cmd/go: retain sums for replacement modules in 'go mod tidy'

Fixes #27868

Change-Id: I6c2d221c4325a2f44625e797a82735d812ee0ec1
Reviewed-on: https://go-review.googlesource.com/c/153817
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modcmd/tidy.go

@@ -77,7 +77,17 @@ func modTidyGoSum() {
 	keep := make(map[module.Version]bool)
 	var walk func(module.Version)
 	walk = func(m module.Version) {
-		keep[m] = true
+		// If we build using a replacement module, keep the sum for the replacement,
+		// since that's the code we'll actually use during a build.
+		//
+		// TODO(golang.org/issue/29182): Perhaps we should keep both sums, and the
+		// sums for both sets of transitive requirements.
+		r := modload.Replacement(m)
+		if r.Path == "" {
+			keep[m] = true
+		} else {
+			keep[r] = true
+		}
 		list, _ := reqs.Required(m)
 		for _, r := range list {
 			if !keep[r] {

src/cmd/go/testdata/script/mod_sum_replaced.txt

@@ -0,0 +1,28 @@
+env GO111MODULE=on
+
+# After 'go get -d', the go.sum file should contain the sum for the module.
+go get -d rsc.io/quote@v1.5.0
+grep 'rsc.io/quote v1.5.0' go.sum
+
+# If we replace the module and run 'go mod tidy', we should get a sum for the replacement.
+go mod edit -replace rsc.io/quote@v1.5.0=rsc.io/quote@v1.5.1
+go mod tidy
+grep 'rsc.io/quote v1.5.1' go.sum
+cp go.sum go.sum.tidy
+
+# 'go mod vendor' should preserve that sum, and should not need to add any new entries.
+go mod vendor
+grep 'rsc.io/quote v1.5.1' go.sum
+cmp go.sum go.sum.tidy
+
+-- go.mod --
+module golang.org/issue/27868
+
+require rsc.io/quote v1.5.0
+
+-- main.go --
+package main
+
+import _ "rsc.io/quote"
+
+func main() {}