Add a test for true positives related to PR104715.

Related to:
PR tree-optimization/104715 - false dangling pointer with strstr

gcc/testsuite/ChangeLog:
	PR tree-optimization/104715
	* gcc.dg/Wdangling-pointer-3.c: New test.

gcc/testsuite/gcc.dg/Wdangling-pointer-3.c

+/* PR tree-optimization/104715 - false dangling pointer with strstr
+   Vertify that using pointers that have become dangling after they were
+   passed to and returned from strstr is diagnosed.
+  { dg-do compile }
+  { dg-options "-Wall" } */
+
+extern char* strstr (const char*, const char*);
+
+void sink (const void*);
+
+void nowarn_strstr_static (const char *s)
+{
+  char *t1;
+
+  {
+    static const char a[] = "abc";
+    t1 = strstr (a, s);
+    sink (t1);
+  }
+
+  sink (t1);
+}
+
+
+void nowarn_strstr_lit (const char *s)
+{
+  char *t2;
+
+  {
+    t2 = strstr ("def", s);
+    sink (t2);
+  }
+
+  sink (t2);
+}
+
+
+void warn_strstr_comp_lit (const char *s)
+{
+  char *t3;
+
+  {
+    const char *a =
+      (char[]){ '1', '\0' };  // { dg-message "unnamed temporary defined here" }
+    t3 = strstr (a, s);
+    sink (t3);
+  }
+
+  sink (t3);            // { dg-warning "using dangling pointer 't3' to an unnamed temporary" }
+}
+
+
+void warn_strstr_arg (const char *s)
+{
+  char *t4;
+
+  {
+    char a[] = "1";     // { dg-message "'a' declared here" }
+    t4 = strstr (a, s);
+    sink (t4);
+  }
+
+  sink (t4);            // { dg-warning "using dangling pointer 't4' to 'a'" }
+}
+
+
+void warn_strstr_arg_plus_cst (const char *s)
+{
+  char *t5;
+
+  {
+    char a[] = "12";    // { dg-message "'a' declared here" }
+    t5 = strstr (a + 1, s);
+    sink (t5);
+  }
+
+  sink (t5);            // { dg-warning "using dangling pointer 't5' to 'a'" }
+}
+
+
+void warn_strstr_arg_plus_var (const char *s, int i)
+{
+  char *t6;
+
+  {
+    char a[] = "123";   // { dg-message "'a' declared here" }
+    t6 = strstr (a + i, s);
+    sink (t6++);
+  }
+
+  sink (t6);            // { dg-warning "using dangling pointer 't6' to 'a'" }
+}