Do not allow use of deprecated Conditions with custom tasks

docs/pipelines.md

@@ -972,6 +972,8 @@ Pipelines do not directly support passing the following items to custom tasks:
 * Service account name
 * Pod templates
 
+A pipeline task that references a custom task cannot reference `Conditions`.
+`Conditions` are deprecated.  Use [`WhenExpressions`](#guard-task-execution-using-whenexpressions) instead.
 
 ## Code examples
 

pkg/apis/pipeline/v1beta1/pipeline_validation.go

@@ -144,6 +144,11 @@ func validatePipelineTask(ctx context.Context, t PipelineTask, taskNames sets.St
 		if t.TaskRef.Kind == "" {
 			errs = errs.Also(apis.ErrInvalidValue("custom task ref must specify kind", "taskRef.kind"))
 		}
+		// Conditions are deprecated so the effort to support them with custom tasks is not justified.
+		// When expressions should be used instead.
+		if len(t.Conditions) > 0 {
+			errs = errs.Also(apis.ErrInvalidValue("custom tasks do not support conditions - use when expressions instead", "conditions"))
+		}
 		// TODO(#3133): Support these features if possible.
 		if t.Retries > 0 {
 			errs = errs.Also(apis.ErrInvalidValue("custom tasks do not support retries", "retries"))

pkg/apis/pipeline/v1beta1/pipeline_validation_test.go

@@ -582,6 +582,20 @@ func TestValidatePipelineTasks_Failure(t *testing.T) {
 			Paths:   []string{"tasks[0].taskRef.kind"},
 		},
 		wc: enableFeature(t, "enable-custom-tasks"),
+	}, {
+		name: "pipelinetask custom task doesn't support conditions",
+		tasks: []PipelineTask{{
+			Name: "foo",
+			Conditions: []PipelineTaskCondition{{
+				ConditionRef: "some-condition",
+			}},
+			TaskRef: &TaskRef{APIVersion: "example.dev/v0", Kind: "Example"},
+		}},
+		expectedError: apis.FieldError{
+			Message: `invalid value: custom tasks do not support conditions - use when expressions instead`,
+			Paths:   []string{"tasks[0].conditions"},
+		},
+		wc: enableFeature(t, "enable-custom-tasks"),
 	}, {
 		name: "pipelinetask custom task doesn't support retries",
 		tasks: []PipelineTask{{