[ASDAlexander77]

Follow-up to #429. A function that can fall off its end beside value returns still inferred T, and falling off returned the return slot as it was left:

function f(c: boolean) { if (c) return "a"; }
f(false) === undefined;   // before: false, type string   after: true, type string | undefined

Change

• bodyFallsOffEnd checks whether the end of the body is reachable, by AST analysis (canCompleteNormally). It's exact for return, throw, blocks, if/else, try/catch/finally, a switch with a default (the end of its last clause, or a break), and while (true) / for (;;) / do .. while (true) without a break that leaves them (BreakOutVisitor: an unlabeled break outside nested loops/switches, or any labeled break). Any other statement is taken to complete. MLIRGen's IR can't answer this: ts.ReturnVal sits inside ts.If regions and isn't a terminator.
• Discovery adds undefined to an inferred type when the end is reachable, as An inferred return type keeps undefined and null #429 does for a bare return;. An explicit return type is never changed.
• A function whose result is T | undefined (inferred or declared), any or unknown now generates return undefined at a reachable end. Explicit T | undefined functions that fell off used to return garbage there.

function	type
if (c) return "a";	string | undefined
if (c) return "a"; else return "b";	string
switch (n) { case 1: return "one"; default: return "many"; }	string
switch (n) { case 1: return "one"; case 2: return "two"; }	string | undefined
while (true) { if (n > 0) return n; n++; }	number
try { ...; throw 1; } catch { return 2; }	s32
if (c) return "x"; throw "e";	string
for (const k of a) { if (k == n) return k; }	s32 | undefined

If the analysis is unsure it says "reachable", which adds | undefined, so a call into a function that never returns (where TypeScript would know never) still counts as completing.

Checked

• DefaultLib's non-generic functions (all 363 in src/lib.ts) have no optional result after this change, so their API doesn't change. Generic ones are instantiated in user code; reduce/reduceRight became V | undefined with An inferred return type keeps undefined and null #429 already, and DefaultLib can I use it in a release environment yet #18 declares them : V.

Tests

• New 00return_falls_off_end.ts (compile + jit): each row above, plus a declared number | undefined.
• 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.

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