Fix generation of fallback validator

This must only happen in case all other validator succeed; otherwise
  we might generate invalid validators.

crates/aiken-lang/src/ast/well_known.rs

@@ -49,6 +49,8 @@ pub const SCRIPT_PURPOSE_CONSTRUCTORS: &[&str] = &[
     SCRIPT_PURPOSE_PROPOSE,
 ];
 
+pub const VALIDATOR_ELSE: &str = "else";
+
 // ----------------------------------------------------------------------------
 // Types
 

crates/aiken-lang/src/gen_uplc.rs

@@ -1575,7 +1575,11 @@ impl<'a> CodeGenerator<'a> {
         otherwise: AirTree,
         depth: usize,
     ) -> AirTree {
-        assert!(tipo.get_generic().is_none());
+        assert!(
+            tipo.get_generic().is_none(),
+            "left-hand side of expect is generic: {}",
+            tipo.to_pretty(0)
+        );
         // Shouldn't be needed but still here just in case
         // this function is called from anywhere else besides assignment
         let tipo = &convert_opaque_type(tipo, &self.data_types, true);

crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__free_vars.snap

@@ -1,6 +1,6 @@
 ---
 source: crates/aiken-project/src/blueprint/validator.rs
-description: "Code:\n\nvalidator {\n  fn generics(redeemer: a, ctx: Void) {\n    True\n  }\n}\n"
+description: "Code:\n\nvalidator generics {\n  mint(redeemer: a, policy_id: ByteArray, transaction: Data) {\n    True\n  }\n}\n"
 ---
 Schema {
     error: Error {
@@ -16,7 +16,7 @@ Schema {
             },
         ],
     },
-    location: 26..37,
+    location: 28..39,
     source_code: NamedSource {
         name: "",
         source: "<redacted>",

crates/aiken-project/src/blueprint/validator.rs

@@ -69,6 +69,11 @@ impl Validator {
             ));
         }
 
+        // NOTE: Only push the fallback if all other validators have been successfully
+        // generated. Otherwise, we may fall into scenarios where we cannot generate validators
+        // (e.g. due to the presence of generics in datum/redeemer), which won't be caught by
+        // the else branch since it lacks arguments.
+        if validators.iter().all(|v| v.is_ok()) {
             validators.push(Validator::create_validator_blueprint(
                 generator,
                 modules,
@@ -78,6 +83,7 @@ impl Validator {
                 &mut program,
                 plutus_version,
             ));
+        }
 
         validators
     }
@@ -92,10 +98,6 @@ impl Validator {
         program: &mut MemoProgram,
         plutus_version: &PlutusVersion,
     ) -> Result<Validator, Error> {
-        let mut args = func.arguments.iter().rev();
-
-        let (_, _, redeemer, datum) = (args.next(), args.next(), args.next(), args.next());
-
         let mut definitions = Definitions::new();
 
         let parameters = def
@@ -122,6 +124,18 @@ impl Validator {
             })
             .collect::<Result<_, _>>()?;
 
+        let (datum, redeemer) = if func.name == well_known::VALIDATOR_ELSE {
+            (None, None)
+        } else {
+            let mut args = func.arguments.iter().rev();
+
+            let (_, _, redeemer, datum) = (
+                args.next(),
+                args.next(),
+                args.next().expect("redeemer is always present"),
+                args.next(),
+            );
+
             let datum = datum
                 .map(|datum| {
                     match datum.tipo.as_ref() {
@@ -160,9 +174,7 @@ impl Validator {
                     schema,
                 });
 
-        let redeemer = redeemer
+            let redeemer = Annotated::from_type(
-            .map(|redeemer| {
-                Annotated::from_type(
                 modules.into(),
                 tipo_or_annotation(module, redeemer),
                 &mut definitions,
@@ -175,19 +187,20 @@ impl Validator {
                     module.code.clone(),
                 ),
             })
-            })
-            .transpose()?
             .map(|schema| Parameter {
-                title: redeemer.map(|redeemer| redeemer.arg_name.get_label()),
+                title: Some(redeemer.arg_name.get_label()),
                 schema,
-            });
+            })?;
+
+            (datum, Some(redeemer))
+        };
 
         Ok(Validator {
             title: format!(
                 "{}.{}_{}",
                 &module.name,
                 &def.name,
-                if func.name == "else" {
+                if func.name == well_known::VALIDATOR_ELSE {
                     "__fallback"
                 } else {
                     &func.name