ensure that the KW_ONLY sentinel is only provided once

https://docs.python.org/3.10/library/dataclasses.html#dataclasses.KW_ONLY

In this section in the documentation it states:

In a single dataclass, it is an error to specify more than one field whose type
is KW_ONLY.

This commit handles raising a mypy failure if this case is detected.

mypy/plugins/dataclasses.py

@@ -341,6 +341,8 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
         # Ensure that arguments without a default don't follow
         # arguments that have a default.
         found_default = False
+        # Ensure that the KW_ONLY sentinel is only provided once
+        found_kw_sentinel = False
         for attr in all_attrs:
             # If we find any attribute that is_in_init, not kw_only, and that
             # doesn't have a default after one that does have one,
@@ -356,6 +358,14 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
                 )
 
             found_default = found_default or (attr.has_default and attr.is_in_init)
+            if found_kw_sentinel and self._is_kw_only_type(attr.type):
+                context = (Context(line=attr.line, column=attr.column) if attr in attrs
+                           else ctx.cls)
+                ctx.api.fail(
+                    'There may not be more than one field with the KW_ONLY type',
+                    context,
+                )
+            found_kw_sentinel = found_kw_sentinel or self._is_kw_only_type(attr.type)
 
         return all_attrs
 

test-data/unit/check-dataclasses.test

@@ -386,6 +386,18 @@ class D(Base):
 
 D("Hello", "World")
 
+[case testDataclassesOrderingKwOnlyWithMultipleSentinel]
+# flags: --python-version 3.10
+from dataclasses import dataclass, field, KW_ONLY
+
+@dataclass
+class Base:
+    x: str
+    _: KW_ONLY
+    y: int = 0
+    __: KW_ONLY  # E: There may not be more than one field with the KW_ONLY type
+    w: int = 1
+
 [builtins fixtures/list.pyi]
 
 [case testDataclassesClassmethods]