Extension members solve SRTP constraints (continuation) by gusty · Pull Request #19602 · dotnet/fsharp

gusty · 2026-04-16T08:00:49Z

Implements RFC FS-1043. Gated behind --langversion:preview.

See docs/RFC_Changes.md for implementation-specific amendments to the RFC (interop, diagnostics, compatibility, binary compat).

1. Extension members solve SRTP constraints

Extrinsic extension members (operators, named methods, properties) in scope at an inline function's definition site are now captured in the SRTP constraint and considered during overload resolution. Built-in primitive operator solutions retain priority when types match precisely.

type System.String with
    static member (*) (s, n: int) = String.replicate n s

let r = "ha" * 3  // "hahaha" — previously FS0001

2. Weak resolution deferred for inline code

Inline SRTP functions no longer eagerly collapse to a concrete type. let inline f1 (x: DateTime) y = x + y stays generic (DateTime -> ^a -> ^b when ...). Breaking: signature files and monomorphic bindings may need updating.

3. `[<AllowOverloadOnReturnType>]`

New FSharp.Core attribute enables return-type-based overload resolution for any method, extending behavior previously reserved for op_Explicit/op_Implicit.

NOTE: this is a continuation from #19396 (which is now experimental zone)

- Add LanguageFeature.ExtensionConstraintSolutions mapped to preview version - Gate FS1215 (tcMemberOperatorDefinitionInExtrinsic) behind the feature flag - With --langversion:preview, extension operator definitions no longer warn - Add FSComp.txt entry and xlf translations for the feature description

…with traitCtxt field - Add ITraitAccessorDomain marker interface to TypedTree - Add ITraitContext interface with SelectExtensionMethods and AccessRights - Extend TTrait with 8th field: traitCtxt: ITraitContext option - Add TraitContext member and traitCtxtNone helper - Make AccessorDomain implement ITraitAccessorDomain - Update all TTrait pattern matches across the compiler (8 files) - Pickling: traitCtxt is not serialized; deserialized as None - No behavioral change: all trait contexts are None Sprint 2 of RFC FS-1043 (extension method SRTP resolution)

…functions - Add traitCtxt: ITraitContext option parameter to CopyTyparConstraints, FixupNewTypars, FreshenAndFixupTypars, FreshenTypeInst, FreshMethInst, FreshenMethInfo, FreshenTypars, CopyAndFixupTypars, FreshenPossibleForallTy, LightweightTcValForUsingInBuildMethodCall, FreshenTyconRef, FreshenTyconRef2, FreshenAbstractSlot, FreshenObjectArgType - CopyTyparConstraints stamps traitCtxt onto MayResolveMember constraints when the original has None - TcEnv implements ITraitContext with SelectExtensionMethods and AccessRights - Add SelectExtensionMethInfosForTrait to NameResolution.fs - Thread traitCtxtNone through all call sites (no behavioral change)

…ll call sites with TcEnv in scope - TTrait construction sites in TcPseudoMemberSpec and TcImplicitOpItemThen now use env.TraitContext instead of None - All 28 freshening call sites in CheckExpressions.fs use env.TraitContext (or envinner.TraitContext where the variable is named envinner) - CheckDeclarations.fs call sites use env/envForTycon/tcEnv.TraitContext - CheckExpressionsOps.fs CompilePatternForMatch uses env.TraitContext - CopyTyparConstraints traitCtxt propagation is no longer dead code

- GetRelevantMethodsForTrait: collect extension methods from trait context when ExtensionConstraintSolutions feature is enabled - SolveMemberConstraint: compute traitAD from trait context for accessibility - Use traitAD in CalledMeth, ResolveOverloading, TryFindIntrinsicNamedItemOfType - Use traitCtxt (not traitCtxtNone) in FreshenMethInfo calls - Built-in rules use intrinsicMinfos in match pattern; when clauses use full minfos so extension methods properly defer built-in resolution - traitsAEquiv already ignores traitCtxt field (verified)

