Raise RuntimeError with better error messages (#15778)

While working on #15776 I've noticed
that some `RuntimeError` do not have enough metadata to understand what
is going on.
CI:
https://github.com/python/mypy/actions/runs/5700479199/job/15450345887

This PR adds more context to error messages.

mypy/erasetype.py

@@ -71,7 +71,7 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
 
     def visit_partial_type(self, t: PartialType) -> ProperType:
         # Should not get here.
-        raise RuntimeError()
+        raise RuntimeError("Cannot erase partial types")
 
     def visit_deleted_type(self, t: DeletedType) -> ProperType:
         return t

mypy/nodes.py

@@ -203,7 +203,7 @@ def str_with_options(self, options: Options) -> str:
         return ans
 
     def accept(self, visitor: NodeVisitor[T]) -> T:
-        raise RuntimeError("Not implemented")
+        raise RuntimeError("Not implemented", type(self))
 
 
 @trait
@@ -213,7 +213,7 @@ class Statement(Node):
     __slots__ = ()
 
     def accept(self, visitor: StatementVisitor[T]) -> T:
-        raise RuntimeError("Not implemented")
+        raise RuntimeError("Not implemented", type(self))
 
 
 @trait
@@ -223,7 +223,7 @@ class Expression(Node):
     __slots__ = ()
 
     def accept(self, visitor: ExpressionVisitor[T]) -> T:
-        raise RuntimeError("Not implemented")
+        raise RuntimeError("Not implemented", type(self))
 
 
 class FakeExpression(Expression):

mypy/patterns.py

@@ -19,7 +19,7 @@ class Pattern(Node):
     __slots__ = ()
 
     def accept(self, visitor: PatternVisitor[T]) -> T:
-        raise RuntimeError("Not implemented")
+        raise RuntimeError("Not implemented", type(self))
 
 
 class AsPattern(Pattern):

mypy/server/astmerge.py

@@ -467,13 +467,13 @@ def visit_overloaded(self, t: Overloaded) -> None:
 
     def visit_erased_type(self, t: ErasedType) -> None:
         # This type should exist only temporarily during type inference
-        raise RuntimeError
+        raise RuntimeError("Cannot handle erased type")
 
     def visit_deleted_type(self, typ: DeletedType) -> None:
         pass
 
     def visit_partial_type(self, typ: PartialType) -> None:
-        raise RuntimeError
+        raise RuntimeError("Cannot handle partial type")
 
     def visit_tuple_type(self, typ: TupleType) -> None:
         for item in typ.items:

mypy/types.py

@@ -260,7 +260,7 @@ def can_be_false_default(self) -> bool:
         return True
 
     def accept(self, visitor: TypeVisitor[T]) -> T:
-        raise RuntimeError("Not implemented")
+        raise RuntimeError("Not implemented", type(self))
 
     def __repr__(self) -> str:
         return self.accept(TypeStrVisitor(options=Options()))