[C2y] Claim conformance and add test coverage for WG14 N3346 (#115516)

This converts some undefined behaviors during initialization to instead
be constraint violations. Clang has always implemented these as
constraints, so no compiler changes were needed.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -303,6 +303,11 @@ C2y Feature Support
   undefined. Clang has always accepted ``const`` and ``volatile`` qualified
   function types by ignoring the qualifiers.
 
+- Updated conformance for `N3346 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3346.pdf>`_
+  which changes some undefined behavior around initialization to instead be
+  constraint violations. This paper adopts Clang's existing practice, so there
+  were no changes to compiler behavior.
+
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 

clang/test/C/C2y/n3346.c

@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -ffreestanding %s
+// RUN: %clang_cc1 -verify=expected,ped -Wall -pedantic -ffreestanding %s
+
+/* WG14 N3346: Yes
+ * Slay Some Earthly Demons VIII
+ *
+ * Updates some undefined behavior during initialization to instead be a
+ * constraint violation.
+ */
+
+// The initializer for a scalar shall be a single expression, optionally
+// enclosed in braces, or it shall be an empty initializer.
+int i = 12, j = {12}, k = {}; // ped-warning {{use of an empty initializer is a C23 extension}}
+
+struct S {
+  int i;
+  float f;
+  int : 0;
+  char c;
+};
+
+void test1(void) {
+  // The initializer for an object that has structure or union type shall be
+  // either a single expression that has compatible type or a brace-enclosed
+  // list of initializers for the elements or named members.
+  struct S s1 = { 1, 1.2f, 'a' };
+  struct S s2 = s1;
+
+  // Despite being structurally identical to S, T is not compatible with S.
+  struct T { int i; float f; int : 0; char c; } t;
+  struct S s3 = t; // expected-error {{initializing 'struct S' with an expression of incompatible type 'struct T'}}
+}
+
+void test2(void) {
+  typedef __WCHAR_TYPE__ wchar_t;
+  typedef __CHAR16_TYPE__ char16_t;
+  typedef __CHAR32_TYPE__ char32_t;
+
+  // The initializer for an array shall be either a string literal, optionally
+  // enclosed in braces, or a brace-enclosed list of initializers for the
+  // elements. An array initialized by character string literal or UTF-8 string
+  // literal shall have a character type as element type. An array initialized
+  // with a wide string literal shall have element type compatible with a
+  // qualified or unqualified wchar_t, char16_t, or char32_t, and the string
+  // literal shall have the corresponding encoding prefix (L, u, or U,
+  // respectively).
+  char str1[] = "string literal";
+  char str2[] = { "string literal" };
+  char str3[] = u8"string literal";
+  char str4[] = { u8"string literal" };
+
+  int str5[] = "this doesn't work";          // expected-error {{array initializer must be an initializer list}}
+  int str6[] = { "this also doesn't work" }; // expected-error {{incompatible pointer to integer conversion initializing 'int' with an expression of type 'char[23]'}}
+
+  wchar_t str7[] = L"string literal";
+  wchar_t str8[] = { L"string literal" };
+  char16_t str9[] = u"string literal";
+  char16_t str10[] = { u"string literal" };
+  char32_t str11[] = U"string literal";
+  char32_t str12[] = { U"string literal" };
+
+  wchar_t str13[] = "nope";      // expected-error {{initializing wide char array with non-wide string literal}}
+  wchar_t str14[] = { "nope" };  // expected-error-re {{incompatible pointer to integer conversion initializing 'wchar_t' (aka '{{.*}}') with an expression of type 'char[5]'}}
+  char16_t str15[] = "nope";     // expected-error {{initializing wide char array with non-wide string literal}}
+  char16_t str16[] = { "nope" }; // expected-error-re {{incompatible pointer to integer conversion initializing 'char16_t' (aka '{{.*}}') with an expression of type 'char[5]'}}
+  char32_t str17[] = "nope";     // expected-error {{initializing wide char array with non-wide string literal}}
+  char32_t str18[] = { "nope" }; // expected-error-re {{incompatible pointer to integer conversion initializing 'char32_t' (aka '{{.*}}') with an expression of type 'char[5]'}}
+}

clang/www/c_status.html

@@ -211,7 +211,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons VIII</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3346.pdf">N3346</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
     </tr>
     <tr>
       <td>Introduce complex literals v. 2</td>