cmd/go/internal/modload: follow dependencies of unpruned roots in dqTracker.path

For #65363.

Change-Id: I82ae1098b00c8772ef8d3aa92197e7d8c66d1b37
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/571800
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/internal/modload/edit.go

@@ -842,6 +842,12 @@ func (t *dqTracker) check(m module.Version, pruning modPruning) dqState {
 // If m is not disqualified, path returns (nil, nil).
 func (t *dqTracker) path(m module.Version, pruning modPruning) (path []module.Version, err error) {
 	for {
+		if rootPruning, isRoot := t.extendedRootPruning[m]; isRoot && rootPruning == unpruned {
+			// Since m is a root, any other module that requires it would cause
+			// its full unpruned dependencies to be included in the module graph.
+			// Those dependencies must also be considered as part of the path to the conflict.
+			pruning = unpruned
+		}
 		dq := t.dqReason[m].from(pruning)
 		if !dq.isDisqualified() {
 			return path, nil

src/cmd/go/testdata/script/mod_get_issue65363.txt

@@ -0,0 +1,77 @@
+! go get -v example.net/a@v0.1.0
+! stderr panic
+stderr 'example.net/d@v0.1.0 requires\n\texample.net/invalid'
+
+-- go.mod --
+module example
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/b v0.1.0 => ./b1
+	example.net/b v0.2.0 => ./b2
+	example.net/c v0.1.0 => ./c1
+	example.net/c v0.2.0 => ./c2
+	example.net/d v0.1.0 => ./d
+)
+
+require (
+	example.net/b v0.1.0
+)
+-- a/go.mod --
+module example.net/a
+
+go 1.18
+
+require example.net/b v0.2.0
+-- a/a.go --
+package a
+
+import _ "example.net/b"
+-- b1/go.mod --
+module example.net/b
+
+go 1.16
+-- b1/b.go --
+package b
+-- b2/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/c v0.2.0
+-- b2/b.go --
+package b
+-- b2/b_test.go --
+package b_test
+
+import _ "example.net/c"
+-- c1/go.mod --
+module example.net/c
+
+go 1.18
+-- c1/c.go --
+package c
+-- c2/go.mod --
+module example.net/c
+
+go 1.18
+
+require example.net/d v0.1.0
+-- c2/c.go --
+package c
+-- c2/c_test.go --
+package c_test
+
+import _ "example.net/d"
+-- d/go.mod --
+module example.net/d
+
+go 1.18
+
+require example.net/invalid v0.1.0
+-- d/d.go --
+package d
+-- d/d_test.go --
+package d
+
+import _ "example.net/invalid"