I've tried a few things, today, and here are my findings: Two sketches that compiled under IDE 1 do not compile under 1 any more, neither under IDE 2. Both use Adafruit_GFX and Adafruit_SSD1306 libraries.
I've to admit that I did a stupid thing, not only I've installed IDE 2 but I've also installed IDE 1.8.15 Hourly Build 2021/05/19 12:35.
The error message is at linking time. All before looks OK and I just copied and pasted what follows:
Linking everything together...
/Users/fabriziobianchi/Library/Arduino15/packages/esp32/tools/xtensa-esp32-elf-gcc/1.22.0-97-gc752ad5-5.2.0/bin/xtensa-esp32-elf-gcc -nostdlib -L/Users/fabriziobianchi/Library/Arduino15/packages/esp32/hardware/esp32/1.0.6/tools/sdk/lib -L/Users/fabriziobianchi/Library/Arduino15/packages/esp32/hardware/esp32/1.0.6/tools/sdk/ld -T esp32_out.ld -T esp32.project.ld -T esp32.rom.ld -T esp32.peripherals.ld -T esp32.rom.libgcc.ld -T esp32.rom.spiram_incompatible_fns.ld -u esp_app_desc -u ld_include_panic_highint_hdl -u call_user_start_cpu0 -Wl,--gc-sections -Wl,-static -Wl,--undefined=uxTopUsedPriority -u __cxa_guard_dummy -u __cxx_fatal_exception -Wl,--start-group /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/sketch/OLED_featherwing.ino.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/SPI/SPI.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Wire/Wire.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/glcdfont.c.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GFX.cpp.o "/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o" /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_SPITFT.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_SSD1306/Adafruit_SSD1306.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_BusIO/Adafruit_BusIO_Register.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_BusIO/Adafruit_I2CDevice.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_BusIO/Adafruit_SPIDevice.cpp.o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-core-cache/core_6a12163587d12953fe494fe5f29ea1de.a -lgcc -lesp_websocket_client -lwpa2 -ldetection -lesp_https_server -lwps -lhal -lconsole -lpe -lsoc -lsdmmc -lpthread -llog -lesp_http_client -ljson -lmesh -lesp32-camera -lnet80211 -lwpa_supplicant -lc -lmqtt -lcxx -lesp_https_ota -lulp -lefuse -lpp -lmdns -lbt -lwpa -lspiffs -lheap -limage_util -lunity -lrtc -lmbedtls -lface_recognition -lnghttp -ljsmn -lopenssl -lcore -lfatfs -lm -lprotocomm -lsmartconfig -lxtensa-debug-module -ldl -lesp_event -lesp-tls -lfd -lespcoredump -lesp_http_server -lfr -lsmartconfig_ack -lwear_levelling -ltcp_transport -llwip -lphy -lvfs -lcoap -lesp32 -llibsodium -lbootloader_support -ldriver -lcoexist -lasio -lod -lmicro-ecc -lesp_ringbuf -ldetection_cat_face -lapp_update -lespnow -lface_detection -lapp_trace -lnewlib -lbtdm_app -lwifi_provisioning -lfreertos -lfreemodbus -lethernet -lnvs_flash -lspi_flash -lc_nano -lexpat -lfb_gfx -lprotobuf-c -lesp_adc_cal -ltcpip_adapter -lstdc++ -lc-psram-workaround -lm-psram-workaround -Wl,--end-group -Wl,-EL -o /var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/OLED_featherwing.ino.elf
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::drawPixel(short, short, unsigned short)':
/Users/fabriziobianchi/Documents/Arduino/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp:284: multiple definition of `Adafruit_GrayOLED::drawPixel(short, short, unsigned short)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:/Users/fabriziobianchi/Documents/Arduino/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp:284: first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, TwoWire*, signed char, unsigned int, unsigned int)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP7TwoWireajj+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, TwoWire*, signed char, unsigned int, unsigned int)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP7TwoWireajj+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, TwoWire*, signed char, unsigned int, unsigned int)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP7TwoWireajj+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, TwoWire*, signed char, unsigned int, unsigned int)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP7TwoWireajj+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, signed char, signed char, signed char, signed char, signed char)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2Ehttaaaaa+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, signed char, signed char, signed char, signed char, signed char)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2Ehttaaaaa+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, signed char, signed char, signed char, signed char, signed char)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2Ehttaaaaa+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, signed char, signed char, signed char, signed char, signed char)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2Ehttaaaaa+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, SPIClass*, signed