Prevent defining stubs on undefined methods in LazyDouble

In Crystal 1.6, a segfault would occur in the spec spec/spectator/mocks/lazy_double_spec.cr:238
I suspect this is a Crystal bug of some kind, but can't reduce it.
The methods produced by `method_missing` don't have a return type including Symbol.
Symbol is excluded from the union of return types (Int32 | String | Nil).
The program segfaults when calling a method on the actual value, which is a symbol.
It ultimately crashes when producing a failure message, which indicates the value it tested doesn't equal the expected value (a symbol of the same value).
Avoid this issue by preventing stubs on undefined/untyped methods.

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Removed
+- Removed support for stubbing undefined (untyped) methods in lazy doubles. Avoids possible segfault.
+
 ## [0.11.3] - 2022-09-03
 ### Fixed
 - Display error block (failure message and stack trace) when using `fail`. [#78](https://gitlab.com/arctic-fox/spectator/-/issues/78)

spec/spectator/mocks/lazy_double_spec.cr

@@ -235,16 +235,9 @@ Spectator.describe Spectator::LazyDouble do
     end
 
     context "with previously undefined methods" do
-      it "can stub methods" do
+      it "raises an error" do
         stub = Spectator::ValueStub.new(:baz, :xyz)
-        dbl._spectator_define_stub(stub)
-        expect(dbl.baz).to eq(:xyz)
-      end
-
-      it "uses a stub only if an argument constraint is met" do
-        stub = Spectator::ValueStub.new(:baz, :xyz, Spectator::Arguments.capture(:right))
-        dbl._spectator_define_stub(stub)
-        expect { dbl.baz }.to raise_error(Spectator::UnexpectedMessage, /baz/)
+        expect { dbl._spectator_define_stub(stub) }.to raise_error(/stub/)
       end
     end
   end
@@ -258,15 +251,6 @@ Spectator.describe Spectator::LazyDouble do
     it "removes previously defined stubs" do
       expect { dbl._spectator_clear_stubs }.to change { dbl.foo }.from(5).to(42)
     end
-
-    it "raises on methods without an implementation" do
-      stub = Spectator::ValueStub.new(:baz, :xyz)
-      dbl._spectator_define_stub(stub)
-      expect(dbl.baz).to eq(:xyz)
-
-      dbl._spectator_clear_stubs
-      expect { dbl.baz }.to raise_error(Spectator::UnexpectedMessage, /baz/)
-    end
   end
 
   describe "#_spectator_calls" do

src/spectator/mocks/lazy_double.cr

@@ -26,6 +26,15 @@ module Spectator
       super(_spectator_double_stubs + message_stubs)
     end
 
+    # Defines a stub to change the behavior of a method in this double.
+    #
+    # NOTE: Defining a stub for a method not defined in the double's type raises an error.
+    protected def _spectator_define_stub(stub : Stub) : Nil
+      return super if Messages.types.has_key?(stub.method)
+
+      raise "Can't define stub #{stub} on lazy double because it wasn't initially defined."
+    end
+
     # Returns the double's name formatted for user output.
     private def _spectator_stubbed_name : String
       "#<LazyDouble #{@name || "Anonymous"}>"