Don't re-report obligations that we know were seen elsewhere

crates/compiler/solve/src/ability.rs

@@ -238,8 +238,10 @@ impl DeferredObligations {
         // Go through and attach generic "type does not implement ability" errors, if they were not
         // part of a larger context.
         for mia in incomplete_not_in_context.into_iter() {
-            if let Err(unfulfilled) = obligation_cache.check_one(subs, mia) {
-                if !reported_in_context.contains(&mia) {
+            // If the obligation is already cached, we must have already reported it in another
+            // context.
+            if !obligation_cache.has_cached(mia) && !reported_in_context.contains(&mia) {
+                if let Err(unfulfilled) = obligation_cache.check_one(subs, mia) {
                     problems.push(TypeError::UnfulfilledAbility(unfulfilled.clone()));
                 }
             }
@@ -274,6 +276,19 @@ impl ObligationCache<'_> {
         }
     }
 
+    fn has_cached(&self, mia: MustImplementAbility) -> bool {
+        match mia.typ {
+            Obligated::Opaque(opaque) => self.impl_cache.contains_key(&ImplKey {
+                opaque,
+                ability: mia.ability,
+            }),
+            Obligated::Adhoc(_) => {
+                // ad-hoc obligations are never cached
+                false
+            }
+        }
+    }
+
     fn check_adhoc(&mut self, subs: &mut Subs, var: Variable, ability: Symbol) -> ObligationResult {
         // Not worth caching ad-hoc checks because variables are unlikely to be the same between
         // independent queries.

crates/reporting/tests/test_reporting.rs

@@ -9109,10 +9109,6 @@ All branches in an `if` must have the same type!
 
     Tip: `A` does not implement `Encoding`. Consider adding a custom
     implementation or `has Encode.Encoding` to the definition of `A`.
-
-    ── INCOMPLETE ABILITY IMPLEMENTATION ───────────────────── /code/proj/Main.roc ─
-
-    The type `A` does not fully implement the ability `Encoding`.
     "###
     );