Incremental false with composite not allowed

src/compiler/diagnosticMessages.json

@@ -4028,6 +4028,10 @@
         "category": "Message",
         "code": 6378
     },
+    "Composite projects may not disable incremental compilation.": {
+        "category": "Error",
+        "code": 6379
+    },
 
     "The expected type comes from property '{0}' which is declared here on type '{1}'": {
         "category": "Message",

src/compiler/program.ts

@@ -2720,6 +2720,9 @@ namespace ts {
                 if (options.declaration === false) {
                     createDiagnosticForOptionName(Diagnostics.Composite_projects_may_not_disable_declaration_emit, "declaration");
                 }
+                if (options.incremental === false) {
+                    createDiagnosticForOptionName(Diagnostics.Composite_projects_may_not_disable_declaration_emit, "declaration");
+                }
             }
 
             verifyProjectReferences();