C++: Only give alert when no def fits arg count

The `cpp/too-few-arguments` query produced alerts for ambiguous
databases where a function had multiple possible declarations, with some
declarations having the right number of parameters and some having too
many. With this change, the query errs on the side of caution in those
cases and does not produce an alert.

This fixes false positives on racket/racket.

The new `hasDefiniteNumberOfParameters` is exactly the negation of the
old `hasZeroParamDecl`.

Author: jbj

cpp/ql/src/Likely Bugs/Underspecified Functions/TooFewArguments.qll

@@ -6,10 +6,23 @@
 
 import cpp
 
+/**
+ * Holds if `fde` has a parameter declaration that's clear on the minimum
+ * number of parameters. This is essentially true for everything except
+ * `()`-declarations.
+ */
+private predicate hasDefiniteNumberOfParameters(FunctionDeclarationEntry fde) {
+  fde.hasVoidParamList()
+  or
+  fde.getNumberOfParameters() > 0
+  or
+  fde.isDefinition()
+}
+
 // True if function was ()-declared, but not (void)-declared or K&R-defined
 private predicate hasZeroParamDecl(Function f) {
   exists(FunctionDeclarationEntry fde | fde = f.getADeclarationEntry() |
-    not fde.hasVoidParamList() and fde.getNumberOfParameters() = 0 and not fde.isDefinition()
+    not hasDefiniteNumberOfParameters(fde)
   )
 }
 
@@ -24,11 +37,18 @@ predicate tooFewArguments(FunctionCall fc, Function f) {
   f = fc.getTarget() and
   not f.isVarargs() and
   not f instanceof BuiltInFunction and
+  // This query should only have results on C (not C++) functions that have a
+  // `()` parameter list somewhere. If it has results on other functions, then
+  // it's probably because the extractor only saw a partial compilation.
   hasZeroParamDecl(f) and
   isCompiledAsC(f.getFile()) and
-  // There is an explicit declaration of the function whose parameter count is larger
-  // than the number of call arguments
-  exists(FunctionDeclarationEntry fde | fde = f.getADeclarationEntry() |
+  // Produce an alert when all declarations that are authoritative on the
+  // parameter count specify a parameter count larger than the number of call
+  // arguments.
+  forex(FunctionDeclarationEntry fde |
+    fde = f.getADeclarationEntry() and
+    hasDefiniteNumberOfParameters(fde)
+  |
     fde.getNumberOfParameters() > fc.getNumberOfArguments()
   )
 }