[Repo Assist] Fix crash in Markdown.ToMd when document contains indirect images (#1094)

* Fix crash in Markdown.ToMd when document contains indirect images

Markdown.ToMd (formatAsMarkdown) called formatSpan in MarkdownUtils.fs
which had a failwith "tbd - IndirectImage" for both arms of IndirectImage.
This caused any call to Markdown.ToMd on a document containing

Fix: implement the two cases by analogy with IndirectLink:
  (preserving the original indirect form)

Add two regression tests in FSharp.Markdown.Tests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 * Add regression test confirming that types whose name matches their enclosing namespace are correctly included in generated API docs. [#944](https://github.com/fsprojects/FSharp.Formatting/issues/944)
+* Fix crash (`failwith "tbd - IndirectImage"`) when `Markdown.ToMd` is called on a document containing reference-style images (`![alt][ref]`). The indirect image is now serialised as `![alt](url)` when the reference is resolved, or `![alt][ref]` when it is not.
 
 ## 22.0.0-alpha.2 - 2026-03-13
 

src/FSharp.Formatting.Markdown/MarkdownUtils.fs

@@ -110,9 +110,9 @@ module internal MarkdownUtils =
         | DirectLink(body, link, _, _)
         | IndirectLink(body, link, _, _) -> "[" + formatSpans ctx body + "](" + link + ")"
 
-        | IndirectImage(_body, _, LookupKey ctx.Links (_link, _), _)
-        | IndirectImage(_body, _link, _, _) -> failwith "tbd - IndirectImage"
-        | DirectImage(_body, _link, _, _) -> sprintf "![%s](%s)" _body _link
+        | IndirectImage(body, _, LookupKey ctx.Links (link, _), _) -> sprintf "![%s](%s)" body link
+        | IndirectImage(body, _, key, _) -> sprintf "![%s][%s]" body key
+        | DirectImage(body, link, _, _) -> sprintf "![%s](%s)" body link
         | Strong(body, _) -> "**" + formatSpans ctx body + "**"
         | InlineCode(body, _) -> "`" + body + "`"
         | Emphasis(body, _) -> "**" + formatSpans ctx body + "**"

tests/FSharp.Markdown.Tests/Markdown.fs

@@ -1201,3 +1201,28 @@ let ``Paragraph between sublists should not be absorbed into first sublist item
     paragraphPos |> should be (greaterThan firstSublistEnd)
     // Second sublist starts after paragraph.
     secondSublistStart |> should be (greaterThan paragraphPos)
+
+// --------------------------------------------------------------------------------------
+// ToMd round-trip: indirect images (issue - failwith "tbd - IndirectImage")
+// --------------------------------------------------------------------------------------
+
+[<Test>]
+let ``ToMd round-trip: indirect image with resolved reference`` () =
+    // Markdown with a reference-style image and a reference definition
+    let input = "![alt text][img-id]\n\n[img-id]: http://example.com/image.png \"Title\""
+    let doc = Markdown.Parse(input)
+    // Should NOT throw (was failing with failwith "tbd - IndirectImage")
+    let result = Markdown.ToMd(doc)
+    // When key is resolved, should render as a direct image
+    result |> should contain "![alt text]("
+    result |> should contain "http://example.com/image.png"
+
+[<Test>]
+let ``ToMd round-trip: indirect image with unresolved reference`` () =
+    // Parse just the image token without a reference definition
+    let input = "![alt text][unknown-ref]"
+    let doc = Markdown.Parse(input)
+    // Should NOT throw (was failing with failwith "tbd - IndirectImage")
+    let result = Markdown.ToMd(doc)
+    // When key is not resolved, should preserve the indirect form
+    result |> should contain "![alt text][unknown-ref]"