Add but not use WrongTypeCombinationError

Relates to #60

src/error/display.rs

@@ -62,6 +62,11 @@ impl fmt::Display for EvalexprError {
             TypeError { expected, actual } => {
                 write!(f, "Expected one of {:?}, but got {:?}.", expected, actual)
             },
+            WrongTypeCombination { operator, actual } => write!(
+                f,
+                "The operator {:?} was called with a wrong combination of types: {:?}",
+                operator, actual
+            ),
             UnmatchedLBrace => write!(f, "Found an unmatched opening parenthesis '('."),
             UnmatchedRBrace => write!(f, "Found an unmatched closing parenthesis ')'."),
             UnmatchedPartialToken { first, second } => {

src/error/mod.rs

@@ -7,7 +7,7 @@
 
 use crate::{token::PartialToken, value::value_type::ValueType};
 
-use crate::value::Value;
+use crate::{operator::Operator, value::Value};
 
 mod display;
 
@@ -112,6 +112,14 @@ pub enum EvalexprError {
         actual: Value,
     },
 
+    /// An operator is used with a wrong combination of types.
+    WrongTypeCombination {
+        /// The operator that whose evaluation caused the error.
+        operator: Operator,
+        /// The types that were used in the operator causing it to fail.
+        actual: Vec<ValueType>,
+    },
+
     /// An opening brace without a matching closing brace was found.
     UnmatchedLBrace,