Fix crash in dataclass protocol with self attribute assignment (#15157)

Fix #15004 

FWIW I don't think dataclass protocols make much sense, but we
definitely should not crash. Also the root cause has nothing to do with
dataclasses, the problem is that a self attribute assignment in a
protocol created a new `Var` (after an original `Var` was created in
class body), which is obviously wrong.

mypy/semanal.py

@@ -3647,7 +3647,7 @@ def analyze_lvalue(
                 has_explicit_value=has_explicit_value,
             )
         elif isinstance(lval, MemberExpr):
-            self.analyze_member_lvalue(lval, explicit_type, is_final)
+            self.analyze_member_lvalue(lval, explicit_type, is_final, has_explicit_value)
             if explicit_type and not self.is_self_member_ref(lval):
                 self.fail("Type cannot be declared in assignment to non-self attribute", lval)
         elif isinstance(lval, IndexExpr):
@@ -3824,7 +3824,9 @@ def analyze_tuple_or_list_lvalue(self, lval: TupleExpr, explicit_type: bool = Fa
                     has_explicit_value=True,
                 )
 
-    def analyze_member_lvalue(self, lval: MemberExpr, explicit_type: bool, is_final: bool) -> None:
+    def analyze_member_lvalue(
+        self, lval: MemberExpr, explicit_type: bool, is_final: bool, has_explicit_value: bool
+    ) -> None:
         """Analyze lvalue that is a member expression.
 
         Arguments:
@@ -3853,12 +3855,18 @@ def analyze_member_lvalue(self, lval: MemberExpr, explicit_type: bool, is_final:
                 and explicit_type
             ):
                 self.attribute_already_defined(lval.name, lval, cur_node)
-            # If the attribute of self is not defined in superclasses, create a new Var, ...
+            if self.type.is_protocol and has_explicit_value and cur_node is not None:
+                # Make this variable non-abstract, it would be safer to do this only if we
+                # are inside __init__, but we do this always to preserve historical behaviour.
+                if isinstance(cur_node.node, Var):
+                    cur_node.node.is_abstract_var = False
             if (
+                # If the attribute of self is not defined, create a new Var, ...
                 node is None
-                or (isinstance(node.node, Var) and node.node.is_abstract_var)
+                # ... or if it is defined as abstract in a *superclass*.
+                or (cur_node is None and isinstance(node.node, Var) and node.node.is_abstract_var)
                 # ... also an explicit declaration on self also creates a new Var.
-                # Note that `explicit_type` might has been erased for bare `Final`,
+                # Note that `explicit_type` might have been erased for bare `Final`,
                 # so we also check if `is_final` is passed.
                 or (cur_node is None and (explicit_type or is_final))
             ):

test-data/unit/check-dataclasses.test

@@ -2037,3 +2037,16 @@ Foo(
     present_5=5,
 )
 [builtins fixtures/dataclasses.pyi]
+
+[case testProtocolNoCrash]
+from typing import Protocol, Union, ClassVar
+from dataclasses import dataclass, field
+
+DEFAULT = 0
+
+@dataclass
+class Test(Protocol):
+    x: int
+    def reset(self) -> None:
+        self.x = DEFAULT
+[builtins fixtures/dataclasses.pyi]

test-data/unit/check-protocols.test

@@ -4054,3 +4054,14 @@ class P(Protocol):
 
 [file lib.py]
 class C: ...
+
+[case testAllowDefaultConstructorInProtocols]
+from typing import Protocol
+
+class P(Protocol):
+    x: int
+    def __init__(self, x: int) -> None:
+        self.x = x
+
+class C(P): ...
+C(0)  # OK