Summary

When a consumer passes extraSupportedExtensions together with extraExtensionsToRemove (e.g. glint registers .gts/.gjs and strips them so foo.gts emits foo.d.ts), tsc -b / build mode can abort during declaration emit with:

Error: Debug Failure. Extension .gts is unsupported:: FileName:: .../foo.gts

Cause

transformTscContent already teaches tsc about the extra extensions in several places — supportedTSExtensions / supportedJSExtensions / allSupportedExtensions, changeExtension, and extensionsToRemove — but not tryGetJSExtensionForFile, which maps a source file to the JS extension it emits (.ts → .js, .tsx → .jsx/.js, …).

In build mode, declaration emit synthesizes module specifiers for imported symbols. When the preferred ending is "js" (e.g. a sibling ./x.js import is preserved in the emitted .d.ts) and the target is one of these extra-extension files, tsc calls getJSExtensionForFile, which falls through to Debug.fail("Extension .xxx is unsupported") and aborts the whole build.

--noEmit checking does not compute those specifiers, so this only surfaces under tsc -b.

Notes

• Only stripped extensions are affected: those in extraExtensionsToRemove behave like .ts → .js. Kept extensions (e.g. Vue's .vue, which appends to foo.vue.d.ts) take a different emit path and don't hit this.
• Downstream report + a temporary monkey-patch that works around it today: Fix tsc -b / ember-tsc --build crash on .gts declaration emit ("Extension .gts is unsupported") typed-ember/glint#1157

Fix

One addition to transformTscContent, keyed off extraExtensionsToRemove, mapping those extensions to .js in tryGetJSExtensionForFile (same wrapper style as the existing changeExtension patch). PR incoming.