fix(lower): null-FuncId path restores full caller state [0100 F2]

lazyLowerFunction's three exit paths (non-null branch, already-promoted
early return, null-FuncId `ns.fn` qualified-alias branch) each duplicated
the caller-state restore, and the null branch's copy had drifted: it
restored every saved field EXCEPT `block_terminated`. A qualified alias
whose body terminates (e.g. a constant-folded `if true { return ... }`)
leaves `block_terminated = true` after lowerFunction; the null path
returned without resetting it, so the flag leaked into the CALLER's body
lowering and the caller's own trailing statements / `return` were rejected
as dead-after-terminator ("function ... body produces no value").

Fix: collapse the three restores into a single `defer` registered right
after the state is saved, so every exit path restores the identical full
set and the class cannot diverge again. Fields restored on all paths:
current_source_file (F1), scope, func_defer_base, block_terminated (F2),
force_block_value, builder.func/current_block/inst_counter. The
foreign-class / jni-env / pack-mono / inline-return fields already had
their own defers and are unchanged.

Regression: examples/0721-modules-qualified-terminating-callee.sx — a
qualified alias `m.foo` folds `if true { return helper(); }` (helper from
m.sx's own import) and is followed by caller statements + the caller's own
`return 0`. Reports "body produces no value" pre-fix; prints
"terminating-callee: ok" / "after" and exits 0 after. 0719 (collision) and
0720 (F1 own-import visibility) stay green. issues/0100 RESOLVED banner
extended with the F2 follow-up.

examples/0721-modules-qualified-terminating-callee.sx

@@ -0,0 +1,31 @@
+// Regression (issue 0100 F2): lowering a QUALIFIED imported function whose
+// body terminates must leave the CALLER's lowering state untouched.
+//
+// `m :: #import …` registers `m.foo` as a module-qualified alias with a unique
+// FuncId (the identity fix, issue 0100 / example 0719) and lowers it through
+// `lazyLowerFunction`'s null-FuncId `lowerFunction` path. `foo`'s body folds
+// `if true { return helper(); }` to an unconditional return, so its lowering
+// ends with `block_terminated = true`. The null-FuncId path used to restore
+// every saved caller field EXCEPT `block_terminated`, so that flag leaked back
+// into `main`, and `main`'s own trailing `print` / `return 0` were treated as
+// dead-after-terminator — the compiler rejected `return 0` with "body produces
+// no value". The fix routes all exit paths through one save/restore defer, so
+// the qualified alias is transparent to the caller. (`helper` also lives in
+// m.sx's own flat import, exercising the F1 source-context restore too.)
+
+#import "modules/std.sx";
+m :: #import "0721-modules-qualified-terminating-callee/m.sx";
+
+report :: (label: string, ok: bool) {
+    if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }
+}
+
+main :: () -> s32 {
+    // Qualified callee whose body terminates via a constant-folded `if true`.
+    x := m.foo();
+    // Caller statements AFTER the call must still be emitted (not dead).
+    report("terminating-callee", x == 7);
+    print("after\n");
+    // The caller's OWN return — rejected pre-fix because block_terminated leaked.
+    return 0;
+}

examples/0721-modules-qualified-terminating-callee/helper.sx

@@ -0,0 +1,3 @@
+// Lives in m.sx's OWN flat import — reachable from `foo` but not from the
+// top-level consumer that imports m.sx under a qualified namespace.
+helper :: () -> s64 { return 7; }

examples/0721-modules-qualified-terminating-callee/m.sx

@@ -0,0 +1,10 @@
+// `foo` is pulled in QUALIFIED by the consumer (`m :: #import …`). Its body
+// terminates via a constant-folded `if true { return … }`, and the `return`
+// calls `helper` from m.sx's OWN flat import. Lowering `foo` as a qualified
+// alias must be transparent to the caller's lowering state — issue 0100 F2.
+#import "helper.sx";
+
+foo :: () -> s64 {
+    if true { return helper(); }
+    return 0;
+}

