C PATCH to fix ICE with -Wformat and zero-length array (PR c/80919)

[Marek Polacek]

In the C FE, zero-length arrays require structural equality, so we can't
compare their canonical types, 'cause they're NULL.  But matching_type_p didn't
know that so we were crashing.  With this patch the ICE is gone and the warning
messages are the same as with e.g. "int a[1]".

Bootstrapped/regtested on x86_64-linux, ok for trunk/7?

2017-06-02  Marek Polacek  <pola...@redhat.com>

        PR c/80919
        * c-format.c (matching_type_p): Return false if any of the types
        requires structural equality.

        * gcc.dg/format/pr80919.c: New test.

diff --git gcc/c-family/c-format.c gcc/c-family/c-format.c
index faef267..732339b 100644
--- gcc/c-family/c-format.c
+++ gcc/c-family/c-format.c
@@ -3278,6 +3278,12 @@ matching_type_p (tree spec_type, tree arg_type)
   gcc_assert (spec_type);
   gcc_assert (arg_type);
 
+  /* If any of the types requires structural equality, we can't compare
+     their canonical types.  */
+  if (TYPE_STRUCTURAL_EQUALITY_P (spec_type)
+      || TYPE_STRUCTURAL_EQUALITY_P (arg_type))
+    return false;
+
   spec_type = TYPE_CANONICAL (spec_type);
   arg_type = TYPE_CANONICAL (arg_type);
 
diff --git gcc/testsuite/gcc.dg/format/pr80919.c 
gcc/testsuite/gcc.dg/format/pr80919.c
index e69de29..510c2e4 100644
--- gcc/testsuite/gcc.dg/format/pr80919.c
+++ gcc/testsuite/gcc.dg/format/pr80919.c
@@ -0,0 +1,16 @@
+/* PR c/80919 */
+/* { dg-do compile } */
+/* { dg-options "-Wall" } */
+
+void
+fn (void)
+{
+  int a[0];
+  __builtin_printf("%d\n", &a); /* { dg-warning "expects argument of type" } */
+  __builtin_printf("%i\n", &a); /* { dg-warning "expects argument of type" } */
+
+  __builtin_printf("%o\n", &a); /* { dg-warning "expects argument of type" } */
+  __builtin_printf("%u\n", &a); /* { dg-warning "expects argument of type" } */
+  __builtin_printf("%x\n", &a); /* { dg-warning "expects argument of type" } */
+  __builtin_printf("%X\n", &a); /* { dg-warning "expects argument of type" } */
+}

        Marek