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fix: #776 — implicit-return functions now produce undefined, not 0 #786

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TheHypnoo merged 2 commits into from
May 15, 2026
+38 −39
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fix: #776 — implicit-return functions now produce undefined, not 0 #786

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5197799
fix: #776 — implicit-return functions now produce undefined, not 0
TheHypnoo May 15, 2026
2811c88
fmt: cargo fmt
TheHypnoo May 15, 2026
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39 changes: 22 additions & 17 deletions crates/perry-codegen/src/codegen.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3256,21 +3256,22 @@ fn compile_function(
stmt::lower_stmts(&mut ctx, &f.body)
.with_context(|| format!("lowering body of '{}'", f.name))?;

// Defensive: a well-typed numeric function always returns via an
// explicit `return`, but we emit `ret double 0.0` as a fallback so
// the LLVM verifier doesn't reject a missing terminator. For
// async functions, the fallback also wraps in a resolved promise
// so callers can await the result.
// A function that falls off the end without an explicit `return`
// returns `undefined` in JS — emit the NaN-boxed TAG_UNDEFINED
// value so the LLVM verifier has a terminator AND user code that
// does `f() === undefined` / `f() !== undefined` observes the
// correct value. For async functions, wrap undefined in a
// resolved promise so callers can await the result.
if !ctx.block().is_terminated() {
let undef = crate::nanbox::double_literal(f64::from_bits(crate::nanbox::TAG_UNDEFINED));
if f.is_async {
let zero = "0.0".to_string();
let handle = ctx
.block()
.call(I64, "js_promise_resolved", &[(DOUBLE, &zero)]);
.call(I64, "js_promise_resolved", &[(DOUBLE, &undef)]);
let boxed = crate::expr::nanbox_pointer_inline_pub(ctx.block(), &handle);
ctx.block().ret(DOUBLE, &boxed);
} else {
ctx.block().ret(DOUBLE, "0.0");
ctx.block().ret(DOUBLE, &undef);
}
}
let ic_globals = std::mem::take(&mut ctx.ic_globals);
Expand Down Expand Up @@ -3598,15 +3599,15 @@ fn compile_closure(
.with_context(|| format!("lowering closure body func_id={}", func_id))?;

if !ctx.block().is_terminated() {
let undef = crate::nanbox::double_literal(f64::from_bits(crate::nanbox::TAG_UNDEFINED));
if is_async {
let zero = "0.0".to_string();
let handle = ctx
.block()
.call(I64, "js_promise_resolved", &[(DOUBLE, &zero)]);
.call(I64, "js_promise_resolved", &[(DOUBLE, &undef)]);
let boxed = crate::expr::nanbox_pointer_inline_pub(ctx.block(), &handle);
ctx.block().ret(DOUBLE, &boxed);
} else {
ctx.block().ret(DOUBLE, "0.0");
ctx.block().ret(DOUBLE, &undef);
}
}
let ic_globals = std::mem::take(&mut ctx.ic_globals);
Expand Down Expand Up @@ -3898,9 +3899,12 @@ fn compile_method(
stmt::lower_stmts(&mut ctx, &method.body).with_context(|| {
format!("lowering body of method '{}::{}'", class.name, method.name)
})?;
// Fall through to the default ret-void at end.
// Fall through to the default ret at end.
if !ctx.block().is_terminated() {
ctx.block().ret(DOUBLE, "0.0");
let undef = crate::nanbox::double_literal(f64::from_bits(
crate::nanbox::TAG_UNDEFINED,
));
ctx.block().ret(DOUBLE, &undef);
}
let _ = std::mem::take(&mut ctx.ic_globals);
let _ = std::mem::take(&mut ctx.typed_parse_rodata);
Expand Down Expand Up @@ -3978,7 +3982,8 @@ fn compile_method(
.with_context(|| format!("lowering body of method '{}::{}'", class.name, method.name))?;

if !ctx.block().is_terminated() {
ctx.block().ret(DOUBLE, "0.0");
let undef = crate::nanbox::double_literal(f64::from_bits(crate::nanbox::TAG_UNDEFINED));
ctx.block().ret(DOUBLE, &undef);
}
let ic_globals = std::mem::take(&mut ctx.ic_globals);
let typed_parse_rodata = std::mem::take(&mut ctx.typed_parse_rodata);
Expand Down Expand Up @@ -5389,15 +5394,15 @@ fn compile_static_method(
.with_context(|| format!("lowering body of static '{}::{}'", class_name, f.name))?;

