[MangelMaxime]

Fixes #5166.

Explicit constructors with a self identifier (new() as x = { ... } then ...) now work on JS, TS and Python, in a class with no base class. Constructors chaining within the class work too.

These stay a compile error, as on main:

• A self identifier on Dart, Rust or Beam.
• A self identifier in a class with a base class, or with [<AttachMembers>].
• Inheriting from a class that has such a constructor, including from an object expression.

Inheriting needs the base then block to run on the derived instance, tracked in #5272.

Known gaps:

• Reading self before initialisation doesn't throw InvalidOperationException.
• A field written out of declaration order that reads a let mutable sees a later field's change to it. Without as, main has the same bug.
• A generic [<DefaultValue>] field doesn't get the type's default. Without as, main has the same bug.
• Inheriting from a precompiled class behind a .fsi isn't rejected, because the signature hides the field the check looks for.

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	33
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	11	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded

[Copilot]

🟡 Changes recommended

Critical constructor-lowering defects break Rust and BEAM explicit constructors and leave inherited self references pointing to a separate base object rather than the derived instance.

4 open issues

• Base then block must initialize the derived instance · New
• Rust misses inherited constructor normalization with a then block · New
• BEAM constructor emission drops bindings and uses the wrong instance · New
• Constructor rewrite changes unit-valued initializer sequences · New

What changed in this PR

This PR addresses #5166 by extending Fable’s shared F# lowering to support explicit constructors with as self bindings and then blocks.

Changes:

• Resolves constructor self references and preserves the constructed object after then execution.
• Supplies defaults for omitted compiler-generated fields and refactors base-constructor handling.
• Adds JavaScript and Python regression tests for side effects, constructor chaining, and inheritance.

File	Description
tests/​Python/​TestType.fs	Adds explicit-constructor regression tests for Python.
tests/​Js/​Main/​TypeTests.fs	Adds corresponding JavaScript constructor tests.
src/​Fable.Transforms/​FSharp2Fable.fs	Implements shared lowering for self bindings, then blocks, and omitted fields.

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

Constructor generation can emit invalid duplicate parameters, while incomplete self-safety analysis can wrongly reject valid overloads or miss unsafe inheritance.

5 open issues

• Ensure inherited initialization parameter names are unique · New
• Track watch dependencies for inspected constructor implementations · New
• Preserve constructor-safety information for precompiled libraries · New
• Traverse LetRec bodies when checking forwarded constructors · New
• Exclude generated initialization writes when detecting constructors that bind s… · New

4 resolved since last review

• BEAM constructor emission drops bindings and uses the wrong instance
• Rust misses inherited constructor normalization with a then block
• Base then block must initialize the derived instance
• Constructor rewrite changes unit-valued initializer sequences

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

The inheritance checks incorrectly reject valid bases by analyzing ignored import/global constructor stubs and mistaking compiler-generated secondary-constructor writes or static initialization flags for explicit self bindings.

3 open issues

• Exclude imported and global bases from self-constructor analysis · New
• Avoid treating generated secondary-constructor writes as self bindings · New
• Ignore static initialization flags in constructor self detection · New

5 resolved since last review

• Ensure inherited initialization parameter names are unique
• Exclude generated initialization writes when detecting constructors that bind s…
• Traverse LetRec bodies when checking forwarded constructors
• Preserve constructor-safety information for precompiled libraries
• Track watch dependencies for inspected constructor implementations

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

The inheritance check incorrectly rejects imported/global derived stubs whose local base has an explicit self constructor, breaking interop stubs even though their derived constructors are discarded.

1 open issue

• Skip inheritance validation for imported or global derived stubs · New

3 resolved since last review

• Ignore static initialization flags in constructor self detection
• Avoid treating generated secondary-constructor writes as self bindings
• Exclude imported and global bases from self-constructor analysis

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

Shared constructor lowering spans FCS expression shapes and three target emitters, requiring .NET/JS/TS/Python runtime validation of object identity, initialization order, and constructor chaining that could not be performed in this review environment.

No open issues 🎉

1 resolved since last review

• Skip inheritance validation for imported or global derived stubs

_{Review effort: Balanced}