bartelink · Aug 10, 2026 · Aug 10, 2026 · Aug 10, 2026 · Aug 10, 2026 · Aug 10, 2026
diff --git a/.fantomasignore b/.fantomasignore
@@ -75,6 +75,7 @@ src/Compiler/SyntaxTree/LexFilter.fs
 
 src/FSharp.Core/array2.fs
 src/FSharp.Core/array3.fs
+src/FSharp.Core/fslib-extra-pervasives.fs
 src/FSharp.Core/Linq.fs
 src/FSharp.Core/local.fs
 src/FSharp.Core/nativeptr.fs

diff --git a/.github/agents/compiler-perf-investigator.md b/.github/agents/compiler-perf-investigator.md
@@ -40,7 +40,7 @@ These are **general investigation instructions** for this agent, a template for
 ### 1. Preparation
 - **Setup:** Clone/generate repo/snippet/etc.
 - **Clear old config:** Remove `global.json` unless needed.
-- **Prepare local compiler:** Use `PrepareRepoForRegressionTesting.fsx` and absolute env paths.
+- **Prepare local compiler:** Build via `dotnet fsi <fsharp-repo>/eng/scripts/BuildWithLocalFSharp.fsx --build-script '<cmd>'` (sets the local-compiler + FSharp.Core shim env).
 
 ### 2. Experiment Matrix
 

diff --git a/.github/agents/expert-reviewer.md b/.github/agents/expert-reviewer.md
@@ -476,47 +476,49 @@ Execute review in five waves, each building on the previous.
 
 ### Wave 5: Deliver Review as Inline Comments
 
-**This wave is mandatory.** Analysis without posting is worthless — the review MUST appear on the PR as posted inline comments at the correct file and line.
+**This wave is mandatory.** Analysis without posting is worthless — the review MUST appear on the PR as inline comments anchored to the exact file and line. A prose review body is the failure this wave exists to prevent: the author wrote the diff, so narrating it back is noise. Every review body and every comment begins with the robot+detective disclosure `🤖🕵️`.
 
-1. **Deduplicate first.** Check existing reviews to avoid duplicate content:
+1. **Deduplicate first.**
    ```bash
    gh api repos/{owner}/{repo}/pulls/{number}/reviews --jq 'length'
    ```
    If reviews from automated accounts already exist, skip posting and apply the label only.
 
-2. **Post findings as a review with inline comments** at the best-fitting file and line. Use the GitHub API to create a single review with all comments:
+2. **Findings exist → one review, inline comments only.** Every finding — *including minor and "nice to have" ones* — is its own inline comment anchored to a file+line, so each becomes an independent, resolvable thread. Never collect findings into the body.
    ```bash
    gh api repos/{owner}/{repo}/pulls/{number}/reviews \
      --method POST \
      --field event=COMMENT \
-     --field body='' \
+     --field 'body=🤖🕵️ AI review — verify independently.' \
      --field 'comments=[
-      {"path":"src/Compiler/Checking/CheckDeclarations.fs","line":2750,"body":"**[Test Coverage]** This code path lacks a test. Add a test exercising this branch."},
-      {"path":"src/Compiler/CodeGen/IlxGen.fs","line":1234,"body":"**[IL Emission]** Call `stripTyEqns` before this match to handle type abbreviations."}
+      {"path":"src/Compiler/CodeGen/IlxGen.fs","line":1234,"body":"🤖🕵️ Type abbreviation slips past the match — wrong branch taken.\n```fsharp\ntype Alias = int\nlet f (x: Alias) = ()  // reaches this case, mishandled\n```"}
      ]'
    ```
-   Each comment needs:
-   - `path`: file path relative to repo root (from the diff)
-   - `line`: line number in the **new** version of the file (right side of diff)
-   - `body`: severity tag in `**[Dimension]**` format, the issue, and suggested fix
+   - Review **`body` = one short line**, the `🤖🕵️` disclosure. Never a summary, never a finding list.
+   - `path`: file path from the diff. `line`: line in the **new** file (right side of diff); use `start_line`+`line` for a range.
+   - Comment body: `🤖🕵️` + one fragment naming the **consequence** (what breaks), then a fenced **user-code** sample — the input/usage that triggers it, or the fix. Show how the problem looks in *user* code; do not restate the product diff.
+   - **Validated, not guessed.** If you have not run it and seen the result, drop the finding. "Would allocate" / "could box" / "may cancel" without a repro → drop.
+   - At most ~6–10 comments, ranked Behavioral > Quality > Nitpick.
 