if !ctx.block().is_terminated() {
let undef = crate::nanbox::double_literal(f64::from_bits(crate::nanbox::TAG_UNDEFINED));
if f.is_async {
let zero = "0.0".to_string();
let handle = ctx
.block()
.call(I64, "js_promise_resolved", &[(DOUBLE, &zero)]);
.call(I64, "js_promise_resolved", &[(DOUBLE, &undef)]);
let boxed = crate::expr::nanbox_pointer_inline_pub(ctx.block(), &handle);
ctx.block().ret(DOUBLE, &boxed);
} else {
ctx.block().ret(DOUBLE, "0.0");
ctx.block().ret(DOUBLE, &undef);
}
}
let ic_globals = std::mem::take(&mut ctx.ic_globals);
Expand Down
15 changes: 10 additions & 5 deletions crates/perry-codegen/src/stmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -106,12 +106,17 @@ pub(crate) fn lower_stmt(ctx: &mut FnCtx<'_>, stmt: &Stmt) -> Result<()> {
Ok(())
}
Stmt::Return(None) => {
// Bare `return;` returns undefined (encoded as 0.0). For
// async functions, wrap undefined in a resolved promise.
// Bare `return;` returns the NaN-boxed `undefined` value
// (TAG_UNDEFINED). For async functions, wrap it in a
// resolved promise.
let undef = crate::nanbox::double_literal(f64::from_bits(crate::nanbox::TAG_UNDEFINED));
if ctx.is_async_fn {
let zero = "0.0".to_string();
let blk = ctx.block();
let handle = blk.call(crate::types::I64, "js_promise_resolved", &[(DOUBLE, &zero)]);
let handle = blk.call(
crate::types::I64,
"js_promise_resolved",
&[(DOUBLE, &undef)],
);
let boxed = crate::expr::nanbox_pointer_inline_pub(blk, &handle);
// Pop open try frames first (see above).
for _ in 0..ctx.try_depth {
Expand All @@ -123,7 +128,7 @@ pub(crate) fn lower_stmt(ctx: &mut FnCtx<'_>, stmt: &Stmt) -> Result<()> {
for _ in 0..ctx.try_depth {
ctx.block().call_void("js_try_end", &[]);
}
ctx.block().ret(DOUBLE, "0.0");
ctx.block().ret(DOUBLE, &undef);
}
Ok(())
}
Expand Down
21 changes: 5 additions & 16 deletions crates/perry-hir/src/lower.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3894,27 +3894,16 @@ fn append_decorator_invocations_inner(
init: Some(call),
});
let msg = format!(
"Class decorator `@{dec_name}` on `{class_name}` returned a replacement \
class. Perry does not install class replacements from decorators (see \
"Class decorator `@{dec_name}` on `{class_name}` returned a value. \
Perry does not install decorator return values as class replacements (see \
docs/src/language/decorators.md). Return `undefined` (or nothing) to keep \
the decorator running for side effects only."
);
// Check `typeof ret === "function"` rather than
// `ret !== undefined`: Perry's lowering for a function
// expression with no explicit `return` currently leaves a
// numeric sentinel in the return slot rather than the
// NaN-boxed undefined value, so `!== undefined` would
// false-positive on side-effect-only decorators. The
// semantic check the maintainer asked for is "did the
// decorator return a class?" — a class is `typeof
// "function"` in JS, so this catches the @Memoize /
// @Throttle / GraphQL-wrapper case while leaving the bare
// `@Injectable` (no return) shape alone.
out.push(Stmt::If {
condition: Expr::Compare {
op: CompareOp::Eq,
left: Box::new(Expr::TypeOf(Box::new(Expr::LocalGet(ret_id)))),
right: Box::new(Expr::String("function".to_string())),
op: CompareOp::Ne,
left: Box::new(Expr::LocalGet(ret_id)),
right: Box::new(Expr::Undefined),
},
// Perry has dedicated HIR variants for built-in errors
// (`Expr::TypeErrorNew`, etc.); the generic
Expand Down
2 changes: 1 addition & 1 deletion test-parity/expected/test_decorators_replacement_unsupported.txt
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Original file line number Diff line number Diff line change
@@ -1,2 +1,2 @@
TypeError: Class decorator `@ReplaceWithOther` on `Original` returned a replacement class. Perry does not install class replacements from decorators (see docs/src/language/decorators.md). Return `undefined` (or nothing) to keep the decorator running for side effects only.
TypeError: Class decorator `@ReplaceWithOther` on `Original` returned a value. Perry does not install decorator return values as class replacements (see docs/src/language/decorators.md). Return `undefined` (or nothing) to keep the decorator running for side effects only.
at <anonymous>
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