Incomprehensible incompatibility around Partial, Conditional types and Generics.

TypeScript Version: 3.9.0-dev.20200322

Search Terms: partial, conditional types

Code

type SafePartial<T> = T extends {} ? Partial<T> : any;

interface QB<TRecord extends {}> {
  insert(record: SafePartial<TRecord>): void;
}


async function insert1<TRecord extends {}>(qb: QB<TRecord>, record: TRecord) {
  await qb.insert(record);
}

Expected behavior:

The code successfully compiles

Actual behavior:

Following error is observed for qb.insert(record):

Argument of type 'TRecord' is not assignable to parameter of type 'SafePartial<TRecord>'. Type '{}' is not assignable to type 'SafePartial<TRecord>'.

The issue goes away if I use Partial instead of the SafePartial above.

The issue also goes away if a concrete type is used instead of generic type:

interface User {
    id: string;
}

async function insert1(qb: QB<User>, record: User) {
  await qb.insert(record);
}

Playground Link: Playground

Start with the linked TypeScript Playground reproduction and compare the generic conditional-type case with the working Partial and concrete-type cases. Trace the compiler's assignability handling for SafePartial and confirm the behavior with a reduced reproduction. Done means the original code compiles without weakening the reported types.