Union type inference lost on assignment

[amacleay]

TypeScript Version: 2.9.0-dev.20180418

Search Terms: union type strictnullchecks inference

Code

interface Person {
  name: string;                           
  address: {                              
    street?: string;
    city: string;                         
  };                                      
}                                         
                                          
declare function getPerson(): Person;     
                                          
function getMinimalAddress(): string {    
  const person = getPerson();             
  if (person.address.street) {            
    const address = person.address;
    return address.street; // ERROR on this line
  } else {                                
    return person.address.city;           
  }                                       
}

Expected behavior:
Passes type checker. Since person.address.street is either null or a string, and since there is a check on line 13 for person.address.street being truthy, we know that address.street is a string.

You can get the type checker to pass by omitting the reassignment:

--- broken.ts   2018-04-18 16:01:26.188724623 -0400                                 
+++ works.ts    2018-04-18 16:01:26.192724616 -0400                                 
@@ -11,8 +11,7 @@                         
 function getMinimalAddress(): string {   
   const person = getPerson();            
   if (person.address.street) {           
-    const address = person.address;      
-    return address.street;               
+    return person.address.street;        
   } else {                               
     return person.address.city;          
   }

Actual behavior:

$ tsc --strictnullChecks broken.ts 
broken.ts(15,5): error TS2322: Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.

Playground Link: playground link

Related Issues:
#22635
#16069

[RyanCavanaugh]

We track these guards based on the expressions used to test them; unfortunately the type checker is not implemented in a way that it actually understands that address and person.address are references to the same object. There's not a lot we can do here short of a wholesale rewrite of the type guard logic, which isn't really in the cards to fix something like this.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.