Fix Issue 23226 - ImportC: allow single-argument _Static_assert(expr) (#23406)

Author: AayushMainali-Github

changelog/dmd.importc-c23-single-arg-static-assert.dd

@@ -0,0 +1,13 @@
+ImportC supports single-argument `_Static_assert(expr)`
+
+C23 makes the message operand of a static assertion optional.
+ImportC now accepts both forms:
+
+-------
+_Static_assert(sizeof(int) == 4);
+_Static_assert(sizeof(int) == 4, "unexpected int size");
+-------
+
+The two-argument form continues to work as before. When the assertion
+fails and no message was supplied, the diagnostic reports that the
+condition is false.

compiler/src/dmd/cparse.d

@@ -3289,8 +3289,13 @@ final class CParser(AST) : Parser!AST
     }
 
     /***********************************
-     * C11 6.7.10
+     * C11 6.7.10 / C23 6.7.12
      * _Static_assert ( constant-expression , string-literal ) ;
+     * _Static_assert ( constant-expression ) ;
+     *
+     * C23 makes the message operand optional. Accepting the single-argument
+     * form cannot change the meaning of any valid C11 program, so it is
+     * enabled unconditionally.
      */
     private AST.StaticAssert cparseStaticAssert()
     {
@@ -3300,13 +3305,22 @@ final class CParser(AST) : Parser!AST
         nextToken();
         check(TOK.leftParenthesis);
         auto exp = cparseConstantExp();
-        check(TOK.comma);
-        if (token.value != TOK.string_)
-            error("string literal expected");
-        auto msg = cparsePrimaryExp();
+        if (token.value == TOK.comma) // C23 6.7.12
+        {
+            nextToken();
+            if (token.value != TOK.string_)
+                error("string literal expected");
+            auto msg = cparsePrimaryExp();
+            check(TOK.rightParenthesis);
+            check(TOK.semicolon);
+            return new AST.StaticAssert(loc, exp, msg);
+        }
         check(TOK.rightParenthesis);
         check(TOK.semicolon);
-        return new AST.StaticAssert(loc, exp, msg);
+        // Use the Expressions* overload so a missing message stays null
+        // rather than a one-element array of null (matches D's parseStaticAssert).
+        AST.Expressions* msgs = null;
+        return new AST.StaticAssert(loc, exp, msgs);
     }
 
     /*************************

compiler/test/compilable/testcstuff1.c

@@ -1,5 +1,10 @@
 // check the expression parser
 
+// https://github.com/dlang/dmd/issues/23226
+// C23 6.7.12 — message operand is optional
+_Static_assert(1);
+_Static_assert(1, "ok");
+
 _Static_assert(0 == 0, "ok");
 _Static_assert(0 != 1, "ok");
 _Static_assert(1 + 2 == 3, "ok");

compiler/test/fail_compilation/failcstuff3.c

@@ -2,6 +2,7 @@
 /* TEST_OUTPUT:
 ---
 fail_compilation/failcstuff3.c(54): Error: redeclaration of `S22061`
+fail_compilation/failcstuff3.c(100): Error: static assert:  `0` is false
 ---
 */
 
@@ -13,3 +14,9 @@ struct S22061
     int field;
 };
 typedef union S22061 S22061;
+
+/***************************************************/
+// https://github.com/dlang/dmd/issues/23226
+// C23 6.7.12 — failing single-argument _Static_assert still diagnoses
+#line 100
+_Static_assert(0);