crash in new addImplementationSuccessElaboration

[sandersn]

Crashes 44 packages in DT.

✔ ~/DefinitelyTyped/types/agent-base [master|✔] 
09:28 $ tsc
/home/nathansa/ts/built/local/tsc.js:105472
                throw e;
                ^

TypeError: Cannot read property 'length' of undefined
    at getSignatureFromDeclaration (/home/nathansa/ts/built/local/tsc.js:53357:89)
    at addImplementationSuccessElaboration (/home/nathansa/ts/built/local/tsc.js:67988:37)
    at resolveCall (/home/nathansa/ts/built/local/tsc.js:67955:25)
    at resolveCallExpression (/home/nathansa/ts/built/local/tsc.js:68284:20)
    at resolveSignature (/home/nathansa/ts/built/local/tsc.js:68657:28)
    at getResolvedSignature (/home/nathansa/ts/built/local/tsc.js:68688:26)
    at checkCallExpression (/home/nathansa/ts/built/local/tsc.js:68814:29)
    at checkExpressionWorker (/home/nathansa/ts/built/local/tsc.js:71378:28)
    at checkExpression (/home/nathansa/ts/built/local/tsc.js:71287:38)
    at checkExpressionStatement (/home/nathansa/ts/built/local/tsc.js:73819:13)

Added in #41001

[sandersn]

calling getSignatureFromDeclaration on a ModuleDeclaration, which has no parameters, so declaration.parameters.length crashes.

[sandersn]

Overloaded ambient function merged with namespace, such that the function implementation is not the last declaration. An easy fix is to skip such merges, although it's probably possible to use a different method to find the implementation declaration.

This crash might only apply in d.ts files, where there IS no implementation.

declare namespace Agent {
  interface Agent { ... }
}
declare function Agent(opts?): Agent.Agent;
declare function Agent(callback, opts?): Agent.Agent;
export = Agent;

[weswigham]

The crash is just because of an unchecked cast - turns out having a .body member isn't enough to guarantee the declaration is function-like, since namespaces also have a .body 😛 - PR is up: #41017

[minestarks]

https://github.com/microsoft/tsserverfuzzer/issues/301