[Basharkhan7776]

Fixes #5634.

When a function has parameter expressions, FunctionDeclarationInstantiation step 28 gives the body's var names a separate varEnv. A var whose name is also a parameter must be initialized from the parameter environment (env.GetBindingValue), not from varEnv.

The bytecompiler read varEnv instead. That binding has no register yet, so compilation emitted ThrowNewReferenceError("access of uninitialized binding") before the body ran. A later emit_store_undefined also discarded the copied value, so a bare var b would have become undefined instead of the parameter's value.

GetBindingValue now uses scopes.parameter_scope(), and DefInitVar stores that value. Parameter-list closures still close over the parameter environment.

function f(a = () => b, b = 2) { var b = 3; return [a(), b]; }
f(); // [2, 3]

[abdulhamid999]

Hi @Basharkhan7776, great work on tracking this down!

The fix cleanly addresses ECMAScript \FunctionDeclarationInstantiation\ step 28.h.iv. Reading from \scopes.parameter_scope()\ rather than the uninitialized \�arEnv\ correctly preserves the parameter binding value, and removing the erroneous \emit_store_undefined\ prevents clobbering that copied value before \DefInitVar.

Tested locally against \main\ via \cargo test -p boa_engine function\ and all tests pass cleanly. The added test cases in \core/engine/src/tests/function.rs\ cover arrow functions, default parameter expressions, and closure capture thoroughly.

Looks solid!

[Basharkhan7776]

Thanks for the review. Glad the parameter-environment read and the removed emit_store_undefined match what you saw locally.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,450	51,451	+1
Ignored	1,648	1,648	0
Failed	480	479	-1
Panics	0	0	0
Conformance	96.03%	96.03%	+0.00%

Fixed tests (1):

test/staging/sm/Function/arguments-parameter-shadowing.js (previously Failed)

Tested main commit: bfaa15a7d60d8c92370a423d4134ab19b1b1fae0
Tested PR commit: 469f17bb98618a1f6bd1e7103e529943f16fad62
Compare commits: bfaa15a...469f17b

[HalidOdat]

Nice catch! Thanks for the contribution! 🚀

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.17%. Comparing base (6ddc2b4) to head (469f17b).
⚠️ Report is 1086 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5647       +/-   ##
===========================================
+ Coverage   47.24%   63.17%   +15.93%     
===========================================
  Files         476      538       +62     
  Lines       46892    60608    +13716     
===========================================
+ Hits        22154    38292    +16138     
+ Misses      24738    22316     -2422     

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