cmd/compile: better error for embedded field referring to missing import

Fixes #27938.

Change-Id: I16263ac6c0b8903b8a16f02e8db0e1a16d1c95b4
Reviewed-on: https://go-review.googlesource.com/c/144261
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/noder.go

@@ -819,13 +819,18 @@ func (p *noder) packname(expr syntax.Expr) *types.Sym {
 		return name
 	case *syntax.SelectorExpr:
 		name := p.name(expr.X.(*syntax.Name))
+		def := asNode(name.Def)
+		if def == nil {
+			yyerror("undefined: %v", name)
+			return name
+		}
 		var pkg *types.Pkg
-		if asNode(name.Def) == nil || asNode(name.Def).Op != OPACK {
+		if def.Op != OPACK {
 			yyerror("%v is not a package", name)
 			pkg = localpkg
 		} else {
-			asNode(name.Def).Name.SetUsed(true)
+			def.Name.SetUsed(true)
-			pkg = asNode(name.Def).Name.Pkg
+			pkg = def.Name.Pkg
 		}
 		return restrictlookup(expr.Sel.Value, pkg)
 	}

test/fixedbugs/issue27938.go

@@ -0,0 +1,23 @@
+// errorcheck
+
+// 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.
+
+// Verify that we get a single non-confusing error
+// message for embedded fields/interfaces that use
+// a qualified identifier with non-existing package.
+
+package p
+
+type _ struct {
+	F sync.Mutex // ERROR "undefined: sync"
+}
+
+type _ struct {
+	sync.Mutex // ERROR "undefined: sync"
+}
+
+type _ interface {
+	sync.Mutex // ERROR "undefined: sync"
+}