Experiment 3

ts/project.test.ts

@@ -137,7 +137,7 @@ describe("Project.add", () => {
   });
 
   it("provides a nice type synonym for plugins that add a field", () => {
-    const plugin: Plugin<{}, { addedField: garn.Package }> = (p) => ({
+    const plugin: Plugin<object, { addedField: garn.Package }> = (p) => ({
       ...p,
       addedField: garn.build``,
     });
@@ -153,7 +153,9 @@ describe("Project.add", () => {
   });
 
   it("provides a nice type synonym for plugins that add multiple fields", () => {
-    const plugin: Plugin<{}, { one: garn.Package; two: garn.Check }> = (p) => ({
+    const plugin: Plugin<object, { one: garn.Package; two: garn.Check }> = (
+      p,
+    ) => ({
       ...p,
       one: garn.build``,
       two: garn.check(""),
@@ -186,10 +188,12 @@ describe("Project.add", () => {
       .add(plugin);
     assertTypeIsExecutable(project.foo);
     assertTypeIsExecutable(project.addedField);
+    // @ts-expect-error - `dep` is missing
+    () => garn.mkProject({ description: "" }, {}).add(plugin);
   });
 
   it("allows to overwrite fields", () => {
-    const plugin: Plugin<{}, { field: Package }> = (p) => ({
+    const plugin: Plugin<object, { field: Package }> = (p) => ({
       ...p,
       field: garn.build``,
     });

ts/project.ts

@@ -22,9 +22,7 @@ export type ProjectData = {
   defaultExecutable?: Executable;
 };
 
-export type Plugin<Dependencies, Additions> = (
-  project: Dependencies
-) => Additions;
+export type Plugin<Input, Output> = (project: Input) => Output;
 
 type ProjectHelpers = {
   /**
@@ -82,10 +80,10 @@ type ProjectHelpers = {
    *   .add(self => self.addExecutable("codegen")`${self.mainPackage}/bin/codegen`)
    * ```
    */
-  add<T extends Project, Additions>(
-    this: T,
-    fn: T extends infer Dependencies ? Plugin<Dependencies, Additions> : never,
-  ): Omit<T, keyof Additions> & Additions;
+  add<Input extends ProjectData, Output>(
+    this: Input,
+    fn: Plugin<Input, Output>,
+  ): Omit<Input, keyof Output> & Output;
 
   /**
    * Adds an `Executable` with the given name to the Project
@@ -219,14 +217,11 @@ const proxyEnvironmentHelpers = (): ProjectHelpers => ({
     return mkShellPackage(defaultEnvironment, s, ...args);
   },
 
-  add<T extends Project, Additions>(
-    this: T,
-    fn: T extends infer Dependencies ? Plugin<Dependencies, Additions> : never,
-  ): Omit<T, keyof Additions> & Additions {
-    return {
-      ...this,
-      ...fn(this),
-    };
+  add<Input extends ProjectData, Output>(
+    this: Input,
+    fn: Plugin<Input, Output>,
+  ): Omit<Input, keyof Output> & Output {
+    return { ...this, ...fn(this) };
   },
 
   addExecutable<T extends ProjectData, Name extends string>(
@@ -259,7 +254,8 @@ const proxyEnvironmentHelpers = (): ProjectHelpers => ({
       [
         `${exportName} exports the return type of "addExecutable", but this is not the proper usage of addExecutable.`,
         'Did you forget the template literal? Example usage: project.addExecutable("executable-name")`shell script to run`',
-      ].join(" "));
+      ].join(" "),
+    );
     return templateLiteralFn;
   },
 
@@ -293,7 +289,8 @@ const proxyEnvironmentHelpers = (): ProjectHelpers => ({
       [
         `${exportName} exports the return type of "addCheck", but this is not the proper usage of addCheck.`,
         'Did you forget the template literal? Example usage: project.addCheck("check-name")`shell script to run`',
-      ].join(" "));
+      ].join(" "),
+    );
     return templateLiteralFn;
   },