                         self.msg.argument_incompatible_with_supertype(
                             i + 1,
                             name,

test-data/unit/check-dataclasses.test

@@ -2523,3 +2523,18 @@ reveal_type(replaced_2)  # N: Revealed type is "__main__.Gen[builtins.int]"
 Gen(2).__replace__(x="not an int")  # E: Argument "x" to "__replace__" of "Gen" has incompatible type "str"; expected "int"
 
 [builtins fixtures/tuple.pyi]
+
+[case testDunderReplaceCovariantOverride]
+# flags: --python-version 3.13
+from dataclasses import dataclass
+
+@dataclass
+class Base:
+    a: object
+
+@dataclass
+class Child(Base):  # E: Argument 1 of "__replace__" is incompatible with supertype "Base"; supertype defines the argument type as "object" \
+                    # N: This violates the Liskov substitution principle \
+                    # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+    a: int
+[builtins fixtures/tuple.pyi]