[BUG] failed to resolve peer dependencies (conflict) when peer dependencies are in range.

[talsi]

Current Behavior:

npm install favors getting higher versions of peer dependencies.

for example:

"@angular/cdk": 10.2.5 has the following peer dependencies

{
  ...
  "@angular/common": "^10.0.0 || ^11.0.0-0",
  "@angular/core": "^10.0.0 || ^11.0.0-0"
  ...
}

"@angular/forms": 10.2.5 has the following peer dependencies

{
  ...
  "@angular/common": "10.2.5",
  "@angular/core": "10.2.5"
  ...
}

if you try to install both packages above you'll get an error because when resolving peer dependencies it chooses "@angular/common": 11.2.11 (and then "@angular/core": 11.2.11) thus results in a conflict

Expected Behavior:

when resolving peer dependencies it should choose "@angular/common": 10.2.5 (and then "@angular/core": 10.2.5) that would have satisfy all deps

Steps To Reproduce:

1. open an empty project / folder
2. run npm init (package.json doesn't list any dependencies)
3. run npm i @angular/cdk@10.2.5 @angular/forms@10.2.5

result:

npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! Found: @angular/core@11.2.11
npm ERR! node_modules/@angular/core
npm ERR!   peer @angular/core@"^10.0.0 || ^11.0.0-0" from @angular/cdk@10.2.5
npm ERR!   node_modules/@angular/cdk
npm ERR!     @angular/cdk@"10.2.5" from the root project
npm ERR!   peer @angular/core@"11.2.11" from @angular/common@11.2.11
npm ERR!   node_modules/@angular/common
npm ERR!     peer @angular/common@"^10.0.0 || ^11.0.0-0" from @angular/cdk@10.2.5
npm ERR!     node_modules/@angular/cdk
npm ERR!       @angular/cdk@"10.2.5" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! @angular/forms@"10.2.5" from the root project
npm ERR!
npm ERR! Conflicting peer dependency: @angular/core@10.2.5
npm ERR! node_modules/@angular/core
npm ERR!   peer @angular/core@"10.2.5" from @angular/forms@10.2.5
npm ERR!   node_modules/@angular/forms
npm ERR!     @angular/forms@"10.2.5" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.

Environment:

npm: 7.11.1

[nlf]

this is one we intend to resolve in some fashion, but it is surprisingly complicated with the current state of arborist.

as a work around for now you can specify @angular/common@10.2.5 as a direct dependency and you'll at least be unblocked and able to install your tree.

[talsi]

@nlf thanks, i really appreciate your suggestion for a workaround but this is merely an example to demonstrate the nature of the issue.

The real issue I'm facing is more complicated than the example above- i actually want to rely on npm to install peer dependencies correctly and to fail when it finds conflicts.

There are however many published libraries that will fail to install without a workaround.
There are also many CLIs and other tools that will not work because of this issue.

[nlf]

yup, totally understood. we're working on it but this one creates a huge amount of complexity so it may be some time before we come up with a solution that resolves as many conflicts as it possibly can without introducing massive slow downs.