issues/0066: RESOLVED — match-value arms lowered against result type

A value-position match's arms are now lowered with `target_type` set to
the merge's `result_type`, so positive and negated integer literals pick
the same width. Fixes the `PHI node operands are not the same type as the
result` failure for `if n == { case 0: 100; else: -1; }`-style returns.

Regression: examples/0043-basic-match-value-mixed-width.sx.
Gates: zig build, zig build test, run_examples.sh -> 345 passed.

examples/0043-basic-match-value-mixed-width.sx

@@ -0,0 +1,31 @@
+// A value-position match (`if subject == { case … }`) returning a small
+// integer type works when arms mix positive and negated literals: every arm
+// value is lowered against the merge's result type, so the phi operands all
+// share one width.
+//
+// Regression (issue 0066): a negated-literal arm (`else: -1`) previously
+// lowered at a narrower width than the positive arms, tripping LLVM's
+// "PHI node operands are not the same type as the result".
+
+#import "modules/std.sx";
+
+sign :: (n: s32) -> s32 {
+    if n == {
+        case 0:  0;
+        else:    if n > 0 then 1 else -1;
+    }
+}
+
+classify :: (n: s32) -> s32 {
+    if n == {
+        case 0:  100;
+        case 1:  10;
+        else:    -1;
+    }
+}
+
+main :: () -> s32 {
+    print("sign: {} {} {}\n", sign(-9), sign(0), sign(9));   // -1 0 1
+    print("classify: {} {} {}\n", classify(0), classify(1), classify(5)); // 100 10 -1
+    0
+}

examples/expected/0043-basic-match-value-mixed-width.exit

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

examples/expected/0043-basic-match-value-mixed-width.stderr

@@ -0,0 +1 @@
+

examples/expected/0043-basic-match-value-mixed-width.stdout

@@ -0,0 +1,2 @@
+sign: -1 0 1
+classify: 100 10 -1

issues/0066-match-arm-negative-literal-phi-width.md

@@ -1,5 +1,11 @@
 # 0066 — match-as-value with a negated-literal arm builds a mismatched phi
 
+> **RESOLVED.** `lowerMatch`'s value path (`has_value_merge`) now lowers each
+> arm body with `target_type = result_type`, so literals and negated literals in
+> the arms pick the merge's width instead of leaking a narrower one. The phi
+> operands are uniform; `coerceToType` still runs afterward as a backstop.
+> Regression: `examples/0043-basic-match-value-mixed-width.sx`.
+
 ## Symptom
 
 A value-position `match` (the `if subject == { case ... }` sugar) returning a

src/ir/lower.zig

@@ -4497,7 +4497,13 @@ pub const Lowering = struct {
             }
 
             if (has_value_merge) {
+                // Lower the arm body against the merge's result type so literals
+                // (and negated literals) in the arm pick the right width — the
+                // phi operands must all match `result_type` (issue 0066).
+                const saved_arm_target = self.target_type;
+                self.target_type = result_type;
                 const maybe_v = self.lowerBlockValue(arm.body);
+                self.target_type = saved_arm_target;
                 self.current_match_tags = saved_match_tags;
                 self.scope = old_scope;
                 arm_scope.deinit();