TSC Build Cache Invalidation

This project is a minimal reproduction of a cache invalidation issue related to subprojects and incremental compilation in the typescript compiler as of version 5.1.3.

Reproduction steps

Install project dependencies
```
npm ci
```
Build the project
```
npx tsc --build
```
Check out the first commit
```
git checkout 53e42026f1c20bb4e7370
```
Build the project again
```
npx tsc --build
```
Check out main
```
git checkout main
```
Build the project again
```
npx tsc --build
```

For convenience, here are all the steps in a single block:

npm ci
npx tsc --build
git checkout 53e4202
npx tsc --build
git checkout main
npx tsc --build

Result

The final build produces the following error output:

build/hello/hello.d.ts:1:23 - error TS2307: Cannot find module 'main/const' or its corresponding type declarations.

1 import { LogFn } from 'main/const';
                        ~~~~~~~~~~~~


Found 1 error.

Expected

No error. The error will go away if you clean tsc's output and do a rebuild:

npx tsc --build --clean
npx tsc --build

Interpretation

The second commit in this repo (201a7af) creates a new subproject in the hello directory, and moves main/const.ts into hello/const.ts so that it becomes part of this subproject. This also requires updating an import in hello/hello.ts to import const.ts from the new location.

The first build on the main branch creates a build/hello/tsconfig.tsbuildinfo cache file for the hello subproject.

The next build, on the initial commit (53e4202), updates build/hello/hello.d.ts to import from main/const. This commit has no awareness of the hello subproject, and it does not touch build/hello/tsconfig.tsbuildinfo.

The final build, back on main, does not rebuild the hello subproject because it sees build/hello/tsconfig.tsbuildinfo and sees that the newest input of the hello subproject is older than this cache file. You can see this by running npx tsc --build --verbose

Output:

> npx tsc --build --verbose
[12:44:45 PM] Projects in this build:
    * src/hello/tsconfig.json
    * src/main/tsconfig.json
    * tsconfig.json

[12:44:45 PM] Project 'src/hello/tsconfig.json' is up to date because newest input 'src/hello/const.ts' is older than output 'build/hello/tsconfig.tsbuildinfo'

[12:44:45 PM] Project 'src/main/tsconfig.json' is out of date because buildinfo file 'build/main/tsconfig.tsbuildinfo' indicates that some of the changes were not emitted

[12:44:45 PM] Building project '/Users/jason/src/tsc-build-cache-invalidation/src/main/tsconfig.json'...

build/hello/hello.d.ts:1:23 - error TS2307: Cannot find module 'main/const' or its corresponding type declarations.

1 import { LogFn } from 'main/const';
                        ~~~~~~~~~~~~


Found 1 error.

This is incorrect because one of the outputs of the hello project is stale, even though none of the inputs are newer than the build cache file.

Notes

This bug is a reduced version of a bug found by Desmos Studio in the process of migrating the Desmos Graphing Calculator to TS subprojects.

Our developers found that they were having to frequently clean the TS build directory when switching back and forth between branches that were either newer than or older than the migration to subprojects.

jwmerrill / tsc-build-cache-invalidation