Revert allowing to remove interface conformance

runtime/contract_update_validation_test.go

@@ -2558,7 +2558,10 @@ func TestRuntimeContractUpdateConformanceChanges(t *testing.T) {
         `
 
 		err := testDeployAndUpdate(t, "Test", oldCode, newCode)
-		require.NoError(t, err)
+		RequireError(t, err)
+
+		cause := getSingleContractUpdateErrorCause(t, err, "Test")
+		assertConformanceMismatchError(t, cause, "Foo")
 	})
 
 	t.Run("Change conformance order", func(t *testing.T) {

runtime/stdlib/cadence_v0.42_to_v1_contract_upgrade_validation_test.go

@@ -528,6 +528,41 @@ func TestContractUpgradeFieldType(t *testing.T) {
 
 		require.NoError(t, err)
 	})
+
+	t.Run("change field types illegally", func(t *testing.T) {
+
+		t.Parallel()
+
+		const importCode = `
+			access(all) contract interface CI {}
+		`
+
+		const oldCode = `
+            import CI from 0x01
+
+            access(all) contract Test {
+                access(all) var a: Capability<CI>?
+                init() {
+                    self.a = nil
+                }
+            }
+        `
+
+		const newCode = `
+            import CI from 0x01
+
+			access(all) contract Test {
+				access(all) var a: Capability<{CI}>?
+				init() {
+					self.a = nil
+				}
+			}
+        `
+
+		err := testContractUpdateWithImports(t, oldCode, importCode, newCode, importCode)
+
+		require.NoError(t, err)
+	})
 }
 
 func TestContractUpgradeIntersectionAuthorization(t *testing.T) {

runtime/stdlib/contract_update_validation.go

@@ -416,29 +416,41 @@ func checkConformance(
 	newDecl *ast.CompositeDeclaration,
 ) {
 
-	// at this point the declaration kinds are known to be the same
-	if oldDecl.DeclarationKind() != common.DeclarationKindEnum {
-		return
-	}
-
-	// Here it is assumed enums will always have one and only one conformance.
-	// This is enforced by the checker.
-	// Therefore, below check for multiple conformances is only applicable
-	// for non-enum type composite declarations. i.e: structs, resources, etc.
-
-	oldConformance := oldDecl.Conformances[0]
-	newConformance := newDecl.Conformances[0]
+	oldConformances := oldDecl.Conformances
+	newConformances := newDecl.Conformances
+
+	// All the existing conformances must have a match. Order is not important.
+	// Having extra new conformance is OK. See: https://github.com/onflow/cadence/issues/1394
+
+	// Note: Removing a conformance is NOT OK. That could lead to type-safety issues.
+	// e.g:
+	//	- Someone stores an array of type `[{I}]` with `T:I` objects inside.
+	//  - Later T’s conformance to `I` is removed.
+	//  - Now `[{I}]` contains objects if `T` that does not conform to `I`.
+
+	for _, oldConformance := range oldConformances {
+		found := false
+		for index, newConformance := range newConformances {
+			err := oldConformance.CheckEqual(newConformance, validator)
+			if err == nil {
+				found = true
+
+				// Remove the matched conformance, so we don't have to check it again.
+				// i.e: optimization
+				newConformances = append(newConformances[:index], newConformances[index+1:]...)
+				break
+			}
+		}
 
-	err := oldConformance.CheckEqual(newConformance, validator)
+		if !found {
+			validator.report(&ConformanceMismatchError{
+				DeclName: newDecl.Identifier.Identifier,
+				Range:    ast.NewUnmeteredRangeFromPositioned(newDecl.Identifier),
+			})
 
-	if err == nil {
-		return
+			return
+		}
 	}
-
-	validator.report(&ConformanceMismatchError{
-		DeclName: newDecl.Identifier.Identifier,
-		Range:    ast.NewUnmeteredRangeFromPositioned(newDecl.Identifier),
-	})
 }
 
 func (validator *ContractUpdateValidator) report(err error) {