examples/expected/0721-modules-qualified-terminating-callee.exit

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

examples/expected/0721-modules-qualified-terminating-callee.stderr

@@ -0,0 +1 @@
+

examples/expected/0721-modules-qualified-terminating-callee.stdout

@@ -0,0 +1,2 @@
+terminating-callee: ok
+after

issues/0100-cross-module-same-name-fn-lowering-collision.md

@@ -69,6 +69,53 @@ Regression: `examples/0720-modules-qualified-own-import.sx` — `calc.compute`
 Reports `'triple' is not visible` on the attempt-1 code; passes after. 0719's
 cross-module dual-`parse` assertion stays green.
 
+## F2 follow-up — null-FuncId path must restore the FULL caller lowering state
+
+The F1 fix patched the **source file** in `lazyLowerFunction`'s null-FuncId
+branch, but that branch still restored only a SUBSET of the caller state the
+non-null branch restores — it omitted `self.block_terminated`. A qualified
+alias whose body terminates (e.g. a constant-folded `if true { return … }`)
+leaves `block_terminated = true` after `lowerFunction`; the null branch then
+returned without resetting it, so the flag leaked into the **caller's** body
+lowering and the caller's own trailing statements / `return` were treated as
+dead-after-terminator:
+
+```
+m :: #import "m.sx";   // m.sx: `#import "helper.sx"; foo :: () -> s64 { if true { return helper(); } return 0; }`
+main :: () -> s32 {
+    x := m.foo();
+    print("after\n");  // dropped
+    return 0;          // → error: body produces no value
+}
+```
+
+**Fix** (`src/ir/lower.zig`): the three exit paths of `lazyLowerFunction` (the
+null-FuncId branch, the already-promoted early return, and the bottom of the
+non-null branch) duplicated the restore, and the null branch's copy drifted.
+They are now collapsed into a **single `defer`** registered right after the
+state is saved, so every exit path restores the identical full set and the
+class can't diverge again. The fields the defer now restores on all paths:
+
+- `current_source_file` (via `setCurrentSourceFile`, which also resyncs
+  `diagnostics.current_source_file`) — F1
+- `scope`
+- `func_defer_base`
+- `block_terminated` — **F2** (was missing on the null path)
+- `force_block_value`
+- `builder.func`
+- `builder.current_block`
+- `builder.inst_counter`
+
+(The `current_foreign_class`, `jni_env_stack_base`, and pack-mono /
+`inline_return_target` fields already had their own `defer`s and apply on all
+paths; they are unchanged.)
+
+Regression: `examples/0721-modules-qualified-terminating-callee.sx` — `m.foo`
+(a qualified alias) folds `if true { return helper(); }` and is followed by
+caller statements + the caller's own `return 0`. Reports `body produces no
+value` on the attempt-2 code; prints `terminating-callee: ok` / `after` and
+exits 0 after. 0719 and 0720 stay green.
+
 ## Symptom
 
 - **Observed:** a program that imports two modules each exporting a
@@ -117,11 +164,15 @@ machinery already existed but was never fed module-qualified entries.
 ## Fix verification
 
 - `zig build` → 0
-- `zig build test` → 0 (incl. LSP corpus sweep, 472 examples)
-- `bash tests/run_examples.sh` → 455 passed, 0 failed
+- `zig build test` → 0 (incl. LSP corpus sweep, 473 examples; 397/397 tests)
+- `bash tests/run_examples.sh` → 456 passed, 0 failed
 - `examples/0719-modules-cli-and-json.sx`: panics pre-fix, passes post-fix.
 - `examples/0720-modules-qualified-own-import.sx`: `'… is not visible'` on
   the attempt-1 code, passes after the F1 fix.
+- `examples/0721-modules-qualified-terminating-callee.sx`: `body produces no
+  value` on the attempt-2 code, passes after the F2 fix.
 
