fix(eslint-plugin): [no-unnecessary-condition] improve optional chain handling

[yeonjuan]

Hi, 😀
IMO, it's a false negative in no-unnecessary-condition

Repro

{
  "rules": {
    "@typescript-eslint/no-unnecessary-condition": ["error"]
  }
}

// case 1
declare const x: { a?: { b: string } };
x?.a?.b;

// case 2
declare const x: { a: { b?: { c: string } } };
x.a?.b?.c;

// case 3
let x: { a?: string };
x?.a;

Expected Result

// case 1
declare const x: { a?: { b: string } };
x?.a?.b;
^^          // lint error 

// case 2
declare const x: { a: { b?: { c: string } } };
x.a?.b?.c;
  ^^        // lint error

// case 3
let x: { a?: string };
x?.a;
^^        // lint error

Actual Result

// case 1
declare const x: { a?: { b: string } };
x?.a?.b; // no error

// case 2
declare const x: { a: { b?: { c: string } } };
x.a?.b?.c; // no error

// case 3
let x: { a?: string };
x?.a;  // no error

Additional Info

It seems "no-unnecessary-condition" checks whether the whole optional member expression type is nullable or not. so it just passed when the last member in the optional member expression is nullable.

I changed the target node to check. Am I miss something? 🤔

[typescript-eslint]

Thanks for the PR, @yeonjuan!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #2111 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2111   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files         171      171           
  Lines        7755     7757    +2     
  Branches     2215     2216    +1     
=======================================
+ Hits         7245     7247    +2     
  Misses        244      244           
  Partials      266      266           

Flag	Coverage Δ
#unittest	93.42% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...slint-plugin/src/rules/no-unnecessary-condition.ts	97.01% <100.00%> (+0.04%)	⬆️

[bradzacher]

Unless I'm mistaken, I believe this is addressing #1977. If I'm right, please add it to the OP 😄

[yeonjuan]

@bradzacher
Well, I found this issue while trying to solve #1977, but it's quite a different issue.
#1977 occurs when a node's nullable type is originated from its parent nullable, but this issue occurs when the OptionalMemberExpression's last member is nullable type. (In my thought)

so this pr can't fix #1977

let x: { a: { b: string } } | null;
x?.a?.b;
   ^^ // a's nullable type is originated from x's type, so this case belong #1977


let x: { a?: string };
x?.a;  // no error
   ^^ // IMO, we should check type of "x" not "a"

[bradzacher]

LGTM - thanks for this!