fix(hy-check): reject typed constant arithmetic and shift overflow with HYB-E6002 (fixes #725)

[aazminu]

Fixes #725

Summary

Previously, HYB-E6002 constant-overflow checks only validated untyped constants against a target's declared type or the default int/float64 type, and const_int_fold_with did not fold fixed-width integer conversion calls (u8(255), i64(1)) or check the folded result of typed constant expressions (top + 1 where const top u8 = 255, u8(255) + 1, -u8(1), u8(1) << 9, i64(1) << 63, -(i64(1) << 63)). Accepted programs therefore emitted Go that failed go build with constant ... overflows ....

Changes

• compiler/crates/hy-check/src/const_overflow.rs:
  • Add Checker::const_int_fold_expr to fold integer constant expressions including local/file const bindings and 1-arg fixed-width integer conversions (u8(...), i64(...), etc., preserving SPEC X4 two's-complement wrapping on sub-64-bit conversions).
  • Add Checker::typed_const_expr_type to determine when a constant expression carries an explicit fixed-width integer type in Go (through typed const declarations, integer conversion calls, unary -, shifts <</>>, and binary arithmetic/bitwise operations).
  • Skip the untyped default-int check in check_default_int_overflow when an initializer is already a typed constant (so u64(1) << 63 stays accepted as u64), and deduplicate HYB-E6002 diagnostics per expression span.
• compiler/crates/hy-check/src/emit_guard.rs:
  • Check typed constant binary expressions (including << and >>) and unary negation (-u8(1)) against their constant type range via check_const_overflow_init, suppressing cascading outer diagnostics when an inner subexpression already failed.
• compiler/crates/hy-check/src/shape.rs:
  • Propagate operand types through Expr::Unary { op: UnOp::Neg } and Expr::Binary { op: BinOp::Shl | BinOp::Shr } in ty_of_expr.
• compiler/crates/hy-check/tests/typed_const_arith_overflow_issue_725.rs:
  • Add regression tests covering the lead reproducer, the full operator table from [soundness] Typed constant arithmetic that overflows its type (const top u8 = 255; top + 1, i64(1) << 63, u8(0) - 1) passes hyb check, then fails go build (X1/X2) #725, and valid typed/untyped controls (u64(1) << 63, (-i64(1)) << 63, (1 << 63) - 1, -(1 << 63), u8(100 + 200), runtime variable wrapping).

Verification

• cargo test -p hy-diag -p hy-syntax -p hy-emit -p hy-check