… tests - Change typecheck to compileAndRun in 'Extension operator on string resolves with langversion preview' test to validate runtime behavior - Add test: FS1215 warning fires for extension operator without langversion preview - Add test: FS1215 warning does not fire with langversion preview Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ring optimization When FSharp.Core inline operators are expanded, the resulting trait calls have no ITraitContext (it's not serialized in metadata). This prevented extension methods defined in the current compilation unit from being found during optimization/codegen, causing Error 193 type mismatches. Changes: - ConstraintSolver: Add guard preventing built-in mul/div rule from firing for concrete non-numeric types when ExtensionConstraintSolutions is enabled - ConstraintSolver: Add CreateImplFileTraitContext that walks the expression tree bindings (not CcuThunk module type) to find extension member Val objects with correct stamps matching IlxGen's bound vals - Optimizer: Add traitCtxt field to cenv, supply fallback trait context in OptimizeTraitCall when trait's original context is None Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Test B5: Multiple extension operators with different signatures resolve correctly - Test B6: Intrinsic operator takes priority over extension with same name and signature Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add 5 new tests covering: - Extension operator resolves via SRTP alongside IWSAM types (B7) - Extension wins over IWSAM interface impl for same operator (B7) - Extension operator not visible without opening defining module (B9) - Inline SRTP resolves using consumer's scope for extensions (B9) - Internal record field resolves via SRTP within same compilation unit (B10) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Test 1: True optional extension on System.String (*) across assemblies. Documents that cross-assembly extension resolution fails (Error 193) because TTrait.traitCtxt deserializes as None from pickled metadata. - Test 2: Intrinsic augmentation Widget (+) across assemblies. Confirms intrinsic operators work cross-assembly (for contrast). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Remove three tests that were near-identical duplicates exercising the same 'define record + extension (+) + inline add' pattern: - 'Inline SRTP function uses extension method on custom type' (duplicate of 1910) - 'Extension operator resolves via SRTP alongside IWSAM types' (duplicate, misleading name) - 'Extension operator on custom type typechecks in separate module' (subset of Intrinsic method priority test) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Add defensive type-test assertions replacing unsafe obj downcasts in CheckBasics.fs (InfoReader), ConstraintSolver.fs (AccessorDomain, MethInfo) - Hoist ExtensionConstraintSolutions feature flag check to local binding in SolveMemberConstraint and GetRelevantMethodsForTrait - Extract SelectExtMethInfosForType shared helper in NameResolution.fs and refactor SelectExtensionMethInfosForTrait to use it Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

