Fix integer overflow in getCrossProductUnionSize causing hang on large template literal types

[Copilot]

Fixes #1346

Analysis

When getCrossProductUnionSize computes the cross product of many union types (e.g., 49 unions of 4 members each = 4^49), the result overflows Go's int (signed 64-bit integer). Specifically, 4^32 = 2^64 wraps to 0 in signed 64-bit arithmetic, causing all subsequent multiplications to produce 0. This means getCrossProductUnionSize returns 0, which is less than the 100,000 limit, so checkCrossProductUnion incorrectly allows the expansion to proceed. The recursive mapType calls then attempt to expand 4^49 combinations, effectively hanging.

In TypeScript (the reference implementation), numbers are 64-bit floats, so 4^49 evaluates to Infinity, which correctly triggers the "too complex" error.

type N = 0 | 1 | 2 | 3;
type T = `${N}${N}${N}...${N}`; // 49 unions of 4 → 4^49 combinations → hangs in tsgo

Fix

Added overflow protection in getCrossProductUnionSize: before each multiplication, check whether size > math.MaxInt / n. If so, return math.MaxInt immediately instead of continuing to multiply and risking overflow. This ensures the limit check in checkCrossProductUnion correctly detects and rejects excessively large cross products.

Copilot Checklist

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[jakebailey]

@copilot 100_000 seems like a magic number, and this definitely requires a comment. Probably also best to return MaxInt?

[Copilot]

Updated in a446575 — now returns math.MaxInt and added a comment explaining the overflow guard and why it's needed (Go int wraps vs JS Infinity).

[Copilot]

Pull request overview

Fixes a hang in the type checker when expanding extremely large template literal types by preventing int overflow in getCrossProductUnionSize, ensuring the existing “too complex” guard (TS2590) reliably triggers.

Changes:

• Add overflow-capping logic to Checker.getCrossProductUnionSize so cross-product size never wraps to 0/negative on large unions.
• Add a new compiler test that previously caused a combinatorial explosion hang, now expected to produce TS2590.
• Add reference baselines for the new test.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/checker.go	Caps cross-product union size computation before int overflow to preserve complexity-limit behavior.
testdata/tests/cases/compiler/templateLiteralTypeTooComplex.ts	New regression test for large template literal type expansion.
testdata/baselines/reference/compiler/templateLiteralTypeTooComplex.errors.txt	Baseline asserting TS2590 is produced.
testdata/baselines/reference/compiler/templateLiteralTypeTooComplex.types	Baseline for type output under the new test.
testdata/baselines/reference/compiler/templateLiteralTypeTooComplex.symbols	Baseline for symbol output under the new test.
testdata/baselines/reference/compiler/templateLiteralTypeTooComplex.js	Baseline for JS emit output under the new test.