Changed operator bool to explicit rather than implicit conversion in Serial

cores/arduino/Client.h

@@ -37,7 +37,7 @@ class Client : public Stream {
   virtual void flush() = 0;
   virtual void stop() = 0;
   virtual uint8_t connected() = 0;
-  virtual operator bool() = 0;
+  virtual explicit operator bool() = 0;
 protected:
   uint8_t* rawIPAddress(IPAddress& addr) { return addr.raw_address(); };
 };

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);

cores/arduino/USBAPI.h

@@ -105,7 +105,7 @@ class Serial_ : public Stream
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t*, size_t);
 	using Print::write; // pull in write(str) and write(buf, size) from Print
-	operator bool();
+	explicit operator bool();
 
 	volatile uint8_t _rx_buffer_head;
 	volatile uint8_t _rx_buffer_tail;

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;