I have not been doing any Arduino work for about a year. The last time I ran the Arduino IDE was in the previous version of MacOS. Now that I have updated to the latest MacOS Version Catalina (10.15.2) I discovered that the IDE will not compile because the avr-g++ compiler is 32 bit and will not execute on the now 64-bit required MacOS.
So I did the obvious thing. I went to the arduino.cc web site and downloaded the latest version of Arduino IDE thinking that this should solve the problem. I updated to version 1.8.11 of the IDE but the 32-bit problem persisted.
I finally decided that the IDE was updating (new Arduino IDE app) but that the avr toolchain was not being updated. To force this update, I deleted the entire avr toolchain from my ~/Library folder. Now, re-running the IDE is all it took: It discovered the toolchain missing and re-installed and installed with the correct 64 bit binaries (which were part of the new Arduino App all the time).
Perplexing -- why was the toolchain not updated with the install? Is it a requirement to delete the existing toolchain before doing a new install of the Arduino App?