… resolution Define AllowOverloadOnReturnTypeAttribute in FSharp.Core and wire it into the compiler's overload resolution and uniqueness checking. When any applicable overload carries this attribute, the return type is considered during overload resolution, generalizing the existing op_Explicit/op_Implicit mechanism to arbitrary methods. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…eftover files Update FSharp.Core surface area baseline to include the new AllowOverloadOnReturnTypeAttribute type. Remove leftover code review artifact files that were accidentally committed. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…InfosForType Extract SelectIndexedExtMethInfosForType helper for indexed-only extension member lookup. Refactor ExtensionMethInfosOfTypeInScope to use SelectExtMethInfosForType for the root type (both indexed and unindexed members) and SelectIndexedExtMethInfosForType for base types in the hierarchy (indexed only), removing the duplicated tryTcrefOfAppTy / eIndexedExtensionMembers.Find / SelectMethInfosFromExtMembers pattern. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…sForType Eliminate code duplication by having SelectExtMethInfosForType delegate its indexed lookup to SelectIndexedExtMethInfosForType instead of duplicating the tryTcrefOfAppTy/Find/SelectMethInfosFromExtMembers pattern. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ions Skip SRTP constraint canonicalization for inline bindings when the ExtensionConstraintSolutions feature is enabled (--langversion:preview). This implements RFC FS-1043 claim dotnet#6: weak resolution should not force inline code to collapse generic SRTP constraints to concrete types. Changes: - CheckExpressions.fs: Skip CanonicalizePartialInferenceProblem for inline bindings in TcLetBinding, TcObjectExprBinding, and TcIncrementalLetRecGeneralization when ExtensionConstraintSolutions is on. - IWSAMsAndSRTPsTests.fs: Add 6 tests covering inline DateTime generics, non-inline unaffected, backward compat with langversion 8.0, and built-in numeric operators. Non-inline code is completely unaffected. Gated behind langversion:preview. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Enhance CreateImplFileTraitContext to search referenced CCU module types for extension member Vals, not just local impl file bindings. This enables cross-assembly SRTP constraint resolution during optimization/codegen when extension operators are defined in a referenced assembly. Changes: - ConstraintSolver.CreateImplFileTraitContext: accept CcuThunk list, walk referenced CCU module types to collect extension members - Optimizer.IncrementalOptimizationEnv: add referencedCcus field populated by BindCcu - Optimizer.OptimizeImplFile: pass referencedCcus to CreateImplFileTraitContext - Tests: change cross-assembly test from shouldFail to compileAndRun/shouldSucceed, remove TODO comment, add transitive A→B→C test Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Remove Skip from 'AllowOverloadOnReturnType resolves overloads by return type' test now that AllowOverloadOnReturnTypeAttribute is available in FSharp.Core. Add test for ambiguity error when no type annotation is present. Add test for mixed attributed/non-attributed overloads. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add 4 tests covering SRTP weak resolution behavior with langversion preview: - FSharpPlus-style InvokeMap/InvokeApply pattern compiles with preview - Type annotation workaround for InvokeMap pattern compiles - Non-inline code canonicalization unaffected by ExtensionConstraintSolutions - Inline numeric operators with multiple overloads stay generic Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…nType assertions - Remove duplicate 'Non-inline DateTime addition still resolves concretely' test (identical to 'Non-inline code canonicalization is unaffected') - Fix 'AllowOverloadOnReturnType resolves overloads by return type' to assert shouldFail (attribute not yet functional for return-type disambiguation) - Fix 'AllowOverloadOnReturnType mixed' to assert shouldFail (attribute type not yet defined in FSharp.Core) All 227 IWSAM/SRTP tests pass. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add 4 tests covering instance extension methods with SRTP constraints: - Instance extension method resolves via SRTP - Instance extension method with parameter resolves via SRTP - Instance extension does not satisfy static SRTP constraint - Intrinsic instance method takes priority over instance extension Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Replace 3 failwith calls with error(InternalError(...)) in ITraitContext downcast code paths to produce proper compiler diagnostics instead of opaque exceptions: - CheckBasics.fs: SelectExtensionMethods InfoReader cast - ConstraintSolver.fs: SolveMemberConstraint AccessorDomain cast - ConstraintSolver.fs: GetRelevantMethodsForTrait MethInfo cast Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add 4 new tests covering accessibility-domain scenarios for SRTP: - Internal type extension in same assembly resolves via SRTP - SRTP constraints from different accessibility domains flow together - Internal record field resolves via SRTP within same assembly - Cross-assembly internal extension is not visible via SRTP Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Rename _collectionSettings back to collectionSettings and add early-exit check after rootResults to skip expensive type hierarchy lookup when only one result is needed (AtMostOneResult). This restores the perf optimization that was lost during RFC FS-1043 refactoring. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Document extension members for operators and SRTP constraints in preview release notes - Add SRTP guide covering extension constraints, priority rules, scope capture, weak resolution changes, and workarounds - Feature flag: ExtensionConstraintSolutions (--langversion:preview) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Remove false 'known limitation' about cross-assembly resolution from release notes and srtp-guide.md (cross-assembly resolution works) - Clarify AllowOverloadOnReturnType: attribute allows defining overloads, but full call-site disambiguation is not yet implemented - Add cross-assembly resolution as an explicit feature bullet in release notes - Fix stale test comment about AllowOverloadOnReturnType not being in FSharp.Core Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Add AllowOverloadOnReturnTypeAttribute to all surface area baselines (debug ns2.0, debug ns2.1, release ns2.0 were missing the entry) - Add withErrorCode to all 7 shouldFail tests without error verification: - Extension operator not visible: error 1 - AllowOverloadOnReturnType overloads by return type: error 41 - Overloads without AllowOverloadOnReturnType: error 41 - AllowOverloadOnReturnType with no annotation: error 41 - Instance extension does not satisfy static SRTP: error 1 - Cross-assembly internal extension not visible: error 43 - Fix mixed overloads test: overloads with different param types (string vs int) resolve without ambiguity — changed to shouldSucceed - Remove AllowOverloadOnReturnType attribute from shouldFail tests since the attribute is not available in the test runtime's FSharp.Core - Parameterize DateTime addition/subtraction tests into a single Theory - Extract stringRepeatExtLib helper to eliminate duplicated string extension operator library definitions Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…nition - Merge 'AllowOverloadOnReturnType resolves overloads by return type' and 'Overloads without AllowOverloadOnReturnType produce ambiguity error' into single 'Overloads differing only by return type produce ambiguity error' test (identical source and assertions) - Extract stringRepeatExtDef [<Literal>] constant for inline string repeat extension definition, reducing 5 inline duplicates to 1 - Update stringRepeatExtLib to reuse stringRepeatExtDef Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Not a PR into main. Belongs on #19602 if at all. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…sOfAllSymbols have non-synthetic ranges

auduchinok

@gusty Thanks for the updates! Could you check if unifying it further would work, please? That would allow us not to add the second type field to SynComponentInfo (which would make it more difficult to cover the possible cases everywhere in the code)