 Regression tests: `examples/0719-modules-cli-and-json.sx` (collision),
-`examples/0720-modules-qualified-own-import.sx` (F1 own-import visibility).
+`examples/0720-modules-qualified-own-import.sx` (F1 own-import visibility),
+`examples/0721-modules-qualified-terminating-callee.sx` (F2 terminating
+qualified callee — caller state transparency).

src/ir/lower.zig

@@ -1590,6 +1590,25 @@ pub const Lowering = struct {
         const saved_block_terminated = self.block_terminated;
         const saved_force_block_value = self.force_block_value;
         const saved_source_file = self.current_source_file;
+        // Lowering a callee must be transparent to the caller's lowering
+        // state: restore the FULL saved context on EVERY exit path through one
+        // defer so the three exits (non-null branch, already-promoted early
+        // return, null-FuncId `ns.fn` alias branch) cannot drift. Notably
+        // `block_terminated` — a qualified alias whose body terminates (e.g. a
+        // constant-folded `if true { return … }`) leaves it true, and leaking
+        // that into the caller marks the caller's own trailing statements
+        // dead-after-terminator (issue 0100 F2). The jni/pack/foreign-class
+        // fields keep their own defers above.
+        defer {
+            self.setCurrentSourceFile(saved_source_file);
+            self.scope = saved_scope;
+            self.func_defer_base = saved_defer_base;
+            self.block_terminated = saved_block_terminated;
+            self.force_block_value = saved_force_block_value;
+            self.builder.func = saved_func;
+            self.builder.current_block = saved_block;
+            self.builder.inst_counter = saved_counter;
+        }
         // The `#jni_env` Ref stack is lexical within ONE function's instruction
         // stream — Refs from the caller don't dereference correctly in this
         // callee's body. Move the visible base to the current top so
@@ -1650,15 +1669,7 @@ pub const Lowering = struct {
                 self.setCurrentSourceFile(src);
             }
             self.lowerFunction(fd, name, false);
-            // Restore builder state
-            self.setCurrentSourceFile(saved_source_file);
-            self.scope = saved_scope;
-            self.func_defer_base = saved_defer_base;
-            self.force_block_value = saved_force_block_value;
-            self.builder.func = saved_func;
-            self.builder.current_block = saved_block;
-            self.builder.inst_counter = saved_counter;
-            return;
+            return; // caller state restored by the top-level defer
         }
 
         if (func_id) |fid| {
@@ -1667,15 +1678,8 @@ pub const Lowering = struct {
             const func = &self.module.functions.items[@intFromEnum(fid)];
             self.setCurrentSourceFile(func.source_file);
             if (!func.is_extern) {
-                // Already promoted (e.g., via lowerComptimeDeps) — skip
-                self.setCurrentSourceFile(saved_source_file);
-                self.scope = saved_scope;
-                self.func_defer_base = saved_defer_base;
-                self.block_terminated = saved_block_terminated;
-                self.force_block_value = saved_force_block_value;
-                self.builder.func = saved_func;
-                self.builder.current_block = saved_block;
-                self.builder.inst_counter = saved_counter;
+                // Already promoted (e.g., via lowerComptimeDeps) — skip.
+                // Caller state restored by the top-level defer.
                 return;
             }
             func.is_extern = false; // promote from extern stub to real function
@@ -1745,16 +1749,7 @@ pub const Lowering = struct {
 
             self.builder.finalize();
         }
-
-        // Restore builder state
-        self.setCurrentSourceFile(saved_source_file);
-        self.scope = saved_scope;
-        self.func_defer_base = saved_defer_base;
-        self.block_terminated = saved_block_terminated;
-        self.force_block_value = saved_force_block_value;
-        self.builder.func = saved_func;
-        self.builder.current_block = saved_block;
-        self.builder.inst_counter = saved_counter;
+        // Caller state restored by the top-level defer.
     }
 
     /// Lower a single function declaration.