livecheck: error on invalid url symbol

Up to now, we haven't been accounting for `#url` symbol arguments in
`livecheck` blocks that don't reference a checkable URL. This can
either be an invalid symbol (e.g., using the `:stable` formula symbol
in a cask) or a valid symbol where the referenced URL doesn't exist
(e.g., using `:head` when there's no `head` URL). [Almost all of the
valid symbols are required URLs but `head` is optional.]

Over the years, we've had a handful of slips where we've used `:url`
in formulae (when it's only valid in casks) and `:stable` in casks
(when it's only valid in formulae). In this scenario,
`livecheck_url_string` is `nil`, so livecheck falls back to
`#checkable_urls`. In this scenario, `stable` and `url` are the first
checkable URLs for formulae and casks (respectively), so the checks
ended up working as expected merely by chance. This isn't obvious in
the output and even the debug output looks normal. It only becomes
apparent that livecheck isn't working as expected if it iterates
through more than one checkable URL before reaching one that works
(not the case in those instances).

With that in mind, this adds an error when a `#url` symbol is used
but it doesn't correspond to a checkable URL. This will account for
both of the mentioned scenarios (invalid symbols and valid ones
referencing a non-existent URL) and prevent livecheck from quietly
proceeding in an unexpected manner.

Library/Homebrew/livecheck/livecheck.rb

@@ -611,7 +611,12 @@ def self.latest_version(
 
       referenced_package = referenced_formula_or_cask || formula_or_cask
 
-      livecheck_url_string = livecheck_url_to_string(livecheck_url, referenced_package) if livecheck_url
+      if livecheck_url
+        livecheck_url_string = livecheck_url_to_string(livecheck_url, referenced_package)
+        if livecheck_url.is_a?(Symbol) && !livecheck_url_string
+          raise ArgumentError, "`url :#{livecheck_url}` does not reference a checkable URL"
+        end
+      end
 
       urls = [livecheck_url_string] if livecheck_url_string
       urls ||= checkable_urls(referenced_package)

Library/Homebrew/test/dev-cmd/livecheck_spec.rb

@@ -20,7 +20,27 @@
       .and be_a_success
   end
 
-  it "gives an error when no arguments are given and there's no watchlist", :integration_test do
+  it "errors when a `livecheck` block URL symbol does not correspond to a checkable URL", :integration_test do
+    content = <<~RUBY
+      desc "Some test"
+      homepage "https://github.com/Homebrew/brew"
+      url "https://brew.sh/test-1.0.0.tgz"
+
+      # This is deliberately wrong, as `:url` should only be used in casks
+      # (the equivalent for formulae is `:stable`).
+      livecheck do
+        url :url
+      end
+    RUBY
+    setup_test_formula("test", content)
+
+    expect { brew "livecheck", "test" }
+      .to output(/`url :url` does not reference a checkable URL/).to_stderr
+      .and not_to_output.to_stdout
+      .and be_a_failure
+  end
+
+  it "errors when no arguments are given and there's no watchlist", :integration_test do
     expect { brew "livecheck", "HOMEBREW_LIVECHECK_WATCHLIST" => ".this_should_not_exist" }
       .to output(/Invalid usage: A watchlist file is required when no arguments are given\./).to_stderr
       .and not_to_output.to_stdout