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Fix pattern matching on recursive tag union variant payloads #8842

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Merged
rtfeldman merged 3 commits into from
Jan 10, 2026
Merged

Fix pattern matching on recursive tag union variant payloads #8842

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3d5f8a4
Fix pattern matching on recursive tag union variant payloads
rtfeldman Dec 31, 2025
b53b727
Merge remote-tracking branch 'origin/main' into fix-issue-8754
rtfeldman Dec 31, 2025
06eb9f8
Merge origin/main into fix-issue-8754
rtfeldman Jan 4, 2026
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7 changes: 5 additions & 2 deletions src/eval/interpreter.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -7147,9 +7147,12 @@ pub const Interpreter = struct {
}
} else if (arg_vars.len == 1) {
const arg_var = arg_vars[0];
const arg_layout = try self.getRuntimeLayout(arg_var);
// Use the variant layout from the actual tag union data, not computed from type.
// This is critical for recursive types where the payload is boxed in memory
// even though the type says it's the recursive type directly.
const variant_layout = acc.getVariantLayout(tag_index);
payload_value = StackValue{
.layout = arg_layout,
.layout = variant_layout,
.ptr = value.ptr,
.is_initialized = true,
.rt_var = arg_var,
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56 changes: 56 additions & 0 deletions src/eval/test/comptime_eval_test.zig
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Expand Up @@ -2701,6 +2701,62 @@ test "encode - custom format type with infallible encoding (empty error type)" {
try testing.expectEqual(@as(u32, 0), summary.crashed);
}

test "issue 8754: pattern matching on recursive tag union variant payload" {
// Regression test for issue #8754: pattern matching on direct recursive tag union
// variant payload was returning the wrong discriminant.
//
// When Wrapper(Tree) is created where Tree := [..., Wrapper(Tree)], the payload is
// stored as a Box. The bug was extractTagValue using getRuntimeLayout(arg_var)
// which returns the non-boxed layout, causing pattern matching on the extracted
// payload to fail.
const src =
\\Tree := [Node(Str, List(Tree)), Text(Str), Wrapper(Tree)]
\\
\\inner : Tree
\\inner = Text("hello")
\\
\\wrapped : Tree
\\wrapped = Wrapper(inner)
\\
\\result = match wrapped {
\\ Wrapper(inner_tree) =>
\\ match inner_tree {
\\ Text(_) => 1
\\ Node(_, _) => 2
\\ Wrapper(_) => 3
\\ }
\\ _ => 0
\\}
;

var result = try parseCheckAndEvalModule(src);
defer cleanupEvalModule(&result);

const summary = try result.evaluator.evalAll();
try testing.expectEqual(@as(u32, 0), summary.crashed);

// Verify 'result' was folded to 1 (matched Text, not Wrapper)
const defs = result.module_env.store.sliceDefs(result.module_env.all_defs);

for (defs) |def_idx| {
const def = result.module_env.store.getDef(def_idx);
const pattern = result.module_env.store.getPattern(def.pattern);

if (pattern == .assign) {
const ident_text = result.module_env.getIdent(pattern.assign.ident);
if (std.mem.eql(u8, ident_text, "result")) {
const expr = result.module_env.store.getExpr(def.expr);
try testing.expect(expr == .e_num);
const value = expr.e_num.value.toI128();
try testing.expectEqual(@as(i128, 1), value);
return; // Test passed
}
}
}

return error.TestExpectedDefNotFound;
}

test "comptime eval - attached methods on tag union type aliases (issue #8637)" {
// Regression test for GitHub issue #8637
// Methods attached to transparent tag union type aliases with type parameters
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