char, signed char, signed char, unsigned int)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP8SPIClassaaaj+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, SPIClass*, signed char, signed char, signed char, unsigned int)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP8SPIClassaaaj+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, SPIClass*, signed char, signed char, signed char, unsigned int)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP8SPIClassaaaj+0x0): multiple definition of `Adafruit_GrayOLED::Adafruit_GrayOLED(unsigned char, unsigned short, unsigned short, SPIClass*, signed char, signed char, signed char, unsigned int)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDC2EhttP8SPIClassaaaj+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::~Adafruit_GrayOLED()':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDD2Ev+0x0): multiple definition of `Adafruit_GrayOLED::~Adafruit_GrayOLED()'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDD2Ev+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::~Adafruit_GrayOLED()':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDD2Ev+0x0): multiple definition of `Adafruit_GrayOLED::~Adafruit_GrayOLED()'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDD2Ev+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::~Adafruit_GrayOLED()':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLEDD0Ev+0x0): multiple definition of `Adafruit_GrayOLED::~Adafruit_GrayOLED()'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLEDD0Ev+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::oled_command(unsigned char)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED12oled_commandEh+0x0): multiple definition of `Adafruit_GrayOLED::oled_command(unsigned char)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED12oled_commandEh+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::invertDisplay(bool)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED13invertDisplayEb+0x0): multiple definition of `Adafruit_GrayOLED::invertDisplay(bool)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED13invertDisplayEb+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::oled_commandList(unsigned char const*, unsigned char)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED16oled_commandListEPKhh+0x0): multiple definition of `Adafruit_GrayOLED::oled_commandList(unsigned char const*, unsigned char)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED16oled_commandListEPKhh+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::clearDisplay()':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED12clearDisplayEv+0x0): multiple definition of `Adafruit_GrayOLED::clearDisplay()'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED12clearDisplayEv+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::_init(unsigned char, bool)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED5_initEhb+0x0): multiple definition of `Adafruit_GrayOLED::_init(unsigned char, bool)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED5_initEhb+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::getPixel(short, short)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED8getPixelEss+0x0): multiple definition of `Adafruit_GrayOLED::getPixel(short, short)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED8getPixelEss+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::getBuffer()':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED9getBufferEv+0x0): multiple definition of `Adafruit_GrayOLED::getBuffer()'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED9getBufferEv+0x0): first defined here
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED.cpp.o: In function `Adafruit_GrayOLED::setContrast(unsigned char)':
Adafruit_GrayOLED.cpp:(.text._ZN17Adafruit_GrayOLED11setContrastEh+0x0): multiple definition of `Adafruit_GrayOLED::setContrast(unsigned char)'
/var/folders/jm/8ykp7mhd767fgdf5t617f21c0000gn/T/arduino-sketch-A4714554AD4B009504077A1887858FB4/libraries/Adafruit_GFX_Library/Adafruit_GrayOLED 2.cpp.o:Adafruit_GrayOLED 2.cpp:(.text._ZN17Adafruit_GrayOLED11setContrastEh+0x0): first defined here
collect2: error: ld returned 1 exit status
Using library SPI at version 1.0 in folder: /Users/fabriziobianchi/Library/Arduino15/packages/esp32/hardware/esp32/1.0.6/libraries/SPI
Using library Wire at version 1.0.1 in folder: /Users/fabriziobianchi/Library/Arduino15/packages/esp32/hardware/esp32/1.0.6/libraries/Wire
Using library Adafruit_GFX_Library at version 1.10.9 in folder: /Users/fabriziobianchi/Documents/Arduino/libraries/Adafruit_GFX_Library
Using library Adafruit_SSD1306 at version 2.4.5 in folder: /Users/fabriziobianchi/Documents/Arduino/libraries/Adafruit_SSD1306
Using library Adafruit_BusIO at version 1.7.3 in folder: /Users/fabriziobianchi/Documents/Arduino/libraries/Adafruit_BusIO
Compilation error: Error: 2 UNKNOWN: exit status 1
One final comment. I didn't touch the above libraries at all