Merge pull request #3203 from onflow/sainati/capability-get-unreachable

Properly handle `Never` type argument to `Capabilities.get`

Author: dsainati1

runtime/capabilities_test.go

@@ -235,6 +235,13 @@ func TestRuntimeCapability_borrowAndCheck(t *testing.T) {
                   assert(!self.account.capabilities.exists(path))
               }
 
+              access(all)
+              fun testNever() {
+                  let path = /public/r
+                  assert(self.account.capabilities.get<Never>(path) == nil)
+                  assert(self.account.capabilities.exists(path))
+              }
+
               access(all)
               fun testSwap(): Int {
                  let ref = self.account.capabilities.get<&R>(/public/r)!.borrow()!
@@ -305,6 +312,11 @@ func TestRuntimeCapability_borrowAndCheck(t *testing.T) {
 			require.NoError(t, err)
 		})
 
+		t.Run("testNever", func(t *testing.T) {
+			_, err := invoke("testNever")
+			require.NoError(t, err)
+		})
+
 		t.Run("testSwap", func(t *testing.T) {
 
 			_, err := invoke("testSwap")

runtime/stdlib/account.go

@@ -3595,8 +3595,13 @@ func newAccountCapabilitiesGetFunction(
 
 			// Get borrow type type argument
 
-			typeParameterPair := invocation.TypeParameterTypes.Oldest()
-			wantedBorrowType, ok := typeParameterPair.Value.(*sema.ReferenceType)
+			typeParameterPairValue := invocation.TypeParameterTypes.Oldest().Value
+			// `Never` is never a supertype of any stored value
+			if typeParameterPairValue.Equal(sema.NeverType) {
+				return interpreter.Nil
+			}
+
+			wantedBorrowType, ok := typeParameterPairValue.(*sema.ReferenceType)
 			if !ok {
 				panic(errors.NewUnreachableError())
 			}