C++: Fix FPs in cpp/wrong-number-format-arguments

cpp/ql/src/Likely Bugs/Format/WrongNumberOfFormatArguments.ql

@@ -16,6 +16,20 @@
 
 import cpp
 
+class SyntaxError extends CompilerError {
+  SyntaxError() { this.getTag().matches("exp_%") }
+
+  predicate affects(Element e) {
+    exists(Location l1, Location l2 |
+      l1 = this.getLocation() and
+      l2 = e.getLocation()
+    |
+      l1.getFile() = l2.getFile() and
+      l1.getStartLine() = l2.getStartLine()
+    )
+  }
+}
+
 from FormatLiteral fl, FormattingFunctionCall ffc, int expected, int given, string ffcName
 where
   ffc = fl.getUse() and
@@ -27,7 +41,10 @@ where
     if ffc.isInMacroExpansion()
     then ffcName = ffc.getTarget().getName() + " (in a macro expansion)"
     else ffcName = ffc.getTarget().getName()
-  )
+  ) and
+  // A typical problem is that string literals are concatenated, but if one of the string
+  // literals is an undefined macro, then this just leads to a syntax error.
+  not exists(SyntaxError e | e.affects(fl))
 select ffc,
   "Format for " + ffcName + " expects " + expected.toString() + " arguments but given " +
     given.toString()

cpp/ql/test/query-tests/Likely Bugs/Format/WrongNumberOfFormatArguments/syntax_errors.c

@@ -0,0 +1,7 @@
+// semmle-extractor-options: --expect_errors
+
+extern int printf(const char *fmt, ...);
+
+void test_syntax_error() {
+    printf("Error code %d: " FMT_MSG, 0, "");
+}