Better error message for bad type parameter deduction.

Cython/Compiler/PyrexTypes.py

@@ -4078,7 +4078,7 @@ def best_match(args, functions, pos=None, env=None):
                 [pattern.type.deduce_template_params(actual) for (pattern, actual) in zip(func_type.args, arg_types)],
                 {})
             if deductions is None:
-                errors.append((func, "Unable to deduce type parameters"))
+                errors.append((func, "Unable to deduce type parameters for %s given %s" % (pattern.type, actual)))
             elif len(deductions) < len(func_type.templates):
                 errors.append((func, "Unable to deduce type parameter %s" % (
                     ", ".join([param.name for param in set(func_type.templates) - set(deductions.keys())]))))