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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/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.

>From 79e1a4f9738d355feb20b9d5bb67fb35f0336c3c Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Fri, 8 Nov 2024 11:56:44 -0500
Subject: [PATCH] [C2y] Claim conformance and add test coverage for WG14 N3346

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.
---
 clang/docs/ReleaseNotes.rst |  5 +++
 clang/test/C/C2y/n3346.c    | 68 +++++++++++++++++++++++++++++++++++++
 clang/www/c_status.html     |  2 +-
 3 files changed, 74 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/C/C2y/n3346.c

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d4bf05651a63eb..77ba5f1d79bcdb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/test/C/C2y/n3346.c b/clang/test/C/C2y/n3346.c
new file mode 100644
index 00000000000000..2f0ef51d56fb76
--- /dev/null
+++ b/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]'}}
+}
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index fa2411e674d768..663cc2e33f5157 100644
--- a/clang/www/c_status.html
+++ b/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>