build failure - odd compiler selection

[Christopher Jones]

Hi All,

Just trying to understand the build failure in

https://build.macports.org/builders/ports-10.8_x86_64_legacy-builder/builds/99778/steps/install-port/logs/stdio <https://build.macports.org/builders/ports-10.8_x86_64_legacy-builder/builds/99778/steps/install-port/logs/stdio>

port in question 

https://github.com/macports/macports-ports/blob/master/devel/folly/Portfile <https://github.com/macports/macports-ports/blob/master/devel/folly/Portfile>

has the compiler selection

compiler.blacklist-append {clang < 920} macports-clang-3.* macports-clang-4.0 \
    macports-clang-5.0
compiler.fallback-append macports-clang-8.0 macports-clang-7.0 macports-clang-6.0

build generates

--->  Configuring folly
Warning: All compilers are either blacklisted or unavailable; defaulting to first fallback option
DEBUG: Preferred compilers: clang macports-clang-3.4 macports-llvm-gcc-4.2 apple-gcc-4.2 macports-clang-8.0 macports-clang-7.0 macports-clang-6.0
DEBUG: Using compiler 'Xcode Clang’
<snip>
-- The CXX compiler identification is AppleClang 5.1.0.5030040
-- The C compiler identification is AppleClang 5.1.0.5030040
why is Xcode clang being selected, when it is already blacklisted ? Do the blacklist settings not get applied to the fallback list ?

cheers Chris
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