sx/issues/0061-dead-code-after-terminator-stmt.md

0061 — dead statements after return / raise emit into a closed block

✅ RESOLVED (2026-06-01). Root cause: lowerBlock / lowerBlockValue (src/ir/lower.zig) broke their statement loop only on the block_terminated flag, which lowerReturn deliberately does NOT set (it would leak past an if cond { return } merge block — see the comment at lowerReturn). So a bare return X; / raise mid-block closed the current LLVM basic block while lowering kept emitting the trailing statements into it. Fix: after each lowerStmt, also stop the loop when currentBlockHasTerminator() is true (CFG-level termination of the current block — correctly false at an if/inline if merge block, so conditional returns still fall through). Regression test: examples/0038-basic-dead-code-after-terminator.sx.

Symptom

Any statement following a block-terminating statement (return, raise) at the same block level is lowered into the basic block after its terminator, so the LLVM verifier aborts:

LLVM verification failed: Terminator found in the middle of a basic block!
label %entry

Observed: a well-formed program with trailing dead code crashes the compiler. Expected: the dead statements are dropped (unreachable); the program compiles and runs.

This blocked ERR E5.1: the canonical failable-closure form from the plan, closure((x) -> (s32, !) { raise error.X; return x; }), has a dead return x; after the unconditional raise and tripped the verifier.

Reproduction

Minimal (non-failable — the bug is general, not error-specific):

#import "modules/std.sx";
main :: () -> s32 { return 0; print("dead\n"); }

Failable facet (the form that blocked E5.1):

#import "modules/std.sx";
E :: error { Neg }
top :: (x: s64) -> (s64, !E) { raise error.Neg; return x; }
main :: () -> s32 { print("r={}\n", top(5) catch e 0); return 0; }

Both abort with "Terminator found in the middle of a basic block". A conditional terminator (if c { return 1; } return 2;) was unaffected — its merge block is fresh and has no terminator.

Investigation prompt

The bug is in block-statement lowering in src/ir/lower.zig: lowerBlock (~line 1455) and lowerBlockValue (~line 1496) iterate blk.stmts and only check the block_terminated flag. lowerReturn (~line 1767) emits a ret/br terminator but intentionally does NOT set block_terminated (setting it would leak past if cond { return } merge blocks and wrongly skip their trailing statements — see the comment there). The fix is to stop the loop when the current basic block has a terminator after lowering a statement, using the existing currentBlockHasTerminator() helper (~line 11725), which is naturally false at a merge block. Verify with both repros above (now compile + run) and confirm examples/0518-packs-pack-value-dispatch.sx (inline-if + return + trailing statements) still produces all its output.