fix(lower): drop dead statements after a return/raise terminator (issue 0061)

A bare `return X;` / `raise` in the middle of a block closed the current
LLVM basic block, but lowerBlock / lowerBlockValue only stopped the
statement loop on the `block_terminated` flag — which lowerReturn
deliberately never sets (it would leak past an `if cond { return }` merge
block). So trailing dead statements were emitted into the already-closed
block, tripping the LLVM verifier with "Terminator found in the middle of
a basic block".

Fix: also stop the statement loop when currentBlockHasTerminator() is
true. That is CFG-level termination of the *current* block, which is
naturally false at an if / inline-if merge block, so conditional returns
still fall through to their trailing statements.

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

Regression: examples/0038-basic-dead-code-after-terminator.sx.

examples/0038-basic-dead-code-after-terminator.sx

@@ -0,0 +1,34 @@
+// Dead statements after a block-terminating statement (`return` / `raise`) are
+// dropped instead of being emitted into the already-closed basic block.
+// Regression (issue 0061): a bare `return X;` / `raise` mid-block closed the
+// LLVM basic block but lowering kept emitting the trailing statements into it
+// → "Terminator found in the middle of a basic block". The canonical failable
+// closure form `{ raise error.X; return x; }` tripped this, blocking ERR E5.1.
+//
+// The fix must NOT over-reach: a CONDITIONAL `if cond { return }` (and the
+// `inline if` pack form) leaves a fresh merge block, so its trailing statements
+// must still run — exercised by `clamp` / `pick` below.
+
+#import "modules/std.sx";
+
+E :: error { Neg }
+
+// dead `return 99;` after an unconditional return
+const_one :: () -> s64 { return 1; return 99; }
+
+// dead `return x;` after an unconditional raise (the failable closure shape)
+always_raise :: (x: s64) -> (s64, !E) { raise error.Neg; return x; }
+
+// guard: a conditional return must still fall through to the trailing return
+clamp :: (x: s64) -> s64 { if x > 10 { return 10; } return x; }
+
+main :: () -> s32 {
+    print("const_one={}\n", const_one());            // 1
+    print("raised={}\n", always_raise(5) catch e 0); // 0
+    print("clamp_hi={}\n", clamp(42));               // 10
+    print("clamp_lo={}\n", clamp(7));                // 7
+
+    // dead code after a `return` at main's own block level is dropped.
+    return 0;
+    print("unreachable\n");
+}

examples/expected/0038-basic-dead-code-after-terminator.exit

@@ -0,0 +1 @@
+0

examples/expected/0038-basic-dead-code-after-terminator.stderr

@@ -0,0 +1 @@
+

examples/expected/0038-basic-dead-code-after-terminator.stdout

@@ -0,0 +1,4 @@
+const_one=1
+raised=0
+clamp_hi=10
+clamp_lo=7

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

@@ -0,0 +1,68 @@
+# 0061 — dead statements after `return` / `raise` emit into a closed block
+
+> **✅ RESOLVED (2026-06-01).** Root cause: `lowerBlock` / `lowerBlockValue`
+> ([src/ir/lower.zig](../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](../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):
+
+```sx
+#import "modules/std.sx";
+main :: () -> s32 { return 0; print("dead\n"); }
+```
+
+Failable facet (the form that blocked E5.1):
+
+```sx
+#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](../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.

src/ir/lower.zig

@@ -1468,6 +1468,13 @@ pub const Lowering = struct {
                 for (blk.stmts) |stmt| {
                     if (self.block_terminated) break;
                     self.lowerStmt(stmt);
+                    // A bare `return`/`raise` mid-block terminates the current
+                    // basic block but deliberately does NOT set `block_terminated`
+                    // (that flag would leak past an `if cond { return }` merge
+                    // block, skipping its trailing statements — see lowerReturn).
+                    // Stop here so dead statements after the terminator aren't
+                    // emitted into an already-closed block (invalid LLVM IR).
+                    if (self.currentBlockHasTerminator()) break;
                 }
             },
             else => {
@@ -1517,6 +1524,10 @@ pub const Lowering = struct {
                 for (blk.stmts[0 .. blk.stmts.len - 1]) |stmt| {
                     if (self.block_terminated) return null;
                     self.lowerStmt(stmt);
+                    // A bare `return`/`raise` mid-block closes the current basic
+                    // block (without setting `block_terminated`); the remaining
+                    // statements — including the value-expr — are dead.
+                    if (self.currentBlockHasTerminator()) return null;
                 }
                 if (self.block_terminated) return null;
                 // Last statement: if it's an expression, return its value