--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/gcc.dg/builtin-object-size-21.c	Mon May 25 07:50:57 2020 +0900
@@ -0,0 +1,51 @@
+/* PR middle-end/92815 - spurious -Wstringop-overflow writing into
+   a flexible array of an extern struct
+   { dg-do compile }
+   { dg-options "-Wall -fdump-tree-optimized" } */
+
+#define PTRDIFF_MAX __PTRDIFF_MAX__
+
+typedef __SIZE_TYPE__ size_t;
+
+#define bos0(expr) __builtin_object_size (expr, 0)
+#define bos1(expr) __builtin_object_size (expr, 1)
+#define bos2(expr) __builtin_object_size (expr, 2)
+#define bos3(expr) __builtin_object_size (expr, 3)
+
+void fail (const char*, ...);
+
+#define A(x, n01, n23)							\
+  ((bos0 (&x) == n01 ? (void)0 : fail (#x, __LINE__, bos0 (&x), n01)),	\
+   (bos1 (&x) == n01 ? (void)0 : fail (#x, __LINE__, bos1 (&x), n01)),	\
+   (bos2 (&x) == n23 ? (void)0 : fail (#x, __LINE__, bos2 (&x), n23)),	\
+   (bos3 (&x) == n23 ? (void)0 : fail (#x, __LINE__, bos3 (&x), n23)))
+
+struct Ax_m3 { char a[PTRDIFF_MAX - 3], ax[]; };
+
+struct Ax_m3 xm3_0 = { { 0 } };
+struct Ax_m3 xm3_1 = { { 0 }, { 1 } };
+struct Ax_m3 xm3_2 = { { 0 }, { 1, 2 } };
+struct Ax_m3 xm3_3 = { { 0 }, { 1, 2, 3 } };
+struct Ax_m3 xm3_4 = { { 0 }, { 1, 2, 3, 3 } };   // { dg-error "too large" }
+
+void test_axm3 (void)
+{
+  A (xm3_0, sizeof xm3_0, sizeof xm3_0);
+  A (xm3_1, sizeof xm3_1 + 1, sizeof xm3_1 + 1);
+  A (xm3_2, sizeof xm3_2 + 2, sizeof xm3_2 + 2);
+  A (xm3_3, (size_t)-1, 0);   // expect failure
+  A (xm3_4, (size_t)-1, 0);   // expect failure
+}
+
+
+struct Ax_mx { char a[PTRDIFF_MAX], ax[]; };
+struct Ax_mx xmx_0 = { { 0 } };
+struct Ax_mx xmx_1 = { { 0 }, { 1 } };            // { dg-error "too large" }
+extern struct Ax_mx xmx_x;
+
+void test_axmx (void)
+{
+  A (xmx_0, (size_t)-1, 0);   // expect failure
+  A (xmx_1, (size_t)-1, 0);   // expect failure
+  A (xmx_x, (size_t)-1, 0);   // expect failure
+}