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();

src/testRunner/unittests/tsbuild/outFile.ts

@@ -416,10 +416,20 @@ namespace ts {
             modifyFs: noop
         });
 
-        // Verify baseline with and without build info
+        // Verify baseline with build info
         verifyOutFileScenario({
             scenario: "when final project is not composite but uses project references",
             modifyFs: fs => replaceText(fs, sources[project.third][source.config], `"composite": true,`, ""),
+            ignoreWithoutBuildInfo: true,
+            ignoreDtsChanged: true,
+            ignoreDtsUnchanged: true,
+            baselineOnly: true
+        });
+
+        // Verify baseline with and without build info
+        verifyOutFileScenario({
+            scenario: "when final project is not composite but incremental",
+            modifyFs: fs => replaceText(fs, sources[project.third][source.config], `"composite": true,`, `"incremental": true,`),
             ignoreDtsChanged: true,
             ignoreDtsUnchanged: true,
             baselineOnly: true