Record constructors (FS-1073) by charlesroddie · Pull Request #19974 · dotnet/fsharp

charlesroddie · 2026-06-19T10:47:23Z

Implements FS-1073. F# records compile to a class whose all-fields constructor is callable from C# (new MyRecord(a, b)) but not from F#, which only permits { Field = … }. This adds a RecordConstructorSyntax preview language feature that surfaces that constructor to F#, with positional and named arguments.

Notes:

Accessibility mirrors { } construction, so a private/internal record's representation is not bypassed (unlike C#'s public IL constructor).
An existing same-named value binding takes precedence over the constructor (FS-1073 scope rule).
Behind --langversion:preview.
Pattern matching is out of scope.

Records compile to a class whose all-fields constructor is callable from C# (new MyRecord(a, b)) but not from F#, which only allows { Field = ... } syntax. This adds a RecordConstructorSyntax preview feature that surfaces that constructor to F# too, supporting positional and named arguments. Implemented via a new MethInfo.RecdAllFieldsCtor case surfaced by InfoReader for record tycons; it elaborates through the existing mkRecordExpr path, so there is no codegen or overload-resolution change. Accessibility mirrors { } construction (the ctor is no more accessible than the record's representation/fields), so the C# behaviour of a public IL constructor bypassing a private record is not inherited. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

github-actions · 2026-06-19T10:48:03Z

❗ Release notes required

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✅ Found changes and release notes in following paths:

Change path Release notes path Description

src/Compiler docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

LanguageFeatures.fsi docs/release-notes/.Language/preview.md

Shorter name; a record has exactly one synthesized constructor, so the 'AllFields' qualifier is not needed to disambiguate. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

T-Gro · 2026-06-22T09:36:45Z

                LanguageFeature.MethodOverloadsCache, previewVersion // Performance optimization for overload resolution
                LanguageFeature.ImplicitDIMCoverage, languageVersion110
                LanguageFeature.ErrorOnMissingSignatureAttribute, previewVersion // Opt-in: turn FS3888 from warning into error
+                LanguageFeature.RecordConstructorSyntax, previewVersion // Allow constructing a record via its all-fields constructor, e.g. MyRecord(a, b)


Can you pls also add tests to https://github.com/dotnet/fsharp/tree/main/tests/projects/CompilerCompat project to see how this works with two dependent projects, one compiled with the feature and other without (both directions).

Since the codegen has been always there and this feature did not alter pickled representation, it should work as long as the consuming project has the LanguageFeature enabled, independent on what the type-authoring project had.

What about the opposite direction, type-authoring project exposes records and also inline functions with the record constructions. And consuming project is on an older compiler. Will the consuming project work with the inlined calls correctly?

Both directions work because the pickled representations stay the same.

This is verified in the this commit but I don't like the code quality here. The new code adds complexity, the existing structure isn't designed to handle it, and overall I don't think it's maintainable. I suggest I revert it and write a clear argument here instead about why it will work.

Its not that bad - the define can be generalized (so that there isn't one per feature eventually) and when the comments are removed, it is just a few lines.

Thanks for adding it 👍

The feature surfaces only the all-fields constructor. A struct record's zero-init default and a [<CLIMutable>] record's IL parameterless .ctor must remain non-callable from F#; both 'Point()' and 'R()' are rejected (FS0501). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…terless ctor A struct record's 'Point()' is default (zero) initialization, not a real constructor; only [<CLIMutable>] emits an actual parameterless .ctor. Name the two tests accordingly. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

rangeOfMethInfo had no RecdCtor case, so it fell through to ArbitraryValRef (None) and GoToDefinition on a positional record-constructor call navigated nowhere. Add a RecdCtor arm returning the record type's range, mirroring DefaultStructCtor. Adds FSharp.Compiler.Service.Tests smoke tests: go-to-definition lands on the record type, the tooltip mentions the type, and find-all-references links the constructor call to the type declaration. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Regression guard: the positional constructor has no field labels, so a [<RequireQualifiedAccess>] record must construct without a spurious diagnostic. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

github-actions · 2026-06-22T13:22:50Z

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: adds record constructor resolution path in type checking/overload resolution

Generated by PR Tooling Safety Check · opus46 5.6M · ◷

…073) Library exposes a record and an inline constructor function; the app constructs records positionally and via the inline function. The new syntax is gated behind RECORD_CTOR_FEATURE / --langversion:preview for local builds, with a classic { } fallback so SDK-compiler scenarios still build. Both branches elaborate to the same record-allocation node, so the pickled representation is unchanged and the matrix exercises both directions (feature-enabled consumer, older consumer). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

The constructor is not a declared member, so it rides on the record's representation visibility through a signature: available when the .fsi exposes the representation, rejected (FS1133, like { }) when the .fsi hides it. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

T-Gro · 2026-06-25T13:43:05Z

+  <!-- When built with the local compiler, exercise the FS-1073 positional record constructor (preview). -->
+  <PropertyGroup Condition="'$(LoadLocalFSharpBuild)' == 'True'">
+    <LangVersion>preview</LangVersion>
+    <DefineConstants>$(DefineConstants);RECORD_CTOR_FEATURE</DefineConstants>


Can you please generalize the define to USES_PREVIEW_COMPILER ?

T-Gro · 2026-06-25T13:43:58Z

            else
-                printfn "SUCCESS: All compiler compatibility tests passed"
-                0
+                // FS-1073 record-constructor cross-compiler checks.


Please get rid of all the comments, then this addition gets fairly compact 👍

…ents Per review: rename the per-feature RECORD_CTOR_FEATURE define to the reusable USES_PREVIEW_COMPILER, and remove the explanatory comments so the addition is compact. No behavior change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

charlesroddie requested a review from a team as a code owner June 19, 2026 10:47

github-project-automation Bot added this to F# Compiler and Tooling Jun 19, 2026

github-project-automation Bot moved this to New in F# Compiler and Tooling Jun 19, 2026

charlesroddie and others added 2 commits June 19, 2026 11:58

Rename MethInfo.RecdAllFieldsCtor to RecdCtor

d3e1804

Shorter name; a record has exactly one synthesized constructor, so the 'AllFields' qualifier is not needed to disambiguate. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Add release notes for record constructors (FS-1073)

402baa2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

github-actions Bot added the AI-Tooling-Check-Scanned-Clean Tooling check: diff analyzed, no interesting infrastructure files label Jun 19, 2026

T-Gro reviewed Jun 22, 2026

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Comment thread src/Compiler/Checking/InfoReader.fs

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Comment thread tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/RecordTypes.fs

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Comment thread tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/RecordTypes.fs

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Comment thread src/Compiler/Checking/InfoReader.fs

charlesroddie and others added 3 commits June 22, 2026 10:46

Test record constructor on a RequireQualifiedAccess record

249c623

Regression guard: the positional constructor has no field labels, so a [<RequireQualifiedAccess>] record must construct without a spurious diagnostic. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

github-actions Bot added the ⚠️ Affects-Compiler-Output Tooling check: PR touches IL emission or codegen label Jun 22, 2026

charlesroddie and others added 2 commits June 22, 2026 15:53

T-Gro reviewed Jun 25, 2026

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Record constructors (FS-1073)#19974

Record constructors (FS-1073)#19974
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charlesroddie:feature/record-positional-ctor

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