Original patch by q66, adjusted for version 2.0.0.
Fix build on systems with unsigned chars by default.

--- a/src/libs/core/music/tkeysignature.cpp	2021-06-05 13:17:35.000000000 +0200
+++ b/src/libs/core/music/tkeysignature.cpp	2021-08-14 18:58:03.380770429 +0200
@@ -25,7 +25,7 @@
 
 
 /*static*/
-const char TkeySignature::scalesDefArr[15][7] = {
+const signed char TkeySignature::scalesDefArr[15][7] = {
     {-1,-1,-1,-1,-1,-1,-1},       // Cb/ab
     {-1,-1,-1, 0,-1,-1,-1},       // Gb/eb
     { 0,-1,-1, 0,-1,-1,-1},       // Db/bb
@@ -43,8 +43,8 @@
     { 1, 1, 1, 1, 1, 1, 1},       // C#/a#
 };
 
-const char TkeySignature::majorKeys[15] = { 0, 4, 1, 5, 2, 6, 3, 0, 4, 1, 5, 2, 6, 3, 0 };
-const char TkeySignature::minorKeys[15] = { 5, 2, 6, 3, 0, 4, 1, 5, 2, 6, 3, 0, 4, 1, 5 };
+const signed char TkeySignature::majorKeys[15] = { 0, 4, 1, 5, 2, 6, 3, 0, 4, 1, 5, 2, 6, 3, 0 };
+const signed char TkeySignature::minorKeys[15] = { 5, 2, 6, 3, 0, 4, 1, 5, 2, 6, 3, 0, 4, 1, 5 };
 
 QString TkeySignature::majorNames[15] = { QString(), QString(), QString(), QString(), QString(),
                                           QString(), QString(), QString(), QString(), QString(),
--- a/src/libs/core/music/tkeysignature.h	2021-06-05 13:17:35.000000000 +0200
+++ b/src/libs/core/music/tkeysignature.h	2021-08-14 19:01:28.596571380 +0200
@@ -49,7 +49,7 @@
        * @p scalesDefArr[7] is C-major/a-minor and has 7 times 0 (no accids).
        * The order os from 7b [0] to 7# [15]
        */
-  static const char scalesDefArr[15][7];
+  static const signed char scalesDefArr[15][7];
 
       /**
        * Those smart arrays keep which note in @p scalesDefArr is 1-st in key scale.
@@ -57,7 +57,7 @@
        * like @p scalesDefArr[8][4] point on accid of a G note.
        * This is faster way then calculate it and takes less memory.
        */
-  static const char majorKeys[15], minorKeys[15];
+  static const signed char majorKeys[15], minorKeys[15];
 
       /**
        * This method fulfills majorNames and minorNames arrays
--- a/src/libs/core/music/tnote.h	2021-06-05 13:17:35.000000000 +0200
+++ b/src/libs/core/music/tnote.h	2021-08-14 18:55:34.210784018 +0200
@@ -68,7 +68,7 @@
          * It can be: @li e_None = 3 @li e_Sharp = 1 @li e_DoubleSharp=2
          * @li e_Flat= -1 @li e_DoubleFlat= -2 @li e_Natural=0
          */
-  enum Ealter : char {
+  enum Ealter : signed char {
     e_Natural = 0, e_Sharp = 1, e_DoubleSharp = 2, e_Flat = -1, e_DoubleFlat = -2, e_None = 3
   };