This PR fixes an issue where `realizeConst` would generate auxiliary
declarations
(like `_sparseCasesOn`) using the original defining module's private
name prefix
rather than the realizing module's prefix. When two modules
independently realized
the same imported constant, they produced identically-named auxiliary
declarations,
causing "environment already contains" errors on diamond import.
The fix re-privatizes the constant name under the current module before
passing it
to `withDeclNameForAuxNaming`, ensuring each realizing module generates
distinctly
named auxiliary declarations.
Fixes#12825
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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