[no-unnecessary-type-assertion] False positive with non-null-assertion

[brainkim]

Repro

{
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "sourceType": "module",
    "createDefaultProgram": true
  },
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}

let resolve: () => void;
const promise = new Promise((resolve1) => (resolve = resolve1));
resolve!();

Expected Result
No errors occur.

Actual Result

3:1  error  This assertion is unnecessary since it does not change the type of the expression  @typescript-eslint/no-unnecessary-type-assertion

Additional Info
Removing the ! will cause tsc --strict to error with the following message:

test.ts(3,1): error TS2454: Variable 'resolve' is used before being assigned.

Related issues:

• False positives for no-unnecessary-type-assertion #187
• [no-unnecessary-type-assertion] false positive when asserting via non-null assertion operator #529
• [no-unnecessary-type-assertion] False positive with null/undefined union type #1061

Versions

package	version
@typescript-eslint/eslint-plugin	2.3.3
@typescript-eslint/parser	2.3.3
TypeScript	3.6.4
ESLint	6.5.1
node	10.16.3
npm	6.9.0

[bradzacher]

I am unable to repro this against master.

[brainkim]

@bradzacher Are you saying this was fixed, or that you were never able to reproduce the issue.

[bradzacher]

I ran the example code as a test against master, and it did not repro.

So either it is fixed in master, or the code does not repro the issue.

[brainkim]

I am still seeing this on my end after updating to 2.7.0 on my current project. Maybe it has to do with tsconfigs? Specifically, the strict option? Here‘s my tsconfig. Don’t really got the time to reproduce the bug in this project right now, and I don’t mind if we leave this issue closed til I or someone else can produce a reliable reproduction.

{
  "files": ["src/crank.ts"],
  "compilerOptions": {
    /* Basic Options */
    "target": "esnext",                       /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */
    // "module": "es2015",                    /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
    "lib": ["esnext", "dom"],                 /* Specify library files to be included in the compilation. */
    // "allowJs": true,                       /* Allow javascript files to be compiled. */
    // "checkJs": true,                       /* Report errors in .js files. */
    "jsx": "react",                           /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    "declaration": true,                      /* Generates corresponding '.d.ts' file. */
    // "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
    // "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
    // "outDir": "./lib",                     /* Redirect output structure to the directory. */
    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "composite": true,                     /* Enable project compilation */
    // "removeComments": true,                /* Do not emit comments to output. */
    // "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    "downlevelIteration": true,               /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */
    "strict": true,                           /* Enable all strict type-checking options. */
    // "noImplicitAny": true,                 /* Raise error on expressions and declarations with an implied 'any' type. */
    // "strictNullChecks": true,              /* Enable strict null checks. */
    // "strictFunctionTypes": true,           /* Enable strict checking of function types. */
    // "strictBindCallApply": true,           /* Enable strict 'bind', 'call', and 'apply' methods on functions. */
    // "strictPropertyInitialization": true,  /* Enable strict checking of property initialization in classes. */
    // "noImplicitThis": true,                /* Raise error on 'this' expressions with an implied 'any' type. */
    // "alwaysStrict": true,                  /* Parse in strict mode and emit "use strict" for each source file. */

    /* Additional Checks */
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */

    /* Module Resolution Options */
    "moduleResolution": "node",               /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": {},                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true                   /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */

    /* Source Map Options */
    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */
    // "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
  }
}