[ASDAlexander77]

JSON.stringify({x: 1}) threw TypeError("JSON.stringify cannot read the fields of an object literal or a tuple"). Uncaught, that ended a program with 0xC0000409 and no output. Now it writes {"x":1}.

Needs ASDAlexander77/TypeScriptCompiler#832 merged first. That compiler PR makes for...in over an object literal or a tuple once for each field of its type, with the key as a literal. It also fixes a generic instance making other instances: on main, JSON.stringify([1, 2]) does not compile when it is a program's first stringify. The release workflow clones this repo's main, so merging this PR first would fail json.ts with the current compiler.

Change

• toValue sends a tuple, or an "object" (an object literal with methods), to a new fieldsToValue.
• fieldsToValue walks the fields with for (const key in value). value[key] is then that field with its own type.
• A tuple of unnamed fields ("0" in value) becomes a JSON array, as in JavaScript.
• An object leaves out function and undefined fields. In an array they become null.
• A BigInt is still a TypeError.

Tests

• json.ts gets a new objects(), run first, so the writer's inner instances are made from inside it.
• It covers:
  • a plain literal
  • every kind of field
  • nested literals
  • an array of literals
  • a function field
  • get_-named value fields
  • a tuple
  • space

Verified locally (Windows x64, built with #832)

• tests.ps1 passes for gc, rc, none and own, in release and debug, compiled and JIT.
• Not run: Linux/WSL. The release run is this change's first Linux test.

Known limits

• { "0": … } is written as an array, and a named tuple as an object. The compiler does not tell the two apart.
• Under -mm=own, a literal holding a class instance ({ when: new Date(0) }) is rejected by the compiler's existing own rule.

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