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

compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts

@@ -3409,19 +3409,19 @@ function lowerJsxElementName(
   const exprLoc = exprNode.loc ?? GeneratedSource;
   if (exprPath.isJSXIdentifier()) {
     const tag: string = exprPath.node.name;
-    if (tag.match(/^[A-Z]/)) {
+    if (tag.match(/^[a-z]/)) {
+      return {
+        kind: 'BuiltinTag',
+        name: tag,
+        loc: exprLoc,
+      };
+    } else {
       const kind = getLoadKind(builder, exprPath);
       return lowerValueToTemporary(builder, {
         kind: kind,
         place: lowerIdentifier(builder, exprPath),
         loc: exprLoc,
       });
-    } else {
-      return {
-        kind: 'BuiltinTag',
-        name: tag,
-        loc: exprLoc,
-      };
     }
   } else if (exprPath.isJSXMemberExpression()) {
     return lowerJsxMemberExpression(builder, exprPath);

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/jsx-underscore-prefixed-tag-is-component.expect.md

@@ -0,0 +1,58 @@
+
+## Input
+
+```javascript
+import {Stringify} from 'shared-runtime';
+
+// Repro for #36601: an underscore-prefixed JSX tag (`<_Bar>`) is a component
+// reference, not a host/builtin element. Before the fix, `lowerJsxElementName`
+// used `/^[A-Z]/` to detect components, so `_Bar` (which is not A-Z) was lowered
+// as a BuiltinTag — emitting the literal string tag "_Bar" instead of loading
+// the `_Bar` binding. The compiled output below must reference the `_Bar`
+// identifier (component), not a string tag.
+function Foo({_Bar}: {_Bar: React.ComponentType<{children: React.ReactNode}>}) {
+  return <_Bar>ok</_Bar>;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Foo,
+  params: [{_Bar: Stringify}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+import { Stringify } from "shared-runtime";
+
+// Repro for #36601: an underscore-prefixed JSX tag (`<_Bar>`) is a component
+// reference, not a host/builtin element. Before the fix, `lowerJsxElementName`
+// used `/^[A-Z]/` to detect components, so `_Bar` (which is not A-Z) was lowered
+// as a BuiltinTag — emitting the literal string tag "_Bar" instead of loading
+// the `_Bar` binding. The compiled output below must reference the `_Bar`
+// identifier (component), not a string tag.
+function Foo(t0) {
+  const $ = _c(2);
+  const { _Bar } = t0;
+  let t1;
+  if ($[0] !== _Bar) {
+    t1 = <_Bar>ok</_Bar>;
+    $[0] = _Bar;
+    $[1] = t1;
+  } else {
+    t1 = $[1];
+  }
+  return t1;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Foo,
+  params: [{ _Bar: Stringify }],
+};
+
+```
+
+### Eval output
+(kind: ok) <div>{"children":"ok"}</div>

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/jsx-underscore-prefixed-tag-is-component.tsx

@@ -0,0 +1,16 @@
+import {Stringify} from 'shared-runtime';
+
+// Repro for #36601: an underscore-prefixed JSX tag (`<_Bar>`) is a component
+// reference, not a host/builtin element. Before the fix, `lowerJsxElementName`
+// used `/^[A-Z]/` to detect components, so `_Bar` (which is not A-Z) was lowered
+// as a BuiltinTag — emitting the literal string tag "_Bar" instead of loading
+// the `_Bar` binding. The compiled output below must reference the `_Bar`
+// identifier (component), not a string tag.
+function Foo({_Bar}: {_Bar: React.ComponentType<{children: React.ReactNode}>}) {
+  return <_Bar>ok</_Bar>;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Foo,
+  params: [{_Bar: Stringify}],
+};