Merge #222: 🐛 Add a check for multiple definitions of the type alias

a673dd4 fix: add a check for multiple definitions of the type alias (LesterEvSe)

Pull request description:

ACKs for top commit:
  KyrylR:
    utACK a673dd4

Tree-SHA512: fa6016b692649d1104fcab67e6911e466535d051a287149b9c7b96e7731046629433f855408a7d94495c6219563fbf92ea3fa7f0b12efee3875366ebb8b6c07a

Author: KyrylR

src/ast.rs

@@ -617,8 +617,11 @@ impl Scope {
     ///
     /// There are any undefined aliases.
     pub fn insert_alias(&mut self, name: AliasName, ty: AliasedType) -> Result<(), Error> {
-        let resolved_ty = self.resolve(&ty)?;
-        self.aliases.insert(name, resolved_ty);
+        if self.aliases.contains_key(&name) {
+            return Err(Error::RedefinedAlias(name));
+        }
+
+        let _ = self.aliases.insert(name, self.resolve(&ty)?);
         Ok(())
     }
 

src/error.rs

@@ -428,6 +428,7 @@ pub enum Error {
     ExpressionNotConstant,
     IntegerOutOfBounds(UIntType),
     UndefinedVariable(Identifier),
+    RedefinedAlias(AliasName),
     UndefinedAlias(AliasName),
     VariableReuseInPattern(Identifier),
     WitnessReused(WitnessName),
@@ -546,6 +547,10 @@ impl fmt::Display for Error {
                 f,
                 "Variable `{identifier}` is not defined"
             ),
+            Error::RedefinedAlias(identifier) => write!(
+                f,
+                "Type alias `{identifier}` was defined multiple times"
+            ),
             Error::UndefinedAlias(identifier) => write!(
                 f,
                 "Type alias `{identifier}` is not defined"