Feature request: better process function exits for type guards and 'if'-s

[zhuravlikjb]

Currently type guards are limited only to bodies of conditionals.

A simple example with 'if':

function f(s: string | number) {
      if (typeof s === "string") {
          // s is 'string'
      }
      else {         
          // s is 'number'
      }
}

But what if I write the same code, but another way:

function f(s: string | number) {
      if (typeof s === "string") {
          // s is 'string'
          // do something
          return;
      }

      // s is 'string | number', according to the compiler
      // but control flow of 'if' block flows directly to a function exit
      // that's why, s is actually 'number' here, if no assignments are made
}

So, the suggestion is to augment the rule for 'if' as follows:

• If the false branch is not present, and all control flow exits of the true branch point exactly to function exits, the type of a variable or parameter is narrowed by a type guard in the code after 'if' statement when false, provided the code after 'if' statement contains no assignments to the variable or parameter.
• If all control flow exists of the false branch point exactly to function exits, the type of a variable or parameter is narrowed by a type guard in the code after 'if' statement when true, provided the code after 'if' statement contains no assignments to the variable or parameter.

[mhegazy]

looks like the same as #2388