Add test coverage for more C DRs

This completes the initial pass over all of the C DRs.

clang/test/C/drs/dr4xx.c

@@ -322,3 +322,63 @@ struct dr492_t {
   };
   int m13;
 } dr492;
+
+/* WG14 DR496: yes
+ * offsetof questions
+ */
+void dr496(void) {
+  struct A { int n, a [2]; };
+  struct B { struct A a; };
+  struct C { struct A a[1]; };
+
+  /* The standard does not require either of these examples to work, but we
+   * support them just the same. The first one is invalid because it's
+   * referencing a member of a different struct, and the second one is invalid
+   * because it references an array of another struct. Clang calculates the
+   * correct offset to each of those fields.
+   */
+  _Static_assert(__builtin_offsetof(struct B, a.n) == 0, "");
+  /* First int below is for 'n' and the second int is for 'a[0]'; this presumes
+   * there is no padding involved.
+   */
+  _Static_assert(__builtin_offsetof(struct B, a.a[1]) == sizeof(int) + sizeof(int), "");
+
+  /* However, we do not support using the -> operator to access a member, even
+   * if that would be a valid expression. FIXME: GCC accepts this, perhaps we
+   * should as well.
+   */
+  (void)__builtin_offsetof(struct C, a->n); /* expected-error {{expected ')'}} \
+                                               expected-note {{to match this '('}}
+                                             */
+
+  /* The DR asked a question about whether defining a new type within offsetof
+   * is allowed. C2x N2350 made this explicitly undefined behavior, but Clang
+   * has always supported defining a type in this location, and GCC also
+   * supports it.
+   */
+   (void)__builtin_offsetof(struct S { int a; }, a);
+}
+
+/* WG14 DR499: yes
+ * Anonymous structure in union behavior
+ */
+void dr499(void) {
+  union U {
+    struct {
+      char B1;
+      char B2;
+      char B3;
+      char B4;
+    };
+    int word;
+  } u;
+
+  /* Validate that B1, B2, B3, and B4 do not have overlapping storage, only the
+   * anonymous structure and 'word' overlap.
+   */
+  _Static_assert(__builtin_offsetof(union U, B1) == 0, "");
+  _Static_assert(__builtin_offsetof(union U, B2) == 1, "");
+  _Static_assert(__builtin_offsetof(union U, B3) == 2, "");
+  _Static_assert(__builtin_offsetof(union U, B4) == 3, "");
+  _Static_assert(__builtin_offsetof(union U, word) == 0, "");
+}

clang/test/C/drs/dr5xx.c

@@ -0,0 +1,38 @@
+/* RUN: %clang_cc1 -std=c89 -verify=expected,c89only -pedantic -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c99 -verify=expected -pedantic -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c11 -verify=expected -pedantic %s
+   RUN: %clang_cc1 -std=c17 -verify=expected -pedantic %s
+   RUN: %clang_cc1 -std=c2x -verify=expected -pedantic %s
+ */
+
+/* WG14 DR502:
+ * Flexible array member in an anonymous struct
+ */
+void dr502(void) {
+  /* This is EXAMPLE 3 from 6.7.2.1 and is intended to show that a flexible
+   * array member can be used when the only other members of the class are from
+   * an anonymous structure member.
+   */
+  struct s {
+    struct { int i; };
+    int a[]; /* c89only-warning {{flexible array members are a C99 feature}} */
+  };
+
+  /* This is a slightly modified example that looks to see whether the
+   * anonymous structure itself can provide a flexible array member for the
+   * containing class.
+   *
+   * The committee does not think this is valid because it would mean the
+   * anonymous structure would have size 0. Additionally, the anonymous
+   * structure has no additional members and so the flexible array member is
+   * not valid within the anonymous structure.
+   */
+  struct t {
+    int i;
+    struct { int a[]; }; /* expected-error {{flexible array member 'a' not allowed in otherwise empty struct}}
+                            c89only-warning {{flexible array members are a C99 feature}}
+                            expected-warning {{'' may not be nested in a struct due to flexible array member}}
+                          */
+  };
+}
+

clang/www/c_dr_status.html

@@ -2632,7 +2632,7 @@ conformance.</p>
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2148.htm#dr_496">496</a></td>
     <td>NAD</td>
     <td>offsetof questions</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="497">
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2148.htm#dr_497">497</a></td>
@@ -2650,7 +2650,7 @@ conformance.</p>
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_499">499</a></td>
     <td>C17</td>
     <td>Anonymous structure in union behavior</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="500">
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_500">500</a></td>
@@ -2668,13 +2668,13 @@ conformance.</p>
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_502">502</a></td>
     <td>NAD</td>
     <td>Flexible array member in an anonymous struct</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="503">
     <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_503">503</a></td>
     <td>NAD</td>
     <td>Hexadecimal floating-point and strtod</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="na" align="center">N/A</td>
   </tr>