@rollup/plugin-babel causes SOURCEMAP_ERROR while reporting THIS_IS_UNDEFINED from unmapped generated helper code

Start with the linked reproduction and run pnpm run build, comparing it with pnpm run build:bare. Trace how @rollup/plugin-babel 6.1.0 handles the sourcemap for src/index.js and generated helper code. Done means the plugin no longer adds SOURCEMAP_ERROR warnings while Rollup still reports THIS_IS_UNDEFINED.

Rollup Plugin Name

@rollup/plugin-babel

Plugin Version

6.1.0

Rollup Version

4.60.1

Operating System

Windows 11, but this does not appear to be Windows-specific.

Link to reproduction

https://github.com/shrinktofit/rollup-babel-sourcemap-bug

Expected Behavior

When Rollup reports THIS_IS_UNDEFINED for generated helper code that has no original source mapping, using @rollup/plugin-babel should not add extra SOURCEMAP_ERROR warnings.

Bare Rollup reports only THIS_IS_UNDEFINED for the same input.

Actual Behavior

With @rollup/plugin-babel in the pipeline, Rollup emits additional SOURCEMAP_ERROR warnings before the original THIS_IS_UNDEFINED warnings:

[onwarn] SOURCEMAP_ERROR src/index.js (1:17): Error when using sourcemap for reporting an error: Can't resolve original location of error.
[onwarn] THIS_IS_UNDEFINED src/index.js (1:17): The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten
[onwarn] SOURCEMAP_ERROR src/index.js (1:25): Error when using sourcemap for reporting an error: Can't resolve original location of error.
[onwarn] THIS_IS_UNDEFINED src/index.js (1:25): The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten

Steps to Reproduce

git clone https://github.com/shrinktofit/rollup-babel-sourcemap-bug.git
cd rollup-babel-sourcemap-bug
pnpm install
pnpm run build

For comparison, bare Rollup does not emit SOURCEMAP_ERROR:

pnpm run build:bare

Notes

The repro simulates TypeScript legacy decorator output:

var __decorate = (this && this.__decorate) || function () {};
export const value = 1;
//# sourceMappingURL=index.js.map

The sourcemap intentionally has no mapping for generated line 1, representing compiler-generated helper code. The mapping starts at generated line 2.

Bare Rollup reports THIS_IS_UNDEFINED without SOURCEMAP_ERROR. Once @rollup/plugin-babel is added, the input sourcemap enters the sourcemap chain and Rollup tries to map the generated helper location back to an original source location. Since helper code has no original mapping, that remapping fails and produces noisy SOURCEMAP_ERROR warnings.

This shape appears in real projects when bundling TypeScript output that contains inline legacy decorator helpers and sourcemaps.