[MangelMaxime]

Fixes #5227.

On JS, TS and Python, then blocks and new() as x now also work in an explicit constructor whose base class has a primary constructor.

[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

Unresolved constructor-generation defects can produce invalid parameter lists, corrupt forwarded arguments, and break initialization order, causing parse errors and incorrect runtime behavior.

7 open issues

• Reserve generated base parameter names to prevent collisions · New
• Sanitize base parameter names for the target language · New
• Include base parameters when matching constructor argument types · New
• Allocate a unique name for the ParamArray argument · New
• Preserve named unit arguments in base constructor calls · New
• Run child field initializers only after the base constructor · New
• Convert ParamArray before evaluating base-call arguments · New

What changed in this PR

Addresses #5227 by forwarding explicit constructor arguments to primary-constructor bases in Fable’s JavaScript, TypeScript, and Python targets, preserving inheritance and base initialization.

Changes:

• Adds primary-base argument forwarding and rejects unsupported multi-level inheritance chains.
• Updates ParamArray forwarding in JS/TS and Python constructor wrappers.
• Adds regression tests for inherited members, type tests, generics, naming collisions, optional arguments, and ParamArray arguments.

File	Description
tests/​Python/​TestType.fs	Adds Python constructor-inheritance regression tests.
tests/​Js/​Main/​TypeTests.fs	Adds shared JS/TS constructor-inheritance regression tests.
src/​Fable.Transforms/​Python/​Fable2Python.Transforms.fs	Converts constructor ParamArrays and forwards variadic arguments.
src/​Fable.Transforms/​FSharp2Fable.Util.fs	Builds primary-base arguments and reports unsupported inheritance chains.
src/​Fable.Transforms/​FSharp2Fable.fs	Forwards base arguments during explicit constructor lowering.
src/​Fable.Transforms/​Babel/​Fable2Babel.fs	Spreads ParamArray arguments in exposed constructor wrappers.

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

Constructor lowering replaces explicit base initialization with defaults, corrupting inherited state, and can evaluate throwing field reads before the base constructor, altering its effects or exceptions.

2 open issues

• Generated constructors overwrite explicit primary-base initialization · New
• Field reads can throw before the base constructor runs · New

7 resolved since last review

• Allocate a unique name for the ParamArray argument
• Include base parameters when matching constructor argument types
• Sanitize base parameter names for the target language
• Reserve generated base parameter names to prevent collisions
• Convert ParamArray before evaluating base-call arguments
• Run child field initializers only after the base constructor
• Preserve named unit arguments in base constructor calls

_{Review effort: Balanced}