As I’ve written above, dialyzer errors properly on Map.put/3, guarding structs if you do use typespecs…
…However noticed that Map.delete/2 would still pass, even if the required key defined in the struct typespec is missing 
Example, the below returns an empty map after all, and still passes as it’s returning a t:
@spec foo() :: t
def foo do
%__MODULE__{
a: 1,
b: "aha",
c: :foo
}
|> Map.delete(:a)
|> Map.delete(:b)
|> Map.delete(:c)
|> Map.delete(:__struct__)
end
It isn’t a struct specific issue, a simple map also has the same issue (no errors):
@spec foo() :: %{required(:a) => non_neg_integer()}
def foo do
%{a: 1}
|> Map.delete(:a)
end
I think it is a dialyzer bug, let’s see with the OTP guys what this behaviour turns out to be, bug or feature 
