Fix compile time format check when the formatting string is a constant slice.

releasenotes.md

@@ -27,6 +27,7 @@
 - Compiler segfault when modifying variable using an inline assembly block inside defer #2450.
 - Compile time switch over type would not correctly compare function pointer types.
 - Regression: Compiler segfault when assigning struct literal with too few members #2483
+- Fix compile time format check when the formatting string is a constant slice.
 
 ### Stdlib changes
 - Added generic `InterfaceList` to store a list of values that implement a specific interface

src/compiler/sema_expr.c

@@ -2180,6 +2180,10 @@ CHECK_FORMAT:;
 	// Check
 	Expr *expr = actual_args[format_index];
 	if (!sema_cast_const(expr) || call->call_expr.va_is_splat) return true;
+	if (!expr_is_const_string(expr))
+	{
+		RETURN_SEMA_ERROR(expr, "The format string must be a constant string.");
+	}
 	assert(expr_is_const_string(expr));
 	const char *data = expr->const_expr.bytes.ptr;
 	size_t len = expr->const_expr.bytes.len;

test/test_suite/attributes/format_check_error.c3

@@ -0,0 +1,51 @@
+import std;
+
+const VERSION = "2.0";
+enum Subcommand : (String name, String desc, String[] args)
+{
+	FOO = {"oekfe", "foek", { "foke", "foekfe"}},
+	BAR = {"foek", "foek", { "foekf", "foekfe"}},
+	BAZ = {"Foekfef", "foek", { "foke", "foekfe" }}
+}
+macro void help (String program_name, bool to_stderr = false)
+{
+    var printfn = to_stderr ? &io::printfn : &io::eprintfn;
+    var printf = to_stderr ? &io::printfn : &io::eprintfn;
+
+    @pool()
+    {
+        printfn("mu2d version " +++ VERSION);
+        printfn("");
+ (void) printfn("Usage: %s [flags] <subcommand> [options] ...", program_name.file_tbasename());
+        printfn("");
+        printfn("Subcommands:");
+        var $ident = "    ";
+        $foreach $subcommand : Subcommand.values:
+            var $line = $ident +++ $subcommand.name;
+            $foreach $arg : $subcommand.args:
+                $line = $line +++ " " +++ $arg;
+            $endforeach
+            printfn($line);
+
+            var $row_counter = 0;
+            $foreach $c : $subcommand.desc:
+                $if $row_counter == 0:
+                    $row_counter += $ident.len * 2;
+                    printf($ident + $ident); // #error: Cannot do the addition 'String' + 'String'
+                $endif
+
+                $if $row_counter + 1 >= 80 &&& $c == ' ':
+                    printf("\n");
+                    $row_counter = 0;
+                $else
+                    printf((String) { $c }); // #error: The format string must be a constant
+                    $row_counter += 1;
+                $endif
+            $endforeach
+        $endforeach
+    };
+}
+fn void main()
+{
+	help("aaa", true);
+}