fixup! Audit and fix up format strings

src/terminal/terminalframebuffer.cc

@@ -545,7 +545,12 @@ std::string Renditions::sgr( void ) const
     } else if ( foreground_color > 37 ) { /* use 256-color set */
       snprintf( col, sizeof( col ), ";38;5;%d", foreground_color - 30 );
     } else { /* ANSI foreground color */
-      snprintf( col, sizeof( col ), ";%d", foreground_color );
+      // Unfortunately, some versions of GCC (notably including GCC 9.3) give
+      // -Wformat warnings when relying on [conv.prom] to promote
+      // foreground_color in calls to printf. Explicitly promote it to silence
+      // the warning.
+      int fg = foreground_color;
+      snprintf( col, sizeof( col ), ";%d", fg );
     }
     ret.append( col );
   }
@@ -559,7 +564,9 @@ std::string Renditions::sgr( void ) const
     } else if ( background_color > 47 ) { /* use 256-color set */
       snprintf( col, sizeof( col ), ";48;5;%d", background_color - 40 );
     } else { /* ANSI background color */
-      snprintf( col, sizeof( col ), ";%d", background_color );
+      // See comment above about explicit promotion; it applies here as well.
+      int bg = background_color;
+      snprintf( col, sizeof( col ), ";%d", bg );
     }
     ret.append( col );
   }