diff --git a/src/cmd/go/testdata/script/mod_gobuild_import.txt b/src/cmd/go/testdata/script/mod_gobuild_import.txt
index a4eb5d6..ae05250 100644
--- a/src/cmd/go/testdata/script/mod_gobuild_import.txt
+++ b/src/cmd/go/testdata/script/mod_gobuild_import.txt
@@ -62,15 +62,31 @@
 	"go/build"
 	"log"
 	"os"
+	"path/filepath"
 	"strings"
 )
 
 func main() {
-	p, err := build.Import(os.Args[1], os.Args[2], 0)
+	// build.Import should support relative and absolute source dir paths.
+	path := os.Args[1]
+	srcDir := os.Args[2]
+	p1, err := build.Import(path, srcDir, 0)
 	if err != nil {
 		log.Fatal(err)
 	}
-	fmt.Printf("%s\n%s\n", p.Dir, strings.Join(p.GoFiles, " "))
+	absSrcDir, err := filepath.Abs(srcDir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	p2, err := build.Import(path, absSrcDir, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if p1.Dir != p2.Dir {
+		log.Fatalf("different packages loaded with relative and absolute paths:\n\t%s\n\t%s", p1.Dir, p2.Dir)
+	}
+
+	fmt.Printf("%s\n%s\n", p1.Dir, strings.Join(p1.GoFiles, " "))
 }
 
 -- $GOPATH/other/go.mod --
diff --git a/src/go/build/build.go b/src/go/build/build.go
index a91551b..f854760 100644
--- a/src/go/build/build.go
+++ b/src/go/build/build.go
@@ -1001,27 +1001,25 @@
 		return errNoModules
 	}
 
+	// Find the absolute source directory. hasSubdir does not handle
+	// relative paths (and can't because the callbacks don't support this).
+	absSrcDir, err := filepath.Abs(srcDir)
+	if err != nil {
+		return errNoModules
+	}
+
 	// If modules are not enabled, then the in-process code works fine and we should keep using it.
-	// TODO(bcmills): This assumes that the default is "auto" instead of "on".
 	switch os.Getenv("GO111MODULE") {
 	case "off":
 		return errNoModules
-	case "on":
-		// ok
-	default: // "", "auto", anything else
-		// Automatic mode: no module use in $GOPATH/src.
-		for _, root := range gopath {
-			sub, ok := ctxt.hasSubdir(root, srcDir)
-			if ok && strings.HasPrefix(sub, "src/") {
-				return errNoModules
-			}
-		}
+	default: // "", "on", "auto", anything else
+		// Maybe use modules.
 	}
 
 	// If the source directory is in GOROOT, then the in-process code works fine
 	// and we should keep using it. Moreover, the 'go list' approach below doesn't
 	// take standard-library vendoring into account and will fail.
-	if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), srcDir); ok {
+	if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), absSrcDir); ok {
 		return errNoModules
 	}
 
@@ -1034,20 +1032,18 @@
 	}
 
 	// Look to see if there is a go.mod.
-	abs, err := filepath.Abs(srcDir)
-	if err != nil {
-		return errNoModules
-	}
+	// Since go1.13, it doesn't matter if we're inside GOPATH.
+	parent := absSrcDir
 	for {
-		info, err := os.Stat(filepath.Join(abs, "go.mod"))
+		info, err := os.Stat(filepath.Join(parent, "go.mod"))
 		if err == nil && !info.IsDir() {
 			break
 		}
-		d := filepath.Dir(abs)
-		if len(d) >= len(abs) {
+		d := filepath.Dir(parent)
+		if len(d) >= len(parent) {
 			return errNoModules // reached top of file system, no go.mod
 		}
-		abs = d
+		parent = d
 	}
 
 	cmd := exec.Command("go", "list", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n", path)