Io-ts: Typescript <3.6 incompatibility

[philipp-schaerer-lambdait]

🐛 Bug report

Current Behavior

When using the latest io-ts version 2.0.2 and typescript version <3.6 the tsc build fails with the error:

ERROR in ../../node_modules/io-ts/lib/index.d.ts:416:9 - error TS1086: An accessor cannot be declared in an ambient context.

416     get type(): C;

But the documentation mentions that typescript 3.5.2+ is supported.
The error seems to occur when a library is being built with Typescript 3.7+ which uses a getter or setter and the main project has a Typescript <3.6 dependency.
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#class-field-mitigations

This is even more a problem while using angular 8.2.x because of the typescript <3.6 dependency.

Expected behavior

The io-ts definition files should be compatible with 3.5.2 or the documentation should be updated.

Reproducible example

I've created a test repository to test this behavior:
https://github.com/Lambda-IT/io-ts-test

Suggested solution(s)

Perhaps it is possible to build the index.d.ts file in a way which is compatible with projects which use typescript version >=3.5.x
Alternatively the documentation could be updated and for angular 8.2 users a typescript update to 3.6+ together with an disableTypeScriptVersionCheck setting could help.

Environment

| Software | Version(s) |
| ---------- | ---------- |
| io-ts | 2.0.3 |
| fp-ts | 2.3.0 |
| TypeScript | 3.5.3 |

[gcanti]

@philipp-schaerer-lambdait thanks for the detailed report

Perhaps it is possible to build the index.d.ts file in a way which is compatible with projects which use typescript version >=3.5.x

Looks like the diff is

// index.d.ts

get type(): C; // <= [email protected] output
readonly type: C; // <= [email protected],3.6.x output

We could force the same output by adding the getter "by hand", check this PR out, what do you think?

[philipp-schaerer-lambdait]

The changes work and the build runs now correctly with the angular 8.2 Project. If the change from private runDefinition: () => C to public runDefinition: () => C is ok for the library code then I don't have any objections to merge this change.

[gcanti]

@philipp-schaerer-lambdait patch released, thanks for reviewing