I've been able to reproduce this with macOS 10.15 Catalina and the Command Line tools for Xcode 12.4 (the last version that ran on Catalina).
The macOS c++ (clang) compiler seems to default to a C++ dialect of gnu++98. Using this dialect, the generated interpreter compiles, but throws the assertion failure for the assignment (a b c) ← 5. I have been able to work around this. Choosing a more recent C++ dialect (which matches the default dialect in the GCC compilers on Linux) seems to address the issue. If you run ./configure -CXXFLAGS="-std=gnu++14" then build the interpreter, the resulting interpreter runs the assignment as expected, and the assertion is not thrown. Is there a minimum dialect of C++ that should be used to compile GNU APL? - Paul Rockwell