[Dimank92]

Summary

Fixes #2560, fixes #2538.

This PR addresses soundness issues in hy-check where typed int variables bypassed type checking when initializing float64 variable declarations, and where built-in copy(dst, src) erroneously accepted slice arguments with mismatched element types (e.g. []float64 and []int).

Soundness Analysis & Defect Resolution

1. Typed Variable Initialization (var / let):

   • In Collector::local, kinds_conflict(&want, &vk) previously allowed typed integer variables to initialize float64 variables because kinds_conflict(Float, Int) returns false to permit untyped constants.
   • We check int_float_conflict = want == OpKind::Float && vk == OpKind::Int && !self.is_untyped_int_const(expr).
   • Initializing a float64 variable with a typed int variable (var i int = 1; var f float64 = i) is now strictly rejected with HYB-E1002.

2. Built-in copy(dst, src) Slices Element Type Matching:

   • In check_builtin_operand_kinds under "copy", kinds_conflict(dst_elem, src_elem) previously permitted copying between []float64 and []int.
   • We enforce strict element type equivalence for slices: dst_elem == src_elem (unless either is OpKind::Unknown), rejecting mismatched slice copies with HYB-E1002.

3. Untyped vs Typed Constants:

   • Untyped integer constants (1, const k = 1; var f float64 = k, k + 1) remain legal and accepted per the Go specification.
   • By tracking is_untyped on Define and file_untyped_consts on Collector, typed constants (const k int = 1; var f float64 = k) are properly rejected with HYB-E1002.

Verification

• Added test suite in compiler/crates/hy-check/tests/var_init_and_copy_slice_soundness.rs:
  • single_var_init_typed_int_to_float_is_rejected
  • let_init_typed_int_to_float_is_rejected
  • copy_slice_mismatched_float_and_int_is_rejected
  • untyped_int_constants_in_float_var_init_accepted
  • typed_int_constants_in_float_var_init_rejected
• Full test suite passing (178/178 unit tests green).

[Dimank92]

Rebased cleanly onto current main (commit d2917dcb):

• Both underlying soundness checks landed on main (var-init in fix(hy-check): reject typed int variables in float variable declarations (fixes #2674) #2678, copy slice checking in fix(check): enforce identical element types in copy builtin (fixes #2655, fixes #2686) #2673).
• Added dedicated 5-case regression test suite in compiler/crates/hy-check/tests/var_init_and_copy_slice_soundness.rs verifying:
  1. untyped_int_constants_in_float_var_init_accepted
  2. copy_slice_mismatched_float_and_int_is_rejected
  3. typed_int_constants_in_float_var_init_rejected
  4. let_init_typed_int_to_float_is_rejected
  5. single_var_init_typed_int_to_float_is_rejected
• All 5/5 regression tests and all 180 unit tests pass with 0 failures. Ready to land!

Bounty claim:
/claim #2560
@algora-pbc /claim #2560
Settlement wallet: 0xC345C07229307d4f1c7935aD2cAe48e390215808