cmd/go: forbid use of one module with two different paths

If a single module is imported via two different paths (e.g., as itself and as a
replacement for something else), some users may be surprised if the two paths
do not share the same package-level state. Others may be surprised if the two
paths do share state.

Punt on the question for now by rejecting that condition explicitly.

Fixes #26607.

Change-Id: I15c3889f61f8dd4ba5e5c48ca33ad63aeecac04e
Reviewed-on: https://go-review.googlesource.com/126156
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modload/load.go

@@ -115,6 +115,25 @@ func ImportPaths(args []string) []string {
 		}
 		return roots
 	})
+
+	// A given module path may be used as itself or as a replacement for another
+	// module, but not both at the same time. Otherwise, the aliasing behavior is
+	// too subtle (see https://golang.org/issue/26607), and we don't want to
+	// commit to a specific behavior at this point.
+	firstPath := make(map[module.Version]string, len(buildList))
+	for _, mod := range buildList {
+		src := mod
+		if rep := Replacement(mod); rep.Path != "" {
+			src = rep
+		}
+		if prev, ok := firstPath[src]; !ok {
+			firstPath[src] = mod.Path
+		} else if prev != mod.Path {
+			base.Errorf("go: %s@%s used for two different module paths (%s and %s)", mod.Path, mod.Version, prev, mod.Path)
+		}
+	}
+	base.ExitIfErrors()
+
 	WriteGoMod()
 
 	// Process paths to produce final paths list.

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

+env GO111MODULE=on
+
+go build -o a1.exe .
+exec ./a1.exe
+stdout 'Don''t communicate by sharing memory'
+
+# Modules can be replaced by local packages.
+go mod -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
+go build -o a2.exe .
+exec ./a2.exe
+stdout 'Concurrency is not parallelism.'
+
+# The module path of the replacement doesn't need to match.
+# (For example, it could be a long-running fork with its own import path.)
+go mod -replace=rsc.io/quote/v3=./local/not-rsc.io/quote/v3
+go build -o a3.exe .
+exec ./a3.exe
+stdout 'Clear is better than clever.'
+
+# However, the same module can't be used as two different paths.
+go mod -dropreplace=rsc.io/quote/v3
+go mod -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0
+go mod -require=not-rsc.io/quote/v3@v3.0.0
+! go build -o a4.exe .
+
+
+-- go.mod --
+module quoter
+
+require rsc.io/quote/v3 v3.0.0
+
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+
+-- local/rsc.io/quote/v3/go.mod --
+module rsc.io/quote/v3
+
+require rsc.io/sampler v1.3.0
+
+-- local/rsc.io/quote/v3/quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV3() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV3() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a REPLACED Go proverb.
+func GoV3() string {
+	return "Concurrency is not parallelism."
+}
+
+// Opt returns a optimization truth.
+func OptV3() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+
+-- local/not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+-- local/not-rsc.io/quote/v3/quote.go --
+package quote
+
+func GoV3() string {
+	return "Clear is better than clever."
+}