-3. **If no significant issues found**, post an approving review:
+3. **No significant issues → approve.** Body is the disclosure plus, optionally, a collapsed list of what was covered — the ONE permitted multi-line body:
    ```bash
-   gh pr review {number} --repo {owner}/{repo} --approve --body "LGTM"
+   gh pr review {number} --repo {owner}/{repo} --approve \
+     --body $'🤖🕵️ LGTM ✅\n\n<details>\n<summary>Dimensions covered (expand)</summary>\n\n- IL emission\n- Binary compatibility\n- Concurrency\n</details>'
    ```
+   The `<details>` block is a bare list of the dimensions/subagents that ran — no prose, no per-dimension write-up.
 
 4. **Apply the label and request human review:**
    ```bash
    gh pr edit {number} --repo {owner}/{repo} --add-label AI-reviewed --add-reviewer T-Gro
    ```
 
 **Delivery rules:**
-- The review body MUST begin with an AI disclosure note (the `🤖` line above). This is non-negotiable — readers must know this is AI-generated, not human-written.
-- At most 10 inline comments — prioritize Behavioral over Quality over Nitpick.
-- Every comment must reference a specific file and line from the diff.
+- Every review body and every inline comment begins with `🤖🕵️`. Non-negotiable — readers must know this is AI-generated.
+- The review **body** is only ever: the one-line `🤖🕵️` disclosure (findings present), or `🤖🕵️ LGTM ✅` + the optional collapsed dimensions list (no findings). Anything else in the body — a summary, a numbered list of notes, praise, "two tiny things" — is a bug.
+- Every finding is an inline comment anchored to a specific file+line from the diff. No exception for "minor" or "nice" notes: if it is worth saying, anchor it.
+- Each comment carries a concrete **user-code** sample and describes **validated** behavior, not a guess.
+- **Banned in any body or comment:** prose summary, restating what the PR does, `**[Dimension]**`/`file:LINE` as a text header in the body, hedge/filler ("consider", "might want to", "non-blocking nit", "for consistency"), and any finding lacking a file+line or a concrete code sample.
 - Never post duplicate content if reviews already exist on the PR.
-- If the PR is too large to review fully, note skipped areas in the review body.
-- **NEVER write a wall-of-text review body.** The body is either `LGTM` (approve) or empty (with inline findings). All analysis stays in your context — only actionable findings get posted as inline comments. A review body longer than 10 characters (other than `LGTM`) is a bug.
 
 ## Folder Hotspot Mapping
 

diff --git a/.github/docs/state-machine.md b/.github/docs/state-machine.md
@@ -334,7 +334,7 @@ gh-aw safe-output defaults (suppressed below): `target: "*"`, `noop.report-as-is
 | `labelops-pr-maintenance` | `add-comment` | 5 | hide-older-comments: true |
 | `labelops-pr-maintenance` | `add-labels` | 3 | allowed: AI-needs-CI-fix-input |
 | `labelops-pr-maintenance` | `dispatch-workflow` | 3 | workflows: labelops-flake-fix |
-| `labelops-pr-security-scan` | `add-labels` | 50 | allowed: 10 labels (⚠️ Affects-* family + Scanned-Clean + Bypassed) |
+| `labelops-pr-security-scan` | `add-labels` | 50 | allowed: 11 labels (⚠️ Affects-* family + Suspicious-Prompting + Scope-Review-Needed + Scanned-Clean + Bypassed) |
 | `labelops-pr-security-scan` | `add-comment` | 25 | hide-older-comments: true |
 | `msbuild-quality-review` | `create-issue` | 1 | title `[msbuild-quality] `, labels: automation+Area-ProjectsAndBuild |
 | `msbuild-quality-review` | `create-pull-request` | 1 | draft: true, title `[msbuild-quality] `, protected-files: fallback-to-issue |
@@ -372,4 +372,4 @@ gh-aw safe-output defaults (suppressed below): `target: "*"`, `noop.report-as-is
 
