Make ALE complain when conflicting command options are used

autoload/ale/linter.vim

@@ -114,6 +114,15 @@ function! ale#linter#PreProcess(linter) abort
         \   . 'must be defined'
     endif
 
+    if (
+    \   has_key(a:linter, 'command')
+    \   + has_key(a:linter, 'command_chain')
+    \   + has_key(a:linter, 'command_callback')
+    \) > 1
+        throw 'Only one of `command`, `command_callback`, or `command_chain` '
+        \   . 'should be set'
+    endif
+
     let l:obj.output_stream = get(a:linter, 'output_stream', 'stdout')
 
     if type(l:obj.output_stream) != type('')

test/test_linter_defintion_processing.vader

@@ -211,3 +211,26 @@ Execute(PreProcess should complain about invalid chain items at higher indices):
   \}
   AssertThrows call ale#linter#PreProcess(g:linter)
   AssertEqual 'The `command_chain` item 1 must define a `callback` function', g:vader_exception
+
+Execute(PreProcess should complain when conflicting command options are used):
+  let g:linter = {
+  \ 'name': 'x',
+  \ 'callback': 'x',
+  \ 'executable': 'x',
+  \ 'command': 'foo',
+  \ 'command_chain': [{'callback': 'foo'}],
+  \}
+  AssertThrows call ale#linter#PreProcess(g:linter)
+  AssertEqual 'Only one of `command`, `command_callback`, or `command_chain` should be set', g:vader_exception
+
+  unlet g:linter.command
+  let g:linter.command_callback = 'foo'
+
+  AssertThrows call ale#linter#PreProcess(g:linter)
+  AssertEqual 'Only one of `command`, `command_callback`, or `command_chain` should be set', g:vader_exception
+
+  let g:linter.command = 'foo'
+  unlet g:linter.command_chain
+
+  AssertThrows call ale#linter#PreProcess(g:linter)
+  AssertEqual 'Only one of `command`, `command_callback`, or `command_chain` should be set', g:vader_exception