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‘R CMD check’ etc now warn when a package exports non-existing S4 classes or methods, also in case of no "methods" presence; reported by Alex Bertram; reproducible example and patch by Sebastian Meyer in PR#16662.
If an external BLAS is specified by ‘--with-blas=foo’ or _via_ environment variable ‘BLAS_LIBS’ is not found, this is now a configuration error. The previous behaviour was not clear from the documentation: it was to continue the search as if ‘--with-blas=yes’ was specified.