Fix #5210 Don't refer to index.html when Haddock will not create it

Cabal/src/Distribution/Simple/Haddock.hs

@@ -821,14 +821,21 @@ renderArgs verbosity tmpFileOpts version comp platform args k = do
         else k (renderedArgs, result)
   where
     outputDir = (unDir $ argOutputDir args)
+    isNotArgContents = isNothing (flagToMaybe $ argContents args)
+    isNotArgIndex = isNothing (flagToMaybe $ argIndex args)
+    isArgGenIndex = fromFlagOrDefault False (argGenIndex args)
+    -- Haddock, when generating HTML, does not generate an index if the options
+    -- --use-contents or --use-index are passed to it. See
+    -- https://haskell-haddock.readthedocs.io/en/latest/invoking.html#cmdoption-use-contents
+    isIndexGenerated = isArgGenIndex && isNotArgContents && isNotArgIndex
     result =
       intercalate ", "
         . map
           ( \o ->
               outputDir
                 </> case o of
                   Html
-                    | fromFlagOrDefault False (argGenIndex args) ->
+                    | isIndexGenerated ->
                         "index.html"
                   Html
                     | otherwise ->

release-notes/Cabal-3.10.2.0.md

@@ -24,3 +24,7 @@ Cabal and Cabal-syntax 3.10.2.0 changelog and release notes
 
   - Regenerate Cabal-syntax's Lexer.hs with Alex 3.2.7.3 which includes a fix for
     an out-of-bound array access (only noticeable with GHC's JavaScript backend).
+
+- The `haddock` command will no longer refer to the creation of a `index.html`
+  file when Haddock does not create that file
+  [#5210](https://github.com/haskell/cabal/issues/5210)

release-notes/cabal-install-3.10.2.0.md

@@ -58,3 +58,7 @@ cabal-install and cabal-install-solver 3.10.2.0 changelog and release notes
 
   - Added Hackage root key for Joachim Breitner
   - Added Hackage root key for Mathieu Boespflug
+
+- The `haddock` command will no longer refer to the creation of a `index.html`
+  file when Haddock does not create that file
+  [#5210](https://github.com/haskell/cabal/issues/5210)