Fix crash on type alias definition inside dataclass declaration (#12792)

Skip processing a type alias node and generate an error.

Fixes #12544.

mypy/plugins/dataclasses.py

@@ -5,7 +5,7 @@
 
 from mypy.nodes import (
     ARG_OPT, ARG_NAMED, ARG_NAMED_OPT, ARG_POS, ARG_STAR, ARG_STAR2, MDEF,
-    Argument, AssignmentStmt, CallExpr,    Context, Expression, JsonDict,
+    Argument, AssignmentStmt, CallExpr,  TypeAlias,  Context, Expression, JsonDict,
     NameExpr, RefExpr, SymbolTableNode, TempNode, TypeInfo, Var, TypeVarExpr,
     PlaceholderNode
 )
@@ -333,6 +333,20 @@ def collect_attributes(self) -> Optional[List[DataclassAttribute]]:
 
             node = sym.node
             assert not isinstance(node, PlaceholderNode)
+
+            if isinstance(node, TypeAlias):
+                ctx.api.fail(
+                    (
+                        'Type aliases inside dataclass definitions '
+                        'are not supported at runtime'
+                    ),
+                    node
+                )
+                # Skip processing this node. This doesn't match the runtime behaviour,
+                # but the only alternative would be to modify the SymbolTable,
+                # and it's a little hairy to do that in a plugin.
+                continue
+
             assert isinstance(node, Var)
 
             # x: ClassVar[int] is ignored by dataclasses.

test-data/unit/check-dataclasses.test

@@ -526,6 +526,34 @@ Application.COUNTER = 1
 
 [builtins fixtures/dataclasses.pyi]
 
+[case testTypeAliasInDataclassDoesNotCrash]
+# flags: --python-version 3.7
+from dataclasses import dataclass
+from typing import Callable
+from typing_extensions import TypeAlias
+
+@dataclass
+class Foo:
+    x: int
+
+@dataclass
+class One:
+    S: TypeAlias = Foo  # E: Type aliases inside dataclass definitions are not supported at runtime
+
+a = One()
+reveal_type(a.S)  # N: Revealed type is "def (x: builtins.int) -> __main__.Foo"
+a.S()  # E: Missing positional argument "x" in call to "Foo"
+reveal_type(a.S(5))  # N: Revealed type is "__main__.Foo"
+
+@dataclass
+class Two:
+    S: TypeAlias = Callable[[int], str]  # E: Type aliases inside dataclass definitions are not supported at runtime
+
+c = Two()
+x = c.S  # E: Member "S" is not assignable
+reveal_type(x)  # N: Revealed type is "Any"
+[builtins fixtures/dataclasses.pyi]
+
 [case testDataclassOrdering]
 # flags: --python-version 3.7
 from dataclasses import dataclass