Disallow abilities on toplevel of scope

compiler/can/src/def.rs

@@ -505,6 +505,7 @@ pub(crate) fn canonicalize_defs<'a>(
             output,
             scope,
             var_store,
+            pattern_type,
             &mut aliases,
             &abilities_in_scope,
         );
@@ -981,12 +982,13 @@ fn canonicalize_pending_value_def<'a>(
     mut output: Output,
     scope: &mut Scope,
     var_store: &mut VarStore,
+    pattern_type: PatternType,
     aliases: &mut VecMap<Symbol, Alias>,
     abilities_in_scope: &[Symbol],
 ) -> DefOutput {
     use PendingValueDef::*;
 
-    match pending_def {
+    let output = match pending_def {
         AnnotationOnly(_, loc_can_pattern, loc_ann) => {
             // Make types for the body expr, even if we won't end up having a body.
             let expr_var = var_store.fresh();
@@ -1129,7 +1131,21 @@ fn canonicalize_pending_value_def<'a>(
                 None,
             )
         }
+    };
+
+    // Disallow ability specializations that aren't on the toplevel (note: we might loosen this
+    // restriction later on).
+    if pattern_type != PatternType::TopLevelDef {
+        if let Loc {
+            value: Pattern::AbilityMemberSpecialization { specializes, .. },
+            region,
+        } = output.def.loc_pattern
+        {
+            env.problem(Problem::NestedSpecialization(specializes, region));
         }
+    }
+
+    output
 }
 
 // TODO trim down these arguments!

compiler/problem/src/can.rs

@@ -139,6 +139,7 @@ pub enum Problem {
         region: Region,
     },
     AbilityUsedAsType(Lowercase, Symbol, Region),
+    NestedSpecialization(Symbol, Region),
 }
 
 #[derive(Clone, Debug, PartialEq)]

reporting/src/error/canonicalize.rs

@@ -46,6 +46,7 @@ const ABILITY_MEMBER_MISSING_HAS_CLAUSE: &str = "ABILITY MEMBER MISSING HAS CLAU
 const ABILITY_MEMBER_HAS_EXTRANEOUS_HAS_CLAUSE: &str = "ABILITY MEMBER HAS EXTRANEOUS HAS CLAUSE";
 const ABILITY_MEMBER_BINDS_MULTIPLE_VARIABLES: &str = "ABILITY MEMBER BINDS MULTIPLE VARIABLES";
 const ABILITY_NOT_ON_TOPLEVEL: &str = "ABILITY NOT ON TOP-LEVEL";
+const SPECIALIZATION_NOT_ON_TOPLEVEL: &str = "SPECIALIZATION NOT ON TOP-LEVEL";
 const ABILITY_USED_AS_TYPE: &str = "ABILITY USED AS TYPE";
 
 pub fn can_problem<'b>(
@@ -779,6 +780,19 @@ pub fn can_problem<'b>(
             title = ABILITY_USED_AS_TYPE.to_string();
             severity = Severity::RuntimeError;
         }
+        Problem::NestedSpecialization(member, region) => {
+            doc = alloc.stack([
+                alloc.concat([
+                    alloc.reflow("This specialization of the "),
+                    alloc.symbol_unqualified(member),
+                    alloc.reflow("ability member is in a nested scope:"),
+                ]),
+                alloc.region(lines.convert_region(region)),
+                alloc.reflow("Specializations can only be defined on the top-level of a module."),
+            ]);
+            title = SPECIALIZATION_NOT_ON_TOPLEVEL.to_string();
+            severity = Severity::Warning;
+        }
     };
 
     Report {

reporting/tests/test_reporting.rs

@@ -9764,4 +9764,35 @@ I need all branches in an `if` to have the same type!
             "", // no problem
         )
     }
+
+    #[test]
+    fn nested_specialization() {
+        new_report_problem_as(
+            "nested_specialization",
+            indoc!(
+                r#"
+                app "test" provides [ main ] to "./platform"
+
+                Default has default : {} -> a | a has Default
+
+                main =
+                    A := {}
+                    default = \{} -> @A {}
+                    default {}
+                "#
+            ),
+            indoc!(
+                r#"
+                ── SPECIALIZATION NOT ON TOP-LEVEL ─────────────────────── /code/proj/Main.roc ─
+
+                This specialization of the `default`ability member is in a nested scope:
+
+                7│      default = \{} -> @A {}
+                        ^^^^^^^
+
+                Specializations can only be defined on the top-level of a module.
+                "#
+            ),
+        )
+    }
 }