fix: error message for bls elements in a type def

closes #840

crates/aiken-lang/src/tipo/error.rs

@@ -259,7 +259,7 @@ You can use '{discard}' and numbers to distinguish between similar names.
         name: String,
     },
 
-    #[error("I found a data type that has a function type in it. This is not allowed.\n")]
+    #[error("I found a type definition that has a function type in it. This is not allowed.\n")]
     #[diagnostic(code("illegal::function_in_type"))]
     #[diagnostic(help("Data-types can't hold functions. If you want to define method-like functions, group the type definition and the methods under a common namespace in a standalone module."))]
     FunctionTypeInData {
@@ -267,6 +267,18 @@ You can use '{discard}' and numbers to distinguish between similar names.
         location: Span,
     },
 
+    #[error("I found a type definition that has an unsupported type in it.\n")]
+    #[diagnostic(code("illegal::type_in_data"))]
+    #[diagnostic(help(
+        r#"Data-types can't contain type {type_info} because it can't be represented as PlutusData."#,
+        type_info = tipo.to_pretty(0).if_supports_color(Stdout, |s| s.red())
+    ))]
+    IllegalTypeInData {
+        #[label]
+        location: Span,
+        tipo: Rc<Type>,
+    },
+
     #[error("I found a discarded expression not bound to a variable.\n")]
     #[diagnostic(code("implicit_discard"))]
     #[diagnostic(help(
@@ -951,6 +963,7 @@ impl ExtraData for Error {
             | Error::DuplicateVarInPattern { .. }
             | Error::ExtraVarInAlternativePattern { .. }
             | Error::FunctionTypeInData { .. }
+            | Error::IllegalTypeInData { .. }
             | Error::ImplicitlyDiscardedExpression { .. }
             | Error::IncorrectFieldsArity { .. }
             | Error::IncorrectFunctionCallArity { .. }

crates/aiken-lang/src/tipo/infer.rs

@@ -545,6 +545,13 @@ fn infer_definition(
                             location: *location,
                         });
                     }
+
+                    if tipo.is_bls381_12_g1() || tipo.is_bls381_12_g2() || tipo.is_ml_result() {
+                        return Err(Error::IllegalTypeInData {
+                            location: *location,
+                            tipo: tipo.clone(),
+                        });
+                    }
                 }
             }