Due to 3rd party use of symbol optimistic_yield without #include <Arduino>, have to resort to preprocessor definition for inlining.

Author: dok-net

cores/esp8266/Arduino.h

@@ -212,16 +212,11 @@ extern "C" uint8 system_get_cpu_freq(void);
 
 void __optimistic_yield(uint32_t intvl_cycles);
 
-void inline optimistic_yield(uint32_t interval_us) __attribute__((always_inline));
-void inline optimistic_yield(uint32_t interval_us) {
-    __optimistic_yield(interval_us *
 #if defined(F_CPU)
-        clockCyclesPerMicrosecond()
+#define optimistic_yield(interval_us) (__optimistic_yield(interval_us * clockCyclesPerMicrosecond()))
 #else
-        getCpuFreqMHz()
+#define optimistic_yield(interval_us) (__optimistic_yield(interval_us * getCpuFreqMHz()))
 #endif
-    );
-}
 
 #define _PORT_GPIO16    1
 #define digitalPinToPort(pin)       (((pin)==16)?(_PORT_GPIO16):(0))

cores/esp8266/core_esp8266_main.cpp

@@ -133,6 +133,17 @@ extern "C" void __optimistic_yield(uint32_t intvl_cycles) {
     }
 }
 
+#undef optimistic_yield
+extern "C" void optimistic_yield(uint32_t interval_us) {
+    __optimistic_yield(interval_us *
+#if defined(F_CPU)
+        clockCyclesPerMicrosecond()
+#else
+        getCpuFreqMHz()
+#endif
+    );
+}
+
 // Replace ets_intr_(un)lock with nestable versions
 extern "C" void IRAM_ATTR ets_intr_lock() {
   if (ets_intr_lock_stack_ptr < ETS_INTR_LOCK_NEST_MAX)