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>

1 files changed, 11 insertions, 7 deletions

diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 5f6fd672ba..a9f711733c 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/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()
+
+	return matches
+}
 
-	// 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.
+// 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