[C2y] Claim support for WG14 N3623 (#162843)

This requires invalid signatures of main to be diagnosed while still
allowing for common extensions. Clang has always supported this.

Author: AaronBallman

clang/test/C/C2y/n3623.c

@@ -0,0 +1,74 @@
+// RUN: %clang_cc1 -verify -std=c2y -DSTD1 %s
+// RUN: %clang_cc1 -verify -std=c2y -DSTD2 %s
+// RUN: %clang_cc1 -verify=gnu1 -std=gnu2y -DGNU1 %s
+// RUN: %clang_cc1 -verify -std=gnu2y -DGNU2 %s
+// RUN: %clang_cc1 -verify=gnu3 -std=gnu2y -DGNU3 %s
+// RUN: %clang_cc1 -verify -std=gnu2y -DGNU4 %s
+// RUN: %clang_cc1 -verify -std=gnu2y -DGNU5 %s
+// RUN: %clang_cc1 -verify -std=gnu2y -DGNU6 %s
+// RUN: %clang_cc1 -verify=gnu7 -std=gnu2y -DGNU7 %s
+// RUN: %clang_cc1 -triple x86_64-apple-darwin12 -verify -std=c2y -DDARWIN1 %s
+// RUN: %clang_cc1 -triple x86_64-pc-win32-mscv -verify -std=c2y -fms-compatibility -DMS1 %s
+// RUN: %clang_cc1 -triple x86_64-pc-win32-mscv -verify -std=c2y -fms-compatibility -DMS2 %s
+// RUN: %clang_cc1 -verify=invalid -std=c2y -DINVALID1 %s
+// RUN: %clang_cc1 -verify=invalid -std=c2y -DINVALID2 %s
+// expected-no-diagnostics
+
+/* WG14 N3623: Yes
+ * Earthly Demon XV: Definition of Main
+ *
+ * This validates that we accept the standard type definitions of main or some
+ * other implementation-defined type.
+ */
+
+typedef __WCHAR_TYPE__ wchar_t;
+
+// These are the signatures required by the standard.
+#if defined(STD1)
+int main(void) {}
+#elif defined(STD2)
+int main(int argc, char *argv[]) {}
+#endif
+
+// GNU extensions.
+#if defined(GNU1)
+void main(void) {} /* gnu1-warning {{return type of 'main' is not 'int'}}
+                      gnu1-note {{change return type to 'int'}}
+                    */
+#elif defined(GNU2)
+const int main(void) {}
+#elif defined(GNU3)
+int main(...) {} /* gnu3-warning {{'main' is not allowed to be declared variadic}} */
+#elif defined(GNU4)
+int main(int, const char **) {}
+#elif defined(GNU5)
+int main(int, char const * const *) {}
+#elif defined(GNU6)
+int main(int, char * const *) {}
+#elif defined(GNU7)
+int main(int) {} /* gnu7-warning {{only one parameter on 'main' declaration}} */
+#endif
+
+// Darwin extensions.
+#if defined(DARWIN1)
+int main(int argc, char *argv[], char *environ[], char **undocumented) {}
+#endif
+
+// Microsoft extensions.
+#if defined(MS1)
+int wmain(int, wchar_t *[]) {}
+#elif defined(MS2)
+int wmain(int, wchar_t *[], wchar_t *[]) {}
+#endif
+
+// Invalid signatures.
+#if defined(INVALID1)
+inline int main(int, char *[]); /* invalid-error {{'main' is not allowed to be declared inline}} */
+int main(int, char *[], char *[], float); /* invalid-error {{too many parameters (4) for 'main': must be 0, 2, or 3}} */
+float main(int); /* invalid-error {{'main' must return 'int'}} */
+_Noreturn int main(int, char *[]); /* invalid-warning {{'main' is not allowed to be declared _Noreturn}}
+                                      invalid-note {{remove '_Noreturn'}}
+                                    */
+#elif defined(INVALID2)
+static int main(void); /* invalid-warning {{'main' should not be declared static}} */
+#endif

clang/www/c_status.html

@@ -339,7 +339,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Earthly Demon XV: Definition of Main</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3623.pdf">N3623</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
 	</tr>
     <tr>
       <td>static_assert without UB</td>