Union type checking during assignment fails for boolean and passes for other primitives

[evxn]

TypeScript Version: 3.7.2

Search Terms:
number string union

Expected behavior:

Compiler should be giving an error on assigning demo1, like it does with demo2.

Actual behavior:

Doesn't seem to properly discriminate between the string and other types that are conceptually unbound: number/RegExp etc.

Related Issues:
#20863
#30506
#9254

Code

type t1 = {
    a: number;
    b: string;
} | {
    a: string;
};

// no error. Invalid assignment is possible
const demo1: t1 = {
    a: '123',
    b: 'asd',
};

type t2 = {
    a: boolean;
    b: string;
} | {
    a: string;
};

// error Error: TS2322: Type '{ a: string; b: string; }' is not assignable to type 't2'.  Object literal may only specify known properties, and 'b' does not exist in type '{ a: string; }'.
const demo2: t2 = {
    a: '123',
    b: 'asd',
};

Output

"use strict";
// no error
const demo1 = {
    a: '123',
    b: 'asd',
};
// error
const demo2 = {
    a: '123',
    b: 'asd',
};

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided