Duplicate suggestion diagnostics returned for import with missing types

[mjbvz]

TypeScript Version: 3.3.0-dev.20181128

Search Terms:

• suggestion
• errors
• geterr

Code
In a node project:

$ npm install querystring

With a ts file:

import { stringify } from 'querystring'

stringify  

Bug:
Duplicate suggestion diagnostics are returned:

[Trace  - 10:48:04 AM] Event received: suggestionDiag (0).
Data: {
    "file": "/Users/matb/projects/san/src/components/index.ts",
    "diagnostics": [
        {
            "start": {
                "line": 1,
                "offset": 27
            },
            "end": {
                "line": 1,
                "offset": 40
            },
            "text": "Could not find a declaration file for module 'querystring'. '/Users/matb/projects/san/node_modules/querystring/index.js' implicitly has an 'any' type.\n  Try `npm install @types/querystring` if it exists or add a new declaration (.d.ts) file containing `declare module 'querystring';`",
            "code": 7016,
            "category": "suggestion"
        },
        {
            "start": {
                "line": 1,
                "offset": 27
            },
            "end": {
                "line": 1,
                "offset": 40
            },
            "text": "Could not find a declaration file for module 'querystring'. '/Users/matb/projects/san/node_modules/querystring/index.js' implicitly has an 'any' type.\n  Try `npm install @types/querystring` if it exists or add a new declaration (.d.ts) file containing `declare module 'querystring';`",
            "code": 7016,
            "category": "suggestion"
        }
    ]
}

[sandersn]

I believe this happens with quite a few suggestions, based on the fourslash tests of suggestion diagnostics.

[sandersn]

This is not hard to isolate. Here's a fourslash test that shows that you get two diagnostics instead of one:

/// <reference path="fourslash.ts" />
// @Filename: foo.ts
//// import { f } from 'm'
//// f

// @Filename: node_modules/m/index.js
//// module.exports.f = function (x) { return x }

verify.getSemanticDiagnostics([])
verify.getSuggestionDiagnostics([{
    range: { fileName: "foo.ts", pos: 18, end: 21 },
    code: 7016,
    message: "Could not find a declaration file for module 'm'. '/tests/cases/fourslash/node_modules/m/index.js' implicitly has an 'any' type.",
}])

[sandersn]

This is a general problem with suggestionDiagnostics; it uses createMultiMap. Normal diagnostics use createDiagnosticCollection, which de-duplicates its entries. createMultiMap does not. (The latter is also uses sorted arrays instead of a map of strings to unsorted arrays.