what have you tested this with?

I'm not sure what kind of testing you are looking for, but the following compiles and prints "Hello PROGMEM" on the ESP8266. It fails to compile on the ESP32:

#include <pgmspace.h>

// Cannot use F() macro outside of a function, use PROGMEM directly.
static const char HELLO_PROGMEM[] PROGMEM = "Hello PROGMEM";

void printFlashString(const __FlashStringHelper* s) {
  Serial.println(s);
}

void setup() {
  delay(1000);
  Serial.begin(115200);
  while(!Serial); // for Arduino Leonardo/Micro

  printFlashString(FPSTR(HELLO_PROGMEM));
}

void loop() {
}

The error is:

.../FlashMemory.ino: In function 'void setup()':
FlashMemory:21:35: error: cannot convert 'const char*' to 'const __FlashStringHelper*' for argument '1' to 'void printString(const __FlashStringHelper*)'
   printString(FPSTR(HELLO_PROGMEM));
                                   ^
exit status 1

If I apply this PR to my ESP32 core on my Arduino IDE 1.8.9 .../packages/esp32/hardware/esp32/1.0.2/cores/esp32/{WString.h,prgmspace.h}, the same program compiles, and prints the correct output on the ESP32.

OK it's not a big deal :) let's merge and see if we get any issues before we cut 1.0.3 final