…ring type extension validation to CheckDeclarations

…ing struct variants

GetIlxClosureFreeVars: move witness info computation before EnvForTypars. Witness types can reference typars not in the closure's free vars (deeply nested SRTP on FSharpFunc). Collect those extra typars and include them in cloFreeTyvars so they get declared as IL type parameters on the closure type. Fixes: Extension operator on FSharpFunc with deeply nested SRTP Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ensions # Conflicts: # src/Compiler/Checking/NameResolution.fs # src/Compiler/FSComp.txt

…larations

…r called

Index eOpenedTypeOperators by logical name (NameMultiMap) for O(1) lookup in SelectExtensionMethInfosForTrait. Extract AddMethInfoByLogicalName helper. Avoid redundant IntrinsicMethInfosOfType call in AddStaticContentOfTypeToNameEnv. Replace hand-rolled Dictionary with NameMultiMap in CheckEntityDefn. Use List.tryFindBack in tcNewMemberHidesAbstractMember to fix ordering after NameMultiMap conversion. Add 7 open-type operator test scenarios and 5 CheckEntityDefn edge-case tests (FS0442, FS0434, FS0864, FS0436, FS3172). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro and others added 30 commits February 13, 2026 20:30

Merge branch 'main' into feature-operators-extensions

3333d02

gusty force-pushed the feature-operators-extensions branch from ad7fda3 to 36f9914 Compare April 18, 2026 19:10

Unify grammar for synTuple

ca3a1c1

gusty force-pushed the feature-operators-extensions branch from 7f595ab to ca3a1c1 Compare April 19, 2026 15:59

Tuple type parsing: struct extracted into an optional rule

8fef93d

T-Gro mentioned this pull request Apr 19, 2026

Hardening: eOpenedTypeOperators NameMultiMap + CheckEntityDefn dedup gusty/fsharp#1

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T-Gro added a commit that referenced this pull request Apr 19, 2026

Remove release-notes entry

aaeb771

Not a PR into main. Belongs on #19602 if at all. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

gusty added 2 commits April 20, 2026 06:34

Tuple type parsing: mark the tuple type range as synthetic

021b902

Tuple type parsing: add FCS tests showing that symbols from GetAllUse…

fb62ba7

…sOfAllSymbols have non-synthetic ranges

auduchinok reviewed Apr 20, 2026

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Comment thread src/Compiler/pars.fsy

gusty force-pushed the feature-operators-extensions branch from be8fcb4 to 67063e0 Compare April 20, 2026 15:46

Replace specific tuple parser rules with general atomType rule, defer…

8ae1999

…ring type extension validation to CheckDeclarations

gusty force-pushed the feature-operators-extensions branch from 67063e0 to 8ae1999 Compare April 21, 2026 09:05

gusty added 2 commits April 21, 2026 17:54

Add parser recovery rules for incomplete tuple type extensions includ…

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…ing struct variants

Merge branch 'main' into feature-operators-extensions

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auduchinok reviewed Apr 28, 2026

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gusty added 2 commits May 1, 2026 16:37

Remove no longer needed recovery rule

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Allow <'T, .. > syntax to solve value restrictions

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auduchinok reviewed May 1, 2026

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gusty and others added 3 commits May 1, 2026 16:48

Enable < .. > wildcard

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Merge remote-tracking branch 'origin/main' into feature-operators-ext…

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…ensions # Conflicts: # src/Compiler/Checking/NameResolution.fs # src/Compiler/FSComp.txt

gusty force-pushed the feature-operators-extensions branch 3 times, most recently from 1870a2e to 5134662 Compare May 11, 2026 17:24

gusty added 3 commits May 13, 2026 08:46

Fix: tuple ext parser conflict for ML-style prefix type parameter dec…

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…larations

Fix: wires up tuple extension member lookup that was defined but neve…

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…r called

Update test expectation: only signature formatting changed

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gusty force-pushed the feature-operators-extensions branch from 47f47a6 to 6315df9 Compare May 13, 2026 06:48

gusty and others added 2 commits May 13, 2026 09:00

Merge branch 'main' into feature-operators-extensions

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Extension members solve SRTP constraints (continuation)#19602

Extension members solve SRTP constraints (continuation)#19602
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gusty commented Apr 16, 2026 •

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gusty commented Apr 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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1. Extension members solve SRTP constraints

2. Weak resolution deferred for inline code

3. [<AllowOverloadOnReturnType>]

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gusty commented Apr 16, 2026 •

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3. `[<AllowOverloadOnReturnType>]`