 ---
 
-> generator-version: f107bba1a1cd61dc · source-shas: 06e56c52,149f0bbe,1af951a0,36b2b857,3775b51d,49b2989b,5e54b0e6,5e9a1344,7dca5b8f,9285c8a0,98d92f32,a5296399,acf12bdf,b5c04ea8,ec5fa486,f107bba1,
+> generator-version: f107bba1a1cd61dc · source-shas: 06e56c52,149f0bbe,1af951a0,36b2b857,3775b51d,420b9d6e,49b2989b,5e9a1344,7dca5b8f,9285c8a0,98d92f32,acf12bdf,b5c04ea8,d3e496db,ec5fa486,f107bba1,
diff --git a/.github/instructions/EngVersioning.instructions.md b/.github/instructions/EngVersioning.instructions.md
@@ -0,0 +1,17 @@
+---
+applyTo:
+  - "eng/Packages.props"
+  - "eng/Versions.props"
+  - "eng/Version.Details.xml"
+  - "eng/Version.Details.props"
+---
+
+These files feed the VMR source-build. The `System.*` dotnet/runtime packages marked *"Necessary for source-build"* in `eng/Version.Details.xml` are overridden at build time by the VMR's generated `PackageVersions.fsharp.props`, which replaces the darc-named `$(System*Version)` properties (e.g. `$(SystemCollectionsImmutableVersion)`) with the live source-built runtime version.
+
+Rules:
+
+- Consume those packages in `eng/Packages.props` through `$(System*Version)` **directly**. Never route them through a renamed or computed alias (e.g. a `$(System*CentralVersion)`) or a version floor — source-build overrides the darc-named property, and a renamed/eagerly-computed intermediate freezes the value so the override never reaches CPM, causing prebuilt packages and `NU1109` downgrades in the source-only build.
+- When a transitive consumer (MSBuild, Roslyn) requires a higher minimum than the flowed baseline, raise the declared version in `eng/Version.Details.{xml,props}` instead of adding a floor. Source-build overrides it back down to the live runtime version; product/PR restore uses the declared value.
+- fsharp PR/CI does **not** run the VMR source-only prebuilt gate (`SB_CentOSStream10_Online_MsftSdk_x64`), so a green PR restore does not prove source-build correctness. Reason about these edits against source-build.
+
+See `docs/postmortems/regression-sourcebuild-cpm-runtime-version-floor.md` for why.
diff --git a/.github/instructions/TypedTreePickle.instructions.md b/.github/instructions/TypedTreePickle.instructions.md
@@ -1,6 +1,7 @@
 ---
 applyTo:
   - "src/Compiler/TypedTree/TypedTreePickle.{fs,fsi}"
+  - "src/Compiler/TypedTree/TypedTree.{fs,fsi}"
   - "src/Compiler/Driver/CompilerImports.{fs,fsi}"
 ---
 
@@ -21,6 +22,12 @@ This means:
 2. **Additions must be invisible to old readers.** New data goes in stream B, where readers that don't know about it get `0` (the default sentinel) past end-of-stream. New readers detect presence via a tag byte they write unconditionally.
 3. **Tag values are forever.** Once a byte value means something in a reader's `match`, that meaning cannot change. Old DLLs encode that value with the old semantics.
 
+## Flag Enums: Reinterpreting a Tag Breaks Old Binaries
+
+The `ValFlags`, `EntityFlags`, and `TyparFlags` types in `TypedTree.fs` pack enum cases into bit patterns exposed as `PickledBits` and serialized verbatim. Adding a case to such an enum must **not** reuse a bit pattern that already exists in shipped metadata with different semantics. Normalizing the new case on the *write* side protects only future binaries — an older compiler already emitted the old pattern into DLLs that exist permanently. If you reuse a pattern, add matching *read*-side normalization (see `ValFlags.OfPickledBits`) that maps the legacy pattern back to its original meaning; otherwise prefer an unused pattern.
+
+For a detailed example of what goes wrong when a serialized flag pattern is reinterpreted, see `docs/postmortems/regression-legacy-inline-metadata-dynamic-invocation.md`.
+
 ## Reading and Writing Must Be Perfectly Aligned
 
 The format uses two parallel byte streams. Every `p_*` (write) function has a corresponding `u_*` (read) function. They must produce and consume the **exact same byte sequence** under **every possible code path** — including paths gated by feature flags, language versions, or target frameworks that your current build may not exercise.

