Add a testcase

src/testRunner/unittests/tsbuild.ts

@@ -259,6 +259,23 @@ namespace ts {
                 });
             });
         });
+
+        describe("tsbuild - downstream prepend projects always get rebuilt", () => {
+            const fs = outFileFs.shadow();
+            const host = new fakes.CompilerHost(fs);
+            const builder = createSolutionBuilder(host, buildHost, ["/src/third"], { dry: false, force: false, verbose: false });
+            clearDiagnostics();
+            builder.buildAllProjects();
+            assertDiagnosticMessages(/*none*/);
+            assert.equal(fs.statSync("src/third/thirdjs/output/third-output.js").mtimeMs, time(), "First build timestamp is correct");
+            tick();
+            replaceText(fs, "src/first/first_PART1.ts", "Hello", "Hola");
+            tick();
+            builder.resetBuildContext();
+            builder.buildAllProjects();
+            assertDiagnosticMessages(/*none*/);
+            assert.equal(fs.statSync("src/third/thirdjs/output/third-output.js").mtimeMs, time(), "Second build timestamp is correct");
+        });
     }
 
     describe("tsbuild - graph-ordering", () => {

tests/projects/outfile-concat/first/tsconfig.json

@@ -9,6 +9,11 @@
     "declaration": true,
     "outFile": "./bin/first-output.js"
   },
+  "files": [
+    "first_PART1.ts",
+    "first_part2.ts",
+    "first_part3.ts"
+  ],
   "references": [
   ]
 }

tests/projects/outfile-concat/third/tsconfig.json

@@ -9,6 +9,9 @@
     "declaration": true,
     "outFile": "./thirdjs/output/third-output.js"
   },
+  "files": [
+    "third_part1.ts"
+  ],
   "references": [
     { "path": "../first", "prepend": true },
     { "path": "../second", "prepend": true },