Check on intersection types determines never type

[jsteuer]

TypeScript Version: up to latest version 3.8-Beta

Search Terms:

• does not exist on type 'never'
• control flow never
• null check never
• type guard null never

Code

let greeting: null | string = null;

/* A error */    function setIt() { greeting = "Hello!" }; setIt()
/* B error */    setTimeout(() => greeting = "Hello!", 0);
/* C error */    const runCallback = (cb: () => void) => cb(); runCallback(() => greeting = "Hello!");
/* D ok    */ // ((() => { greeting = "Hello!" })())
/* E ok    */ // greeting = Math.random() > 0.5 ? "Hello!" : null;

if (greeting) {
    greeting.toUpperCase(); // Property 'toUpperCase' does not exist on type 'never'.
}
if (greeting != null && greeting !== null) {
    greeting.toUpperCase(); // Property 'toUpperCase' does not exist on type 'never'.
}
if (typeof greeting === "string") {
    greeting.toUpperCase(); // Property 'toUpperCase' does not exist on type 'never'.
}

Expected behavior:
greeting should have type string Inside all if-blocks - not never. This should be true for
all (activated exclusively) test cases (A to E).

Actual behavior:
greeting has type never inside the if-blocks (for test cases A, B and C). It appears that the assignments to greeting are not recognized. For test cases D and E everything is working as expected.

Playground Link: https://www.typescriptlang.org/play/?ts=3.8-Beta&ssl=1&ssc=1&pln=17&pc=2#code/DYUwLgBA5gTi4EsB2UBcEkFdjAgHwgGcwZkoIBeDbYAbgFgAoJgegCoIBBCEGGAexgQ2LCGIgAzTEgDGYBPyRFwASTAAKAJQQA3tDiIUlCACIAEiBz8AhCYgBfWsrBqtrDgCEefQcNHjCcAAVBABbEH5MDS1KAD59eHkjKnNLYBsTABoIAAZNBkZ2CABhbwEhEXEIGUViCBhpYoBDHAAjJpkAa2N1GVb0GIp4gDd+BAATbSHq1q0nBqRmto7O9UH42ESyY1SrW3z3CAARCH5u8UqWUTX13QTDchSLPbt7TS1NQ4BRU-OxS9EmwexgAsk0wAALAB0MCaSHG-FCMXiOShAFYIAB+UzPdK2CDoLA4ApMBASCDqIFJKDaHRMKpUshQsD8ACqAAd2bxmoE5hArhAAAoCLkwMAATwgAHIWRzRTyQFKIAiQIQMPxICAAB4IOqKCASrnSpAgYa8KVQpj2UnkykGakQaxUIm4ABkrvuDqdzpotPp4kZKGZbM53KavPy-NEwv4ool0tloZgCqVKrVSA1PB1eqUhpAxtN5stjGtjDJFLz-HJgceFBSxFIKBMfsYDPtTMT8vDID5ApjcclMpDXcCqf4qvVmuzkH1eYLZpgFqtQA

Related Issues: Maybe to #36777

[RyanCavanaugh]

Duplicate #9998

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.