FunctionLike and SignatureDeclaration wind up being identical types once all sub-unions are flattened. You can test this out with:

import ts = require("typescript");
let a: ts.FunctionLike = <any>null;
let b: ts.SignatureDeclaration = <any>null;
a = b;
b = a;

My suggested fix would be to just use SignatureDeclaration everywhere.

(Ref: #16988)