[repo-assist] docs: document known Debug-mode F# compiler bug affecting taskSeq (#473)

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What

Adds a Known Issues section to README.md documenting an upstream F# compiler regression discovered while investigating #473: Debug builds on .NET SDK 10.0.400+ / F# 10+ can silently truncate a taskSeq { } sequence that do!/awaits between yields. MoveNextAsync simply returns false after the first await — no exception, no warning.

Root cause

This is not a bug in FSharp.Control.TaskSeq. I reproduced the issue locally against both the published 1.1.1 NuGet package and this repo's current main branch source (SDK 10.0.400), and confirmed:

• It only reproduces in Debug (-c Debug) — Release (-c Release / -p:Optimize=true) always works correctly.
• It is already tracked upstream as dotnet/fsharp#20466 ("Incorrect state machine compilation in debug"), filed by a compiler-team contributor using this exact repo as the repro case.
• A fix is already in progress: dotnet/fsharp#20469 — "Debug compilation could route inline builders containing low-level resumable state machines through generated helper methods. That helper boundary hid nested ResumableCode definitions from lowering, causing incorrect execution for composed builders such as TaskSeq." This matches exactly how TaskSeqBuilder.fs's Bind/WhileAsync/For compose ResumableCode values.

Since this can't be fixed in library source (it's a compiler-lowering issue), the most useful contribution right now is documenting it clearly so users hitting it (like the reporter of #473, whose tests silently passed on only the first item because dotnet test defaults to Debug) can find a workaround quickly.

Change

• README.md: new "Known issues" section with a short description, links to the upstream issue/fix PR, and the workaround (build/test in Release, or set <Optimize>true</Optimize>).
• release-notes.txt: added an Unreleased bullet per repo convention.

Test Status

Documentation-only change; no source code modified. dotnet fantomas . --check not applicable (no .fs/.fsx files touched). No build or test run required per repo guidelines for docs-only changes.

Closes nothing directly, but relates to #473.

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Warning

Protected Files — Push Permission Denied

This was originally intended as a pull request, but the patch modifies protected files. A human must create the pull request manually.

Protected files

• README.md

The push was rejected because GitHub Actions does not have workflows permission to push these changes, and is never allowed to make such changes, or other authorization being used does not have this permission.

Create the pull request manually

# Download the patch from the workflow run
gh run download 34605897835 -n agent -D /tmp/agent-34605897835

# Create a new branch
git checkout -b repo-assist/improve-doc-debug-compiler-bug-473-17ff1f7bcf07f13d main

# Apply the patch (--3way handles cross-repo patches)
git am --3way /tmp/agent-34605897835/aw-repo-assist-improve-doc-debug-compiler-bug-473.patch

# Push the branch and create the pull request
git push origin repo-assist/improve-doc-debug-compiler-bug-473-17ff1f7bcf07f13d
gh pr create --title '[repo-assist] docs: document known Debug-mode F# compiler bug affecting taskSeq (#473)' --base main --head repo-assist/improve-doc-debug-compiler-bug-473-17ff1f7bcf07f13d --repo fsprojects/FSharp.Control.TaskSeq

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