[ASDAlexander77]

An inferred return type is the union of what the function returns, as in TypeScript, so undefined and null are no longer dropped:

function	before	after
if (c) return "a"; return undefined;	string (=== undefined false)	string | undefined
if (c) return undefined; return "a";	string | undefined	same
if (c) return "a"; return;	string	string | undefined
if (c) return 1; return undefined;	s32	s32 | undefined
if (c) return { v: "x" }; return undefined;	{v: string}	{v: string} | undefined
if (c) return "a"; return null;	compile error (strict null)	string | null

Change

• processReturnType filtered out a later return undefined / return null, and mergeType absorbs undefined into the other type (canCastFromTo(undefined -> T)), per field of a tuple too. The new MLIRTypeHelper::mergeReturnType makes one side being undefined or null a union, and merges tuples field by field. It's used only by processReturnType, so mergeType's other callers (array literals, conditionals, generic arguments) don't change.
• A bare return; beside value returns adds undefined to the inferred type (PassResult::hasBareReturn; never over an explicit return type), and in a function that returns a value it returns undefined. A bare return in a void function or a constructor is unchanged.
• Generators keep the old merge. The rewritten next() returns Iterator<T>'s {value: T, done}, and a finished generator's value is T's zero by design (00generator_done_value_zero.ts). Making it T | undefined broke every Iterator<T> match (56 tests). next() is marked InternalFlags::GeneratorNext and goes through PassResult::iteratorResult.

Visible in DefaultLib

reduce / reduceRight (inferred return type, return undefined on an empty array with no initial value) now return V | undefined. Their call sites still work: assigning to number, printing, arithmetic, and an empty array really gives undefined. But arithmetic on the result warns casting from optional (undefined) value. TypeScript declares reduce as returning V; declaring : V there in DefaultLib would keep the old signature.

Not changed

• Falling off the end (function f(c) { if (c) return "a"; }) still infers string. It needs a reachability check, and getting it wrong would add | undefined to functions that always return.
• A finished generator's value (see above).

Tests

• New 00return_inference_undefined.ts (compile + jit): both orders, bare return, integer, object, null, a void function with a bare return, a declared T | undefined, a constructor.
• Release suite: 3343/3343 passed.
• DefaultLib (main b67a213) rebuilt release/gc with this compiler: 158/158 compile, 158/158 jit.
• The new test passes under gc, rc and none.
• Not run: Debug, x86, -mm=own.

🤖 Generated with Claude Code