fix(compiler): treat underscore-prefixed JSX tags as components

[ousamabenyounes]

Summary

Fixes #36601

The React Compiler was using a regex check /^[A-Z]/ to determine whether a JSX tag is a component or a builtin (host) element. This meant that tags prefixed with underscore (e.g., <_Bar>) were incorrectly treated as builtin elements.

Per the JSX specification, only tags starting with a lowercase letter (a-z) should be treated as host/builtin elements. Tags starting with anything else (uppercase letters, underscore, dollar sign, etc.) should be treated as component references.

The Bug

Given this input:

function Foo({ _Bar }: { _Bar: React.ComponentType }) {
    return (
      <_Bar>
        {() => <div />}
      </_Bar>
    );
}

The compiler was incorrectly treating <_Bar> as a builtin tag, which caused the children {() => <div />} to be incorrectly extracted into a separate _temp function — silently changing the runtime behavior from what worked fine before React Compiler.

The Fix

Changed the condition in lowerJsxElementName() in BuildHIR.ts from:

if (tag.match(/^[A-Z]/)) {
  // component
} else {
  // builtin tag
}

To:

if (tag.match(/^[a-z]/)) {
  // builtin tag
} else {
  // component
}

This matches the React JSX transform convention where a JSX tag is a host element only if its name starts with a lowercase ASCII letter.

Testing

Existing test fixtures (builtin-jsx-tag-lowered-between-mutations, global-jsx-tag-lowered-between-mutations) continue to work correctly since:

• <div> (lowercase) → still treated as builtin ✓
• <View> (uppercase) → still treated as component ✓

---

Test verification (RED → GREEN)

Added fixture compiler/.../fixtures/compiler/jsx-underscore-prefixed-tag-is-component.tsx (+ generated .expect.md).

Command: yarn snap -p '*underscore*'

GREEN — with the fix (/^[a-z]/): _Bar is lowered as a component (reactive dependency), sprout eval matches.

1 Tests, 1 Passed, 0 Failed

RED — fix reverted to /^[A-Z]/:

FAIL: jsx-underscore-prefixed-tag-is-component
Found differences in evaluator results
Non-forget (expected): (kind: ok) <div>{"children":"ok"}</div>
Forget:                (kind: exception) _Bar is not defined
1 Tests, 0 Passed, 1 Failed

Without the patch, <_Bar> is lowered as a BuiltinTag (host string), so the compiled output references an undefined _Bar and throws at runtime — diverging from the non-compiled reference. The fixture catches exactly this regression.