Bugfix :: SRTP - ApplyDefaults for TyparConstraint.MayResolveMember

EXECUTIVE_SUMMARY.md

@@ -0,0 +1,77 @@
+# FSharpPlus curryN Regression Fix - Executive Summary
+
+## The Bug
+
+FSharpPlus `curryN` pattern worked in F# SDK 8.0 but failed in SDK 9.0+ with error FS0030 (value restriction):
+
+```fsharp
+let _x1 = curryN f1 100  // SDK 8: OK | SDK 9+: FS0030 error
+```
+
+## Root Cause
+
+In `CheckDeclarations.fs`, the `ApplyDefaults` function processes unsolved type variables at the end of type checking. The existing code only solved typars with `StaticReq <> None`:
+
+```fsharp
+if (tp.StaticReq <> TyparStaticReq.None) then
+    ChooseTyparSolutionAndSolve cenv.css denvAtEnd tp
+```
+
+**The problem**: Some SRTP (Statically Resolved Type Parameter) typars have `MayResolveMember` constraints but `StaticReq=None`. These typars were being skipped, leaving them unsolved. When `CheckValueRestriction` ran next, it found unsolved typars and reported FS0030.
+
+## Why This Is a Bug
+
+The `ApplyDefaults` code checks `StaticReq <> None` to identify SRTP typars that need solving. However, a typar may participate in an SRTP constraint (having a `MayResolveMember` constraint) without having `StaticReq` set. This can happen when:
+
+1. The typar is the **result type** of an SRTP method call, not the head type
+2. The typar is constrained through SRTP constraints but isn't directly marked with `^`
+
+**Key insight from instrumentation:**
+```
+  - ? (StaticReq=None, Constraints=[MayResolveMember, CoercesTo])  ← HAS SRTP CONSTRAINT!
+[ApplyDefaults] After processing: 17 still unsolved  ← SRTP typar SKIPPED because StaticReq=None
+```
+
+The condition `tp.StaticReq <> None` was too narrow - it missed typars that have SRTP constraints but no explicit static requirement.
+
+## Regression Analysis - Git Blame Evidence
+
+**Root Cause: PR #15181 (commit `b73be1584`) - Nullness Checking Feature**
+
+The regression was introduced by `FreshenTypar` added in PR #15181:
+
+```fsharp
+// src/Compiler/Checking/NameResolution.fs:1600-1604
+let FreshenTypar (g: TcGlobals) rigid (tp: Typar) =
+    let clearStaticReq = g.langVersion.SupportsFeature LanguageFeature.InterfacesWithAbstractStaticMembers
+    let staticReq = if clearStaticReq then TyparStaticReq.None else tp.StaticReq  // ← BUG!
+    ...
+```
+
+**The Mechanism:**
+
+1. **SDK 8**: `FreshenTypar` did not exist. When typars were freshened, `StaticReq` was preserved from the original typar.
+
+2. **SDK 9+**: When `InterfacesWithAbstractStaticMembers` is enabled (always on), `FreshenTypar` **clears `StaticReq` to `None`** unconditionally.
+
+3. **Effect**: SRTP typars still have `MayResolveMember` constraints, but lose their `StaticReq` marker.
+
+4. **Consequence**: `ApplyDefaults` checks `if tp.StaticReq <> None` → returns false → typar never solved → FS0030 error.
+
+**The fix** adds an alternative check for `MayResolveMember` constraints directly, making `ApplyDefaults` robust against this `StaticReq` clearing.
+
+## The Fix
+
+Added a check for `MayResolveMember` constraints in addition to `StaticReq`:
+
+```fsharp
+let hasSRTPConstraint = tp.Constraints |> List.exists (function TyparConstraint.MayResolveMember _ -> true | _ -> false)
+if (tp.StaticReq <> TyparStaticReq.None) || hasSRTPConstraint then
+    ChooseTyparSolutionAndSolve cenv.css denvAtEnd tp
+```
+
+## Verification
+
+- ✅ New curryN-style regression test passes
+- ✅ FSharpPlus `curryN` pattern compiles without type annotations
+- ✅ No regressions introduced in SRTP-related tests

azure-pipelines-PR.yml

@@ -914,3 +914,17 @@ stages:
           buildScript: build.sh
           displayName: FSharpPlus_Linux
           useVmImage: $(UbuntuMachineQueueName)
+        - repo: fsprojects/FSharpPlus
+          commit: 2648efe
+          buildScript: build.cmd
+          displayName: FsharpPlus_NET10
+         # remove this before merging
+        - repo: fsprojects/FSharpPlus
+          commit: 2648efe
+          buildScript: dotnet msbuild build.proj -t:Build;Pack;Test;AllDocs
+          displayName: FsharpPlus_NET10_FullTestSuite 
+        - repo: fsprojects/FSharpPlus
+          commit: 2648efe
+          buildScript: build.sh
+          displayName: FsharpPlus_Net10_Linux
+          useVmImage: $(UbuntuMachineQueueName)

docs/release-notes/.FSharp.Compiler.Service/10.0.200.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Fixed SRTP resolution regression causing FS0030 value restriction errors with FSharpPlus curryN-style patterns in .NET 9 SDK. ([PR #19218](https://github.com/dotnet/fsharp/pull/19218))
 * Type relations cache: optimize key generation ([Issue #19116](https://github.com/dotnet/fsharp/issues/18767)) ([PR #19120](https://github.com/dotnet/fsharp/pull/19120))
 * Fixed QuickParse to correctly handle optional parameter syntax with `?` prefix, resolving syntax highlighting issues. ([Issue #11008753](https://developercommunity.visualstudio.com/t/F-Highlighting-fails-on-optional-parame/11008753)) ([PR #XXXXX](https://github.com/dotnet/fsharp/pull/XXXXX))
 * Fix `--preferreduilang` switch leaking into `fsi.CommandLineArgs` when positioned after script file ([PR #19151](https://github.com/dotnet/fsharp/pull/19151))