fix: pydantic dataclass can inherit from stdlib dataclass and arbitrary_types_allowed is supported (#2051)

* fix: pydantic dataclasses can inherit from stdlib dataclasses

closes #2042

* docs: add some documentation

* fix: support arbitrary_types_allowed with stdlib dataclass

closes #2054

* docs: add documentation for custom types

docs/examples/dataclasses_arbitrary_types_allowed.py

@@ -0,0 +1,42 @@
+import dataclasses
+
+import pydantic
+
+
+class ArbitraryType:
+    def __init__(self, value):
+        self.value = value
+
+    def __repr__(self):
+        return f'ArbitraryType(value={self.value!r})'
+
+
+@dataclasses.dataclass
+class DC:
+    a: ArbitraryType
+    b: str
+
+
+# valid as it is a builtin dataclass without validation
+my_dc = DC(a=ArbitraryType(value=3), b='qwe')
+
+try:
+    class Model(pydantic.BaseModel):
+        dc: DC
+        other: str
+
+    Model(dc=my_dc, other='other')
+except RuntimeError as e:  # invalid as it is now a pydantic dataclass
+    print(e)
+
+
+class Model(pydantic.BaseModel):
+    dc: DC
+    other: str
+
+    class Config:
+        arbitrary_types_allowed = True
+
+
+m = Model(dc=my_dc, other='other')
+print(repr(m))

docs/examples/dataclasses_stdlib_inheritance.py

@@ -0,0 +1,27 @@
+import dataclasses
+
+import pydantic
+
+
+@dataclasses.dataclass
+class Z:
+    z: int
+
+
+@dataclasses.dataclass
+class Y(Z):
+    y: int = 0
+
+
+@pydantic.dataclasses.dataclass
+class X(Y):
+    x: int = 0
+
+
+foo = X(x=b'1', y='2', z='3')
+print(foo)
+
+try:
+    X(z='pika')
+except pydantic.ValidationError as e:
+    print(e)