Avoid infinite loop with duplicate anonymous union fields (PR c/86690).

If a struct contains an anonymous union and both have a field with the
same name, detect_field_duplicates_hash() will replace one of them
with NULL.  If compilation doesn't stop immediately, it may later call
lookup_field() on the union, which falsely assumes the union's
LANG_SPECIFIC array is sorted, and may loop indefinitely because of
this.

2018-08-03  Bogdan Harjoc  <harjoc@gmail.com>

	PR c/86690
gcc/c:
	* c-typeck.c (lookup_field): Do not use TYPE_LANG_SPECIFIC after
	errors.

gcc/testsuite:
	* gcc.dg/union-duplicate-field.c: New test.

From-SVN: r263294

gcc/c/ChangeLog

+2018-08-03  Bogdan Harjoc  <harjoc@gmail.com>
+
+	PR c/86690
+	* c-typeck.c (lookup_field): Do not use TYPE_LANG_SPECIFIC after
+	errors.
+
 2018-08-01  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/86650

gcc/c/c-typeck.c

@@ -2207,9 +2207,14 @@ lookup_field (tree type, tree component)
   /* If TYPE_LANG_SPECIFIC is set, then it is a sorted array of pointers
      to the field elements.  Use a binary search on this array to quickly
      find the element.  Otherwise, do a linear search.  TYPE_LANG_SPECIFIC
-     will always be set for structures which have many elements.  */
+     will always be set for structures which have many elements.
 
-  if (TYPE_LANG_SPECIFIC (type) && TYPE_LANG_SPECIFIC (type)->s)
+     Duplicate field checking replaces duplicates with NULL_TREE so
+     TYPE_LANG_SPECIFIC arrays are potentially no longer sorted.  In that
+     case just iterate using DECL_CHAIN.  */
+
+  if (TYPE_LANG_SPECIFIC (type) && TYPE_LANG_SPECIFIC (type)->s
+      && !seen_error ())
     {
       int bot, top, half;
       tree *field_array = &TYPE_LANG_SPECIFIC (type)->s->elts[0];

gcc/testsuite/ChangeLog

+2018-08-03  Bogdan Harjoc  <harjoc@gmail.com>
+
+	PR c/86690
+	* gcc.dg/union-duplicate-field.c: New test.
+
 2018-08-03  Jason Merrill  <jason@redhat.com>
 
 	PR c++/86706

gcc/testsuite/gcc.dg/union-duplicate-field.c

+/* { dg-do compile } */
+/* { dg-options "-std=c99" } */
+
+int a0;
+
+struct S
+{
+    int a1;
+    union {
+        int a0;
+        int a1; /* { dg-error "duplicate member" } */
+        int a2, a3, a4, a5, a6, a7, a8, a9;
+        int a10, a11, a12, a13, a14, a15;
+    };
+};
+
+int f()
+{
+    struct S s;
+    return s.a0;
+}