diff --git a/Directory.Build.props b/Directory.Build.props
@@ -74,6 +74,8 @@
        Cannot live in eng/TargetFrameworks.props because it's imported before Arcade. -->
   <PropertyGroup Condition="'$(DotNetBuildSourceOnly)' == 'true'">
     <FSharpNetCoreProductTargetFramework>$(NetCurrent)</FSharpNetCoreProductTargetFramework>
+    <!-- Source-build targets only $(NetCurrent); collapse the shipped-net pin to it (its ref pack is always present). -->
+    <FSharpCoreShippedNetTargetFramework>$(NetCurrent)</FSharpCoreShippedNetTargetFramework>
   </PropertyGroup>
 
   <!-- It's important we set TFM before this, since FSharpTests.Directory.Build.props uses it.

diff --git a/Directory.Build.targets b/Directory.Build.targets
@@ -10,6 +10,13 @@
     <NoWarn>$(NoWarn);NU1507</NoWarn>
   </PropertyGroup>
 
+  <!-- net10.0 ships in the FSharp.Core package only; in-repo .NET consumers keep binding netstandard2.1. -->
+  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and '$(Configuration)' != 'Proto'">
+    <ProjectReference Update="@(ProjectReference)">
+      <SetTargetFramework Condition="'%(ProjectReference.Filename)' == 'FSharp.Core' and '%(ProjectReference.SetTargetFramework)' == ''">TargetFramework=netstandard2.1</SetTargetFramework>
+    </ProjectReference>
+  </ItemGroup>
+
   <!-- Disable R2R when building source-only and not targeting the current SDK bundled TFM. -->
   <PropertyGroup>
     <PublishReadyToRun Condition="'$(DotNetBuildSourceOnly)' == 'true' and

diff --git a/FSharpBuild.Directory.Build.targets b/FSharpBuild.Directory.Build.targets
@@ -97,12 +97,23 @@
     </ItemGroup>
   </Target>
 
-      <!-- SDK targets override -->
-  <PropertyGroup Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')">
-    <FSharpBuildAssemblyFileOverride>$(ProtoOutputPath)\fsc\FSharp.Build.dll</FSharpBuildAssemblyFileOverride>
-  </PropertyGroup>  
-  <UsingTask TaskName="FSharpEmbedResourceText" AssemblyFile="$(FSharpBuildAssemblyFileOverride)" Override="true" Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')" />
-  <UsingTask TaskName="FSharpEmbedResXSource" AssemblyFile="$(FSharpBuildAssemblyFileOverride)" Override="true" Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')" />   
+  <!-- SDK targets override: the SDK's released FSharp.Build cannot generate the classified resource
+       accessors FSComp.txt is compiled with, so the tasks come from the Bootstrap, or from this repo's own
+       build for projects opting in - FSharp.Build itself cannot, its FSBuild.txt would need its own output
+       before it is built. No Exists check, so a missing assembly is an error rather than a silent fallback
+       to the SDK's. TaskHostFactory keeps the node from locking what it loads. -->
+  <PropertyGroup>
+    <FSharpBuildTasksAssembly Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)' != 'true' AND Exists('$(ProtoOutputPath)')">$(ProtoOutputPath)\fsc\FSharp.Build.dll</FSharpBuildTasksAssembly>
+    <FSharpBuildTasksAssembly Condition="'$(FSharpBuildTasksAssembly)' == '' AND '$(UseBootstrappedFSharpBuildTasks)' == 'true'">$(ArtifactsDir)bin\FSharp.Build\$(Configuration)\netstandard2.0\FSharp.Build.dll</FSharpBuildTasksAssembly>
+  </PropertyGroup>
+  <UsingTask TaskName="FSharpEmbedResourceText" AssemblyFile="$(FSharpBuildTasksAssembly)" Override="true" TaskFactory="TaskHostFactory" Condition="'$(FSharpBuildTasksAssembly)' != ''" />
+  <UsingTask TaskName="FSharpEmbedResXSource" AssemblyFile="$(FSharpBuildTasksAssembly)" Override="true" TaskFactory="TaskHostFactory" Condition="'$(FSharpBuildTasksAssembly)' != ''" />
+
+  <Target Name="BuildFSharpBuildForResourceGeneration"
+          BeforeTargets="GenerateFSharpTextResources"
+          Condition="'$(UseBootstrappedFSharpBuildTasks)' == 'true' AND '$(FSharpBuildTasksAssembly)' != '' AND !Exists('$(FSharpBuildTasksAssembly)')">
+    <MSBuild Projects="$(RepoRoot)src\FSharp.Build\FSharp.Build.fsproj" RemoveProperties="TargetFramework" />
+  </Target>
 
 
   <Target Name="BeforeResGen"