[Dimank92]

Summary

Fixes #2575.

This PR addresses soundness issues in hy-check where typed int variables bypassed type checking when returned from functions declaring float64 return types and when sent to channels of type chan float64.

Soundness Analysis & Defect Resolution

1. Return Statements:

   • In report_return_conflict, kinds_conflict(want, got) alone permits int values to be returned for float64 result types because kinds_conflict(Float, Int) evaluates to false.
   • We introduce val: &Expr to report_return_conflict and check int_float_conflict = *want == OpKind::Float && *got == OpKind::Int && !self.is_untyped_int_const(val).
   • If a typed int variable (e.g. var i int = 1; return i) is returned in single or multi-return positions, it is strictly rejected with HYB-E1002.

2. Channel Send Statements:

   • In Stmt::Send and SelectArmKind::Send, sending to chan float64 previously omitted primitive element assignability checks, allowing typed int variables to bypass HYB-E3011.
   • We enforce check_chan_send_operand_kinds, rejecting typed integer variables sent to chan float64 with HYB-E1002.

3. Untyped vs Typed Constants:

   • Untyped integer constants (1, const k = 1; return k, ch <- k, k + 1) remain legal and accepted per Go semantics.
   • Crucially, unlike naive checks that accept any const identifier, we track is_untyped on Define and file_untyped_consts on Collector. This ensures typed constants (e.g. const k int = 1; return k / ch <- k) are properly rejected in compliance with the Go language specification.

Verification

• Added test suite in compiler/crates/hy-check/tests/return_and_send_float_int_soundness.rs:
  • single_return_typed_int_to_float_is_rejected
  • multi_return_typed_int_to_float_is_rejected
  • channel_send_typed_int_to_float_chan_is_rejected
  • select_send_typed_int_to_float_chan_is_rejected
  • untyped_int_constants_in_return_and_send_are_accepted
  • typed_int_constants_in_return_and_send_are_rejected
• Full test suite passing (178/178 unit tests green).