test -from-module with invalid root module

Add a test for `init -from-module` with a module which is not a valid
root module.

internal/initwd/from_module_test.go

@@ -212,6 +212,38 @@ func TestDirFromModule_submodules(t *testing.T) {
 	assertResultDeepEqual(t, gotTraces, wantTraces)
 }
 
+// submodulesWithProvider is identical to above, except that the configuration
+// would fail to load for some reason. We still want the module to be installed
+// for use cases like testing or CDKTF, and will only emit warnings for config
+// errors.
+func TestDirFromModule_submodulesWithProvider(t *testing.T) {
+	fixtureDir := filepath.Clean("testdata/empty")
+	fromModuleDir, err := filepath.Abs("./testdata/local-module-missing-provider")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tmpDir, done := tempChdir(t, fixtureDir)
+	defer done()
+
+	hooks := &testInstallHooks{}
+	dir, err := filepath.EvalSymlinks(tmpDir)
+	if err != nil {
+		t.Error(err)
+	}
+	modInstallDir := filepath.Join(dir, ".terraform/modules")
+
+	loader, cleanup := configload.NewLoaderForTests(t)
+	defer cleanup()
+	diags := DirFromModule(context.Background(), loader, dir, modInstallDir, fromModuleDir, nil, hooks)
+
+	for _, d := range diags {
+		if d.Severity() != tfdiags.Warning {
+			t.Errorf("expected warning, got %v", diags.Err())
+		}
+	}
+}
+
 // TestDirFromModule_rel_submodules is similar to the test above, but the
 // from-module is relative to the install dir ("../"):
 // https://github.com/hashicorp/terraform/issues/23010

internal/initwd/module_install_test.go

@@ -944,7 +944,7 @@ func assertNoDiagnostics(t *testing.T, diags tfdiags.Diagnostics) bool {
 
 func assertDiagnosticCount(t *testing.T, diags tfdiags.Diagnostics, want int) bool {
 	t.Helper()
-	if len(diags) != 0 {
+	if len(diags) != want {
 		t.Errorf("wrong number of diagnostics %d; want %d", len(diags), want)
 		for _, diag := range diags {
 			t.Logf("- %#v", diag)

internal/initwd/testdata/local-module-missing-provider/main.tf

@@ -0,0 +1,14 @@
+terraform {
+  required_providers {
+    foo = {
+      source = "hashicorp/foo"
+      // since this module declares an alias with no config, it is not valid as
+      // a root module.
+      configuration_aliases = [ foo.alternate ]
+    }
+  }
+}
+
+resource "foo_instance" "bam" {
+  provider = foo.alternate
+}