foo.ts is resolved before foo.d.ts even if the latter is in files[]

Affects at least TS 2.7.2 and 2.6.2. This problem appears when compiling files separately, like we do under Bazel.

Imagine this simple app

src/lib.ts

export const a = 1;

src/main.ts

import {a} from './lib';

Imagine lib.ts was compiled separately, so there already exists dist/lib.d.ts

export declare const a = 1;

Now, I want to compile main.ts as a separate program. Given src/tsconfig.json

{
  "compilerOptions": {
    "rootDirs": [
      ".",
      "../dist"
    ],
    "outDir": "../dist",
    "declaration": true
  },

  "files": [
    "main.ts",
    // lib was compiled separately
    "../dist/lib.d.ts"
  ]
}

We see that lib.d.ts is already in the program before resolution begins. However the compiler resolves the import statement to the lib.ts file instead, adding it to the program, and tries to emit on top of an input, so the error is

$ ./node_modules/.bin/tsc -p src
error TS5055: Cannot write file '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/dist/lib.d.ts' because it would overwrite input file.

Okay, we didn’t want lib.ts in the program, so we should just use --noResolve to prevent that, but it’s also broken:

$ ./node_modules/.bin/tsc -p src --noResolve --traceResolution
======== Resolving module './lib' from '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/main.ts'. ========
Module resolution kind is not specified, using 'NodeJs'.
'rootDirs' option is set, using it to resolve relative module name './lib'.
Checking if '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/' is the longest matching prefix for '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib' - 'true'.
Checking if '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/dist/' is the longest matching prefix for '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib' - 'false'.
Longest matching prefix for '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib' is '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/'.
Loading 'lib' from the root dir '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/', candidate location '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib'.
Loading module as file / folder, candidate module location '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib', target file type 'TypeScript'.
File '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib.ts' exist - use it as a name resolution result.
======== Module name './lib' was successfully resolved to '/usr/local/google/home/alexeagle/Projects/repro_ts_resolving_ts/src/lib.ts'. ========
src/main.ts(1,17): error TS2307: Cannot find module './lib'.

So far our workaround at Google is one of:

• use Bazel sandboxing to make the inputs appear in different places
• use our own custom compiler which elides any emit to files we don’t expect

However under Bazel we sometimes cannot sandbox (eg. on Windows) and cannot use our custom compiler (eg. when we are compiling it) so we are stuck.

I found that ts.CompilerOptions.suppressOutputPathCheck could be a workaround, but tsc doesn’t allow that flag from the command line or tsconfig.json (it’s not in optionDeclarations in commandLineParser.ts)

[mprobst]

Alternative idea: when --noResolve is true, just never look at the file system and only use the list of source files as the source of truth on what files are present? Then in the example above, ../dist/lib.ts would consistently not be found, and compilation would succeed by finding lib.d.ts (or fail if that’s not around).

[alexeagle]

I think the way to fix this is not to always try to resolve imports using other rootDirs, only to check if the rootFiles already satisfy an import.