From d89fcd9b2cc3783fa6bf0aef4dca3fd3c169c29b Mon Sep 17 00:00:00 2001
From: cce <51567+cce@users.noreply.github.com>
Date: Fri, 9 Oct 2026 15:04:01 -0400
Subject: [PATCH 1/2] tests: regenerate anydecl_gen.go after array-decode
 removal

anydecl_gen.go was generated by #39 before #37 landed, so it still contained the struct-from-array decode fallback.
---
 tests/anydecl_gen.go | 65 ++++++++++++--------------------------------
 1 file changed, 17 insertions(+), 48 deletions(-)

diff --git a/tests/anydecl_gen.go b/tests/anydecl_gen.go
index 1f79685..948d94c 100644
--- a/tests/anydecl_gen.go
+++ b/tests/anydecl_gen.go
@@ -66,70 +66,39 @@ func (z *AnyDeclNeighbor) UnmarshalMsgWithState(bts []byte, st msgp.UnmarshalSta
 	var zb0001 int
 	var zb0002 bool
 	zb0001, zb0002, bts, err = msgp.ReadMapHeaderBytes(bts)
-	if _, ok := err.(msgp.TypeError); ok {
-		zb0001, zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)
+	if err != nil {
+		err = msgp.WrapError(err)
+		return
+	}
+	if zb0002 {
+		(*z) = AnyDeclNeighbor{}
+	}
+	for zb0001 > 0 {
+		zb0001--
+		field, bts, err = msgp.ReadMapKeyZC(bts)
 		if err != nil {
 			err = msgp.WrapError(err)
 			return
 		}
-		if zb0001 > 0 {
-			zb0001--
+		switch string(field) {
+		case "n":
 			(*z).Name, bts, err = msgp.ReadStringBytes(bts)
 			if err != nil {
-				err = msgp.WrapError(err, "struct-from-array", "Name")
+				err = msgp.WrapError(err, "Name")
 				return
 			}
-		}
-		if zb0001 > 0 {
-			zb0001--
+		case "c":
 			(*z).Count, bts, err = msgp.ReadUint64Bytes(bts)
 			if err != nil {
-				err = msgp.WrapError(err, "struct-from-array", "Count")
+				err = msgp.WrapError(err, "Count")
 				return
 			}
-		}
-		if zb0001 > 0 {
-			err = msgp.ErrTooManyArrayFields(zb0001)
-			if err != nil {
-				err = msgp.WrapError(err, "struct-from-array")
-				return
-			}
-		}
-	} else {
-		if err != nil {
-			err = msgp.WrapError(err)
-			return
-		}
-		if zb0002 {
-			(*z) = AnyDeclNeighbor{}
-		}
-		for zb0001 > 0 {
-			zb0001--
-			field, bts, err = msgp.ReadMapKeyZC(bts)
+		default:
+			err = msgp.ErrNoField(string(field))
 			if err != nil {
 				err = msgp.WrapError(err)
 				return
 			}
-			switch string(field) {
-			case "n":
-				(*z).Name, bts, err = msgp.ReadStringBytes(bts)
-				if err != nil {
-					err = msgp.WrapError(err, "Name")
-					return
-				}
-			case "c":
-				(*z).Count, bts, err = msgp.ReadUint64Bytes(bts)
-				if err != nil {
-					err = msgp.WrapError(err, "Count")
-					return
-				}
-			default:
-				err = msgp.ErrNoField(string(field))
-				if err != nil {
-					err = msgp.WrapError(err)
-					return
-				}
-			}
 		}
 	}
 	o = bts

From 54e8a239430e22080a6fe4f5b36cbd2a3832ed1a Mon Sep 17 00:00:00 2001
From: cce <51567+cce@users.noreply.github.com>
Date: Fri, 9 Oct 2026 15:04:09 -0400
Subject: [PATCH 2/2] CI: fail if tests/ generated code is out of date

Add a make test-gen target that regenerates tests/ with the msgp built from the tree, and have the Build workflow run it and fail if the working tree is dirty afterwards.
---
 .github/workflows/build.yml |  9 +++++++++
 Makefile                    | 10 +++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 23489b8..552a325 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -12,3 +12,12 @@ jobs:
         with:
           go-version-file: go.mod
       - run: make test
+      - name: Check tests/ generated code is up to date
+        run: |
+          make test-gen
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "::error::Generated code is out of date; run 'make test-gen' and commit the result."
+            git status --porcelain
+            git diff
+            exit 1
+          fi
diff --git a/Makefile b/Makefile
index f601ee0..51c6ea6 100644
--- a/Makefile
+++ b/Makefile
@@ -11,7 +11,7 @@ SHELL := /bin/bash
 
 BIN = $(GOBIN)/msgp
 
-.PHONY: clean wipe install get-deps bench all
+.PHONY: clean wipe install get-deps bench all test-gen
 
 $(BIN): */*.go
 	@go install ./...
@@ -25,6 +25,14 @@ test: all
 	go test -covermode=atomic -coverprofile=cover.out ./...
 	cd tests && go test ./...
 
+# Regenerate tests/*_gen*.go with the msgp built from this tree, rather than
+# whatever msgp happens to be on PATH. CI runs this and fails on any diff.
+test-gen:
+	@bindir=$$(mktemp -d) && \
+	go build -o "$$bindir/msgp" . && \
+	(cd tests && PATH="$$bindir:$$PATH" go generate ./...); \
+	status=$$?; rm -rf "$$bindir"; exit $$status
+
 bench: all
 	go test -bench ./...