foo.c:2:22: error: unknown type name 'a2'
    2 | int Concat(char a1[],a2[])
      |                      ^~
foo.c:22:2: warning: return type defaults to 'int' [-Wimplicit-int]
   22 |  main()
      |  ^~~~
foo.c: In function 'main':
foo.c:27:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   27 |  gets(s1);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c:28:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   28 |  gets(s2);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c:29:2: warning: implicit declaration of function 'Concat' [-Wimplicit-function-declaration]
   29 |  Concat(s1,s2);
      |  ^~~~~~
foo.c:30:13: warning: format '%s' expects a matching 'char *' argument [-Wformat=]
   30 |  printf("%s%s",s1);
      |            ~^
      |             |
      |             char *
foo.c:27:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   27 |  gets(s1);
      |  ^~~~~~~~
foo.c:28:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   28 |  gets(s2);
      |  ^~~~~~~~

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