Allow calling of user-defined function call operators

Currently it's not possible to call user-defined function call
operators, at least not without specifying operator() directly:
```
(gdb) l 1
1       struct S {
2         int operator() (int x) { return x + 5; }
3       };
4
5       int main () {
6         S s;
7
8         return s(23);
9       }
(gdb) p s(10)
Invalid data type for function to be called.
(gdb) p s.operator()(10)
$1 = 15
```

This now looks if an user-defined call operator is available when
trying to 'call' a struct value, and calls it instead, making this
possible:
```
(gdb) p s(10)
$1 = 15
```

The change in operation::evaluate_funcall is to make sure the type
fields are only used for function types, only they use them as the
argument types.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=12213
Approved-By: Tom Tromey <tom@tromey.com>

1 files changed, 31 insertions, 3 deletions

diff --git a/gdb/eval.c b/gdb/eval.c
index 6b752e7..40640ae 100644
--- a/gdb/eval.c
+++ b/gdb/eval.c
@@ -588,14 +588,38 @@ evaluate_subexp_do_call (expression *exp, enum noside noside,
 {
   if (callee == NULL)
     error (_("Cannot evaluate function -- may be inlined"));
+
+  type *ftype = callee->type ();
+
+  /* If the callee is a struct, there might be a user-defined function call
+     operator that should be used instead.  */
+  std::vector<value *> vals;
+  if (overload_resolution
+      && exp->language_defn->la_language == language_cplus
+      && check_typedef (ftype)->code () == TYPE_CODE_STRUCT)
+    {
+      /* Include space for the `this' pointer at the start.  */
+      vals.resize (argvec.size () + 1);
+
+      vals[0] = value_addr (callee);
+      for (int i = 0; i < argvec.size (); ++i)
+	vals[i + 1] = argvec[i];
+
+      int static_memfuncp;
+      find_overload_match (vals, "operator()", METHOD, &vals[0], nullptr,
+			   &callee, nullptr, &static_memfuncp, 0, noside);
+      if (!static_memfuncp)
+	argvec = vals;
+
+      ftype = callee->type ();
+    }
+
   if (noside == EVAL_AVOID_SIDE_EFFECTS)
     {
       /* If the return type doesn't look like a function type,
 	 call an error.  This can happen if somebody tries to turn
 	 a variable into a function call.  */
 
-      type *ftype = callee->type ();
-
       if (ftype->code () == TYPE_CODE_INTERNAL_FUNCTION)
 	{
 	  /* We don't know anything about what the internal
@@ -666,9 +690,13 @@ operation::evaluate_funcall (struct type *expect_type,
   struct type *type = callee->type ();
   if (type->code () == TYPE_CODE_PTR)
     type = type->target_type ();
+  /* If type is a struct, num_fields would refer to the number of
+     members in the type, not the number of arguments.  */
+  bool type_has_arguments
+    = type->code () == TYPE_CODE_FUNC || type->code () == TYPE_CODE_METHOD;
   for (int i = 0; i < args.size (); ++i)
     {
-      if (i < type->num_fields ())
+      if (type_has_arguments && i < type->num_fields ())
 	vals[i] = args[i]->evaluate (type->field (i).type (), exp, noside);
       else
 	vals[i] = args[i]->evaluate_with_coercion (exp, noside);