[Clang] Don't assume unexpanded PackExpansions' size when expanding packs (#120380) CheckParameterPacksForExpansion() previously assumed that template arguments don't include PackExpansion types when attempting another pack expansion (i.e. when NumExpansions is present). However, this assumption doesn't hold for type aliases, whose substitution might involve unexpanded packs. This can lead to incorrect diagnostics during substitution because the pack size is not yet determined. To address this, this patch calculates the minimum pack size (ignoring unexpanded PackExpansionTypes) and compares it to the previously expanded size. If the minimum pack size is smaller, then there's still a chance for future substitution to expand it to a correct size, so we don't diagnose it too eagerly. Fixes #61415 Fixes #32252 Fixes #17042
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