Typescript error with required properties

It seems like AllowImplicit is confusing Typescript into thinking that required properties are optional.

interface A {
  a: string;
}

const aDecoder: Decoder<A> = object({
  a: string
});

Output:

Type 'Decoder<{ a?: string; }, unknown>' is not assignable to type 'Decoder<A, unknown>'.
  Type '{ a?: string; }' is not assignable to type 'A'.
    Property 'a' is optional in type '{ a?: string; }' but required in type 'A'.ts(2322)

Live example: https://codesandbox.io/s/nostalgic-sinoussi-lkrq1?file=/src/index.ts

[djlauk]

I narrowed it down to the strict setting in tsconfig.json.

• when "strict": false (the default, when not set), the problem exists
• when "strict": true the problem is gone

Steps to reproduce

Step 1: Create npm project

cd /tmp
mkdir repro-decoders-750
cd repro-decoders-750
npm init -y
npm i -D typescript@4.4.4
npm i decoders@1.25.4

Step 2: Create source file

Create file repro.ts:

import { object, string } from "decoders";
import type { Decoder } from "decoders";

export interface Person {
  name: string;
}

export const personDecoder: Decoder<Person> = object({
  name: string,
});

Step 3: Create tsconfig.json files

Create tsconfig.bad.json:

{
  "compilerOptions": {
    "moduleResolution": "node",
    "module": "es2020",
    "lib": ["es2020", "DOM"],
    "target": "es2020",
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "strict": false
  }
}

Create tsconfig.ok.json:

{
  "compilerOptions": {
    "moduleResolution": "node",
    "module": "es2020",
    "lib": ["es2020", "DOM"],
    "target": "es2020",
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true
  }
}

Step 4: Demonstrate behaviour

Demostrate different behaviour:

npx tsc --noemit -p tsconfig.ok.json
# no output = no compiler errors = all ok

npx tsc --noemit -p tsconfig.bad.json
repro.ts:8:14 - error TS2322: Type 'Decoder<{ name?: string; }, unknown>' is not assignable to type 'Decoder<Person, unknown>'.
  Type '{ name?: string; }' is not assignable to type 'Person'.
    Property 'name' is optional in type '{ name?: string; }' but required in type 'Person'.

8 export const personDecoder: Decoder<Person> = object({
               ~~~~~~~~~~~~~


Found 1 error.

See difference in config:

diff -u tsconfig.*
--- tsconfig.bad.json	2021-10-29 15:37:58.000000000 +0200
+++ tsconfig.ok.json	2021-10-29 15:38:00.000000000 +0200
@@ -8,6 +8,6 @@
     "esModuleInterop": true,
     "skipLibCheck": true,
     "forceConsistentCasingInFileNames": true,
-    "strict": false
+    "strict": true
   }
 }

[stevekrouse]

Thanks!!