Confusing error message when calling value whose type is a union of types with call signatures

[DanielRosenwasser]

From @StrahilKazlachev on Gitter on November 12, 2015 9:04 AM

interface MyType {
    x;y;z;
}

function transform(items: MyType[] | string[]) {
    return items.map(val => val);
        // ~~~~~~~~~~~~~~~~~~~~~ Cannot invoke an expression whose type lacks a call signature.
}

We should consider giving a more helpful error message if the type is a union, all of whose types have call signatures. Something like

Cannot invoke an expression whose type is a union with incompatible call signatures.

or hopefully something better than that

[zbjornson]

This message seems to have just manifested after upgrading to 2.1.0-dev.20160729. Glad to find the translation in this issue :).