diff --git a/clang/test/C/C99/n717.c b/clang/test/C/C99/n717.c
new file mode 100644
index 0000000000000..25010b4137065
--- /dev/null
+++ b/clang/test/C/C99/n717.c
@@ -0,0 +1,75 @@
+// RUN: %clang_cc1 -verify -std=c99 %s
+// RUN: %clang_cc1 -verify -std=c99 -fno-dollars-in-identifiers %s
+
+/* WG14 N717: Clang 17
+ * Extended identifiers
+ */
+
+// Used as a sink for UCNs.
+#define M(arg)
+
+// C99 6.4.3p1 specifies the grammar for UCNs. A \u must be followed by exactly
+// four hex digits, and \U must be followed by exactly eight.
+M(\u1)    // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\u12)   // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\u123)  // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\u1234) // Okay
+M(\u12345)// Okay, two tokens (UCN followed by 5)
+
+M(\U1)         // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U12)        // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U123)       // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U1234)      // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}} \
+                  expected-note {{did you mean to use '\u'?}}
+M(\U12345)     // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U123456)    // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U1234567)   // expected-warning {{incomplete universal character name; treating as '\' followed by identifier}}
+M(\U12345678)  // Okay
+M(\U123456789) // Okay-ish, two tokens (valid-per-spec-but-actually-invalid UCN followed by 9)
+
+// Now test the ones that should work. Note, these work in C17 and earlier but
+// are part of the basic character set in C23 and thus should be diagnosed in
+// that mode. They're valid in a character constant, but not valid in an
+// identifier, except for U+0024 which is allowed if -fdollars-in-identifiers
+// is enabled.
+// FIXME: These three should be handled the same way, and should be accepted
+// when dollar signs are allowed in identifiers, rather than rejected, see
+// GH87106.
+M(\u0024) // expected-error {{character '$' cannot be specified by a universal character name}}
+M(\U00000024) // expected-error {{character '$' cannot be specified by a universal character name}}
+M($)
+
+// These should always be rejected because they're not valid identifier
+// characters.
+// FIXME: the diagnostic could be improved to make it clear this is an issue
+// with forming an identifier rather than a UCN.
+M(\u0040) // expected-error {{character '@' cannot be specified by a universal character name}}
+M(\u0060) // expected-error {{character '`' cannot be specified by a universal character name}}
+M(\U00000040) // expected-error {{character '@' cannot be specified by a universal character name}}
+M(\U00000060) // expected-error {{character '`' cannot be specified by a universal character name}}
+
+// UCNs outside of identifiers are handled in Phase 5 of translation, so we
+// cannot use the macro expansion to test their behavior.
+
+// This is outside of the range of values specified by ISO 10646.
+const char *c1 = "\U00110000"; // expected-error {{invalid universal character}}
+// This does not fall outside of the range
+const char *c2 = "\U0010FFFF";
+
+// These should always be accepted because they're a valid in a character
+// constant.
+int c3 = '\u0024';
+int c4 = '\u0040';
+int c5 = '\u0060';
+
+int c6 = '\U00000024';
+int c7 = '\U00000040';
+int c8 = '\U00000060';
+
+// Valid lone surrogates.
+M(\uD799)
+const char *c9 = "\U0000E000";
+
+// Invalid lone surrogates, which are excluded explicitly by 6.4.3p2.
+M(\uD800) // expected-error {{invalid universal character}}
+const char *c10  = "\U0000DFFF"; // expected-error {{invalid universal character}}
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index bc27b20ce6487..7ee1d2b507e88 100644
--- a/clang/www/c_status.html
+++ b/clang/www/c_status.html
@@ -203,7 +203,7 @@ <h2 id="c99">C99 implementation status</h2>
     <tr>
       <td>extended identifiers</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n717.htm">N717</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Clang 17</td>
     </tr>
     <tr>
       <td>hexadecimal floating-point constants</td>