UNITY_OUTPUT_CHAR can emit a redundant declaration, we scope the extern declartion to the unity.c

src/unity.c

@@ -7,6 +7,11 @@
 #include "unity.h"
 #include <stddef.h>
 
+#ifndef UNITY_OUTPUT_CHAR_USE_PUTC
+//If defined as something else, make sure we declare it here so it's ready for use
+extern int UNITY_OUTPUT_CHAR(int);
+#endif
+
 #define UNITY_FAIL_AND_BAIL   { Unity.CurrentTestFailed  = 1; longjmp(Unity.AbortFrame, 1); }
 #define UNITY_IGNORE_AND_BAIL { Unity.CurrentTestIgnored = 1; longjmp(Unity.AbortFrame, 1); }
 /// return prematurely if we are already in failure or ignore state

src/unity_internals.h

@@ -291,9 +291,12 @@ typedef UNITY_DOUBLE_TYPE _UD;
 //Default to using putchar, which is defined in stdio.h
 #include <stdio.h>
 #define UNITY_OUTPUT_CHAR(a) putchar(a)
-#else
-//If defined as something else, make sure we declare it here so it's ready for use
-extern int UNITY_OUTPUT_CHAR(int);
+// We need to flag the output char function uses putc in
+//  unity.c the extern function is not declared then.
+// Previously the extern was declared in this header but
+//  when redundant function declaration compiler flag is enabled
+//  it wont compile.
+#define UNITY_OUTPUT_CHAR_USE_PUTC
 #endif
 
 #ifndef UNITY_PRINT_EOL