diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index f077708..68e0b65 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -5,7 +5,6 @@
 package modload
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"go/build"
@@ -52,6 +51,41 @@
 	return e.QueryErr
 }
 
+// An AmbiguousImportError indicates an import of a package found in multiple
+// modules in the build list, or found in both the main module and its vendor
+// directory.
+type AmbiguousImportError struct {
+	ImportPath string
+	Dirs       []string
+	Modules    []module.Version // Either empty or 1:1 with Dirs.
+}
+
+func (e *AmbiguousImportError) Error() string {
+	locType := "modules"
+	if len(e.Modules) == 0 {
+		locType = "directories"
+	}
+
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.ImportPath, locType)
+
+	for i, dir := range e.Dirs {
+		buf.WriteString("\n\t")
+		if i < len(e.Modules) {
+			m := e.Modules[i]
+			buf.WriteString(m.Path)
+			if m.Version != "" {
+				fmt.Fprintf(&buf, " %s", m.Version)
+			}
+			fmt.Fprintf(&buf, " (%s)", dir)
+		} else {
+			buf.WriteString(dir)
+		}
+	}
+
+	return buf.String()
+}
+
 // Import finds the module and directory in the build list
 // containing the package with the given import path.
 // The answer must be unique: Import returns an error
@@ -96,7 +130,7 @@
 		mainDir, mainOK := dirInModule(path, targetPrefix, ModRoot(), true)
 		vendorDir, vendorOK := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
 		if mainOK && vendorOK {
-			return module.Version{}, "", fmt.Errorf("ambiguous import: found %s in multiple directories:\n\t%s\n\t%s", path, mainDir, vendorDir)
+			return module.Version{}, "", &AmbiguousImportError{ImportPath: path, Dirs: []string{mainDir, vendorDir}}
 		}
 		// Prefer to return main directory if there is one,
 		// Note that we're not checking that the package exists.
@@ -136,16 +170,7 @@
 		return mods[0], dirs[0], nil
 	}
 	if len(mods) > 0 {
-		var buf bytes.Buffer
-		fmt.Fprintf(&buf, "ambiguous import: found %s in multiple modules:", path)
-		for i, m := range mods {
-			fmt.Fprintf(&buf, "\n\t%s", m.Path)
-			if m.Version != "" {
-				fmt.Fprintf(&buf, " %s", m.Version)
-			}
-			fmt.Fprintf(&buf, " (%s)", dirs[i])
-		}
-		return module.Version{}, "", errors.New(buf.String())
+		return module.Version{}, "", &AmbiguousImportError{ImportPath: path, Dirs: dirs, Modules: mods}
 	}
 
 	// Look up module containing the package, for addition to the build list.
diff --git a/src/cmd/go/testdata/script/mod_ambiguous_import.txt b/src/cmd/go/testdata/script/mod_ambiguous_import.txt
new file mode 100644
index 0000000..9f9669c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_ambiguous_import.txt
@@ -0,0 +1,49 @@
+env GO111MODULE=on
+
+cd $WORK
+
+# An import provided by two different modules should be flagged as an error.
+! go build ./importx
+stderr '^importx[/\\]importx.go:2:8: ambiguous import: found package example.com/a/x in multiple modules:\n\texample.com/a v0.1.0 \('$WORK'[/\\]a[/\\]x\)\n\texample.com/a/x v0.1.0 \('$WORK'[/\\]ax\)$'
+
+# However, it should not be an error if that import is unused.
+go build ./importy
+
+# An import provided by both the main module and the vendor directory
+# should be flagged as an error only when -mod=vendor is set.
+# TODO: This error message is a bit redundant.
+mkdir vendor/example.com/m/importy
+cp $WORK/importy/importy.go vendor/example.com/m/importy/importy.go
+go build example.com/m/importy
+! go build -mod=vendor example.com/m/importy
+stderr '^can.t load package: package example.com/m/importy: ambiguous import: found package example.com/m/importy in multiple directories:\n\t'$WORK'[/\\]importy\n\t'$WORK'[/\\]vendor[/\\]example.com[/\\]m[/\\]importy$'
+
+-- $WORK/go.mod --
+module example.com/m
+go 1.14
+require (
+	example.com/a v0.1.0
+	example.com/a/x v0.1.0
+)
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/a/x v0.1.0 => ./ax
+)
+-- $WORK/importx/importx.go --
+package importx
+import _ "example.com/a/x"
+-- $WORK/importy/importy.go --
+package importy
+import _ "example.com/a/y"
+-- $WORK/a/go.mod --
+module example.com/a
+go 1.14
+-- $WORK/a/x/x.go --
+package x
+-- $WORK/a/y/y.go --
+package y
+-- $WORK/ax/go.mod --
+module example.com/a/x
+go 1.14
+-- $WORK/ax/x.go --
+package x