cmd/go/internal/modcmd: error out if one module with two different paths

If a single module is imported via two different paths, go mod tidy
should have reported this error instead of deferring it until go build.

Fixes #34650.

Change-Id: I9d09df1551b3e2083ed9f0bc77f2989073057717
Reviewed-on: https://go-review.googlesource.com/c/go/+/199598
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -211,11 +211,17 @@ func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 
 	// One last pass to finalize wildcards.
 	updateMatches(matches, false)
+	checkMultiplePaths()
+	WriteGoMod()
 
-	// 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.
+	return matches
+}
+
+// checkMultiplePaths verifies that a given module path is used as itself
+// or as a replacement for another module, but not both at the same time.
+//
+// (See https://golang.org/issue/26607 and https://golang.org/issue/34650.)
+func checkMultiplePaths() {
 	firstPath := make(map[module.Version]string, len(buildList))
 	for _, mod := range buildList {
 		src := mod
@@ -229,9 +235,6 @@ func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 		}
 	}
 	base.ExitIfErrors()
-	WriteGoMod()
-
-	return matches
 }
 
 // pathInModuleCache returns the import path of the directory dir,
@@ -383,6 +386,7 @@ func loadAll(testAll bool) []string {
 	}
 	all := TargetPackages("...")
 	loaded.load(func() []string { return all })
+	checkMultiplePaths()
 	WriteGoMod()
 
 	var paths []string

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

@@ -47,6 +47,12 @@ grep 'rsc.io/sampler v1.2.0' go.mod
 cd outside
 go list -m all
 stdout 'rsc.io/sampler v1.3.0'
+cd ..
+
+# The same module can't be used as two different paths.
+cd multiple-paths
+! go mod tidy
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
 
 -- go.mod --
 module example.com/tidy
@@ -109,3 +115,23 @@ package b
 module golang.org/issue/30166/b
 
 require golang.org/issue/30166/a v0.0.0
+-- multiple-paths/main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+-- multiple-paths/go.mod --
+module quoter
+
+require (
+	rsc.io/quote/v3 v3.0.0
+	not-rsc.io/quote/v3 v3.0.0
+)
+
+replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0

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

@@ -21,6 +21,11 @@ stdout '.*[/\\]vendor[/\\]rsc.io[/\\]quote[/\\]v3'
 ! stderr 'finding'
 ! stderr 'lookup disabled'
 
+# The same module can't be used as two different paths.
+cd multiple-paths
+! go mod vendor
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
+
 -- go.mod --
 module example.com/replace
 
@@ -37,3 +42,20 @@ module not-rsc.io/quote/v3
 
 -- local/not-rsc.io/quote/v3/quote.go --
 package quote
+
+-- multiple-paths/main.go --
+package main
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+func main() {
+	fmt.Println(quote.GoV3())
+}
+-- multiple-paths/go.mod --
+module quoter
+require (
+	rsc.io/quote/v3 v3.0.0
+	not-rsc.io/quote/v3 v3.0.0
+)
+replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0