Improve error message for incompatible signatures in union type from typed/untyped function call

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Please read the FAQ first, especially the "Common Feature Requests" section. Cannot invoke an expression whose type lacks a call signature. Type '(() => string) | (T & Function)' has no compatible call signatures. [2349] --> ## Search Terms untyped function call error message union type incompatible signature <!-- List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily --> ## Example This code errors as follows: ```ts function foo<T>(x: T | (() => string)) { if (typeof x === "function") { let a = x(); // ^ Cannot invoke an expression whose type lacks a call signature. //Type '(() => string) | (T & Function)' has no compatible call signatures. [2349] } } ``` which feels strange given that both of the following are fine: ```ts function fooL<T>(x: T) { if (typeof x === "function") { let a = x(); } } function fooR(x: () => string) { if (typeof x === "function") { let a = x(); } } ``` If an evaluation contexts accepts values of type `A` and values of type `B`, then it should accepts values of type `A | B`. I *think* what is going on is that the call signature from `T & Function` is *untyped*, which is incompatible with the *typed* signature of `() => string`. I don't think this is very obvious to a user, and they end up seeing an application that works for both branches of a union, but not their composition. ## Suggestion I think making the example work is out of scope. My suggestion is to improve the error message, something like: ```ts function foo<T>(x: T | (() => string)) { if (typeof x === "function") { let a = x(); // ^ Cannot invoke an expression whose type lacks a call signature. // Type '(() => string) | (T & Function)' has n…