--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/gcc.dg/format/proto.c	Mon May 25 07:50:57 2020 +0900
@@ -0,0 +1,89 @@
+/* PR c/93812 - ICE on redeclaration of an attribute format function without
+   protoype
+   It's not clear that attribute format should be accepted on functions
+   without a prototype.  If it's decided that it shouldn't be the tests
+   here will need to be adjusted.
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+#define FMT(n1, n2) __attribute__((__format__(__printf__, n1, n2)))
+
+// Exercise function declarations.
+FMT (1, 2) void print1 ();
+
+FMT (2, 3) void print2 ();
+           void print2 ();
+
+FMT (3, 4) void print3 ();
+FMT (3, 4) void print3 ();
+
+FMT (1, 2) void print4 ();
+           void print4 (void);              // { dg-warning "'format' attribute cannot be applied to a function that does not take variable arguments" }
+
+           void print5 ();
+FMT (1, 2) void print5 (void);              // { dg-warning "\\\[-Wattributes" }
+
+FMT (1, 2) void print6 ();
+          void print6 (const char*, ...);   // { dg-error "conflicting types" }
+
+           void print7 (const char*, ...);
+FMT (1, 2) void print7 ();                  // { dg-error "conflicting types" }
+
+
+// Exercise function calls.
+void test_print (void)
+{
+  print1 ("%i %s", 123, "");
+  print1 ("%s %i", 123, 123);               // { dg-warning "\\\[-Wformat" }
+
+  print2 (0, "%s %i", "", 123);
+  print2 (1, "%i %s", "", 123);             // { dg-warning "\\\[-Wformat" }
+
+  print3 (0, 1, "%s %i", "", 123);
+  print3 (1, 2, "%i %s", "", 123);          // { dg-warning "\\\[-Wformat" }
+
+  // Just verify there's no ICE.
+  print4 ();
+  print5 ();
+  print6 ("%i %s", 123, "");
+}
+
+
+// Exercise declarations of pointers to functions.
+FMT (1, 2) void (*pfprint1)();
+
+FMT (2, 3) void (*pfprint2)();
+           void (*pfprint2)();
+
+FMT (3, 4) void (*pfprint3)();
+FMT (3, 4) void (*pfprint3)();
+
+FMT (1, 2) void (*pfprint4)();
+           void (*pfprint4)(void);              // { dg-warning "'format' attribute cannot be applied to a function that does not take variable arguments" }
+
+           void (*pfprint5)();
+FMT (1, 2) void (*pfprint5)(void);              // { dg-warning "\\\[-Wattributes" }
+
+FMT (1, 2) void (*pfprint6)();
+           void (*pfprint6)(const char*, ...);   // { dg-error "conflicting types" }
+
+           void (*pfprint7)(const char*, ...);
+FMT (1, 2) void (*pfprint7)();                  // { dg-error "conflicting types" }
+
+// Exercise calls via function pointers.
+void test_pfprint (void)
+{
+  pfprint1 ("%i %s", 123, "");
+  pfprint1 ("%s %i", 123, 123);             // { dg-warning "\\\[-Wformat" }
+
+  pfprint2 (0, "%s %i", "", 123);
+  pfprint2 (1, "%i %s", "", 123);           // { dg-warning "\\\[-Wformat" }
+
+  pfprint3 (0, 1, "%s %i", "", 123);
+  pfprint3 (1, 2, "%i %s", "", 123);        // { dg-warning "\\\[-Wformat" }
+
+  // Just verify there's no ICE.
+  pfprint4 ();
+  pfprint5 ();
+  pfprint6 ("%i %s", 123, "");
+}