https://src.fedoraproject.org/rpms/mscgen/blob/rawhide/f/mscgen-0.20-language.patch

Description: Use %parse-param to ensure that yyparse is generated
 with the proper prototype.

# Fixes language.c:464:5: error: conflicting types for 'yyparse'
# https://code.google.com/p/mscgen/issues/detail?id=83
--- a/src/language.y
+++ b/src/language.y
@@ -48,7 +48,7 @@ int yylex_destroy(void);
  *  Error handling function.  The TOK_XXX names are substituted for more
  *  understandable values that make more sense to the user.
  */
-void yyerror(const char *str)
+void yyerror(void *unused, const char *str)
 {
     static const char *tokNames[] = { "TOK_OCBRACKET",          "TOK_CCBRACKET",
                                       "TOK_OSBRACKET",          "TOK_CSBRACKET",
@@ -224,6 +224,8 @@ Msc MscParse(FILE *in)
 
 %}
 
+%parse-param {void *YYPARSE_PARAM}
+
 %token TOK_STRING TOK_QSTRING TOK_EQUAL TOK_COMMA TOK_SEMICOLON TOK_OCBRACKET TOK_CCBRACKET
        TOK_OSBRACKET TOK_CSBRACKET TOK_MSC
        TOK_ATTR_LABEL TOK_ATTR_URL TOK_ATTR_ID TOK_ATTR_IDURL