Changed operator bool to explicit rather than implicit conversion in Serial

Author: pratikpc

cores/arduino/HardwareSerial.h

@@ -132,7 +132,7 @@ class HardwareSerial : public Stream
     inline size_t write(unsigned int n) { return write((uint8_t)n); }
     inline size_t write(int n) { return write((uint8_t)n); }
     using Print::write; // pull in write(str) and write(buf, size) from Print
-    operator bool() { return true; }
+    explicit constexpr operator bool() { return true; }
 
     // Interrupt handlers - Not intended to be called externally
     inline void _rx_complete_irq(void);

libraries/SoftwareSerial/src/SoftwareSerial.h

@@ -103,7 +103,7 @@ class SoftwareSerial : public Stream
   virtual int read();
   virtual int available();
   virtual void flush();
-  operator bool() { return true; }
+  explicit constexpr operator bool() { return true; }
 
   using Print::write;