Add map decoration across modules test

This test verifies that map value group decoration chains properly
across module boundaries, demonstrating the correct behavior for
named value groups as implemented in uber-go/dig#381.

Author: jquirke

.claude/settings.local.json

@@ -0,0 +1,19 @@
+{
+  "permissions": {
+    "allow": [
+      "WebFetch(domain:github.com)",
+      "Bash(gh issue view:*)",
+      "Bash(go test:*)",
+      "Bash(go run:*)",
+      "Bash(go mod why:*)",
+      "Bash(go list:*)",
+      "Read(//Users/qjeremy/go/pkg/mod/github.com/jquirke/[email protected]/**)",
+      "Read(//Users/qjeremy/github/jquirke/**)",
+      "Bash(git checkout:*)",
+      "Bash(sed:*)",
+      "Bash(go vet:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}

decorate_test.go

@@ -853,4 +853,87 @@ func TestMapValueGroupsDecoration(t *testing.T) {
 		}
 		assert.ElementsMatch(t, []string{"alpha", "beta", "gamma"}, finalKeys)
 	})
+
+	t.Run("map decoration across modules", func(t *testing.T) {
+		t.Parallel()
+
+		type DecorationInput struct {
+			fx.In
+			Processors map[string]testProcessor `group:"processors"`
+		}
+
+		type DecorationOutput struct {
+			fx.Out
+			Processors map[string]testProcessor `group:"processors"`
+		}
+
+		var outerProcessors map[string]testProcessor
+		var innerProcessors map[string]testProcessor
+
+		app := fxtest.New(t,
+			fx.Provide(
+				fx.Annotate(
+					func() testProcessor { return &testBasicProcessor{name: "auth"} },
+					fx.ResultTags(`name:"auth" group:"processors"`),
+				),
+				fx.Annotate(
+					func() testProcessor { return &testBasicProcessor{name: "billing"} },
+					fx.ResultTags(`name:"billing" group:"processors"`),
+				),
+			),
+			fx.Decorate(func(input DecorationInput) DecorationOutput {
+				enhanced := make(map[string]testProcessor)
+				for name, processor := range input.Processors {
+					enhanced[name] = &testEnhancedProcessor{
+						wrapped: processor,
+						prefix:  "[OUTER]",
+					}
+				}
+				return DecorationOutput{Processors: enhanced}
+			}),
+			fx.Invoke(fx.Annotate(
+				func(processors map[string]testProcessor) {
+					outerProcessors = processors
+				},
+				fx.ParamTags(`group:"processors"`),
+			)),
+			fx.Module("mymodule",
+				fx.Decorate(func(input DecorationInput) DecorationOutput {
+					enhanced := make(map[string]testProcessor)
+					for name, processor := range input.Processors {
+						enhanced[name] = &testEnhancedProcessor{
+							wrapped: processor,
+							prefix:  "[INNER]",
+						}
+					}
+					return DecorationOutput{Processors: enhanced}
+				}),
+				fx.Invoke(fx.Annotate(
+					func(processors map[string]testProcessor) {
+						innerProcessors = processors
+					},
+					fx.ParamTags(`group:"processors"`),
+				)),
+			),
+		)
+		defer app.RequireStart().RequireStop()
+
+		t.Logf("Outer processors:")
+		for name, p := range outerProcessors {
+			t.Logf("  %s: %s", name, p.Process("data"))
+		}
+		t.Logf("Inner processors:")
+		for name, p := range innerProcessors {
+			t.Logf("  %s: %s", name, p.Process("data"))
+		}
+
+		// Test that map decoration chains across modules
+		require.Len(t, outerProcessors, 2)
+		assert.Equal(t, "[OUTER] auth: data", outerProcessors["auth"].Process("data"))
+		assert.Equal(t, "[OUTER] billing: data", outerProcessors["billing"].Process("data"))
+
+		require.Len(t, innerProcessors, 2)
+		assert.Equal(t, "[INNER] [OUTER] auth: data", innerProcessors["auth"].Process("data"))
+		assert.Equal(t, "[INNER] [OUTER] billing: data", innerProcessors["billing"].Process("data"))
+	})
 }