cmd/compile: fix error msg mentioning different packages with same name

This is a regression from 1.6. The respective code in importimport
(export.go) was not exactly replicated with the new importer. Also
copied over the missing cyclic import check.

Added test cases.

Fixes #16133.

Change-Id: I1e0a39ff1275ca62a8054874294d400ed83fb26a
Reviewed-on: https://go-review.googlesource.com/24312Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

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

@@ -239,14 +239,20 @@ func (p *importer) pkg() *Pkg {
 		Fatalf("importer: package path %q for pkg index %d", path, len(p.pkgList))
 	}
 
+	// see importimport (export.go)
 	pkg := importpkg
 	if path != "" {
 		pkg = mkpkg(path)
 	}
 	if pkg.Name == "" {
 		pkg.Name = name
+		numImport[name]++
 	} else if pkg.Name != name {
-		Fatalf("importer: conflicting package names %s and %s for path %q", pkg.Name, name, path)
+		Yyerror("importer: conflicting package names %s and %s for path %q", pkg.Name, name, path)
+	}
+	if incannedimport == 0 && myimportpath != "" && path == myimportpath {
+		Yyerror("import %q: package depends on %q (import cycle)", importpkg.Path, path)
+		errorexit()
 	}
 	p.pkgList = append(p.pkgList, pkg)
 

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

@@ -479,6 +479,10 @@ func pkgtype(s *Sym) *Type {
 	return s.Def.Type
 }
 
+// numImport tracks how often a package with a given name is imported.
+// It is used to provide a better error message (by using the package
+// path to disambiguate) if a package that appears multiple times with
+// the same name appears in an error message.
 var numImport = make(map[string]int)
 
 func importimport(s *Sym, path string) {

test/fixedbugs/issue16133.dir/a1.go

+package a
+
+type X string
+
+func NewX() X {
+	return ""
+}

test/fixedbugs/issue16133.dir/a2.go

+package a
+
+type X string
+
+func NewX() X {
+	return ""
+}

test/fixedbugs/issue16133.dir/b.go

+package b
+
+import "./a2"
+
+type T struct {
+	X a.X
+}

test/fixedbugs/issue16133.dir/c.go

+package p
+
+import (
+	"./a1"
+	"./b"
+)
+
+var _ = b.T{
+	X: a.NewX(), // ERROR `cannot use "a1"\.NewX\(\)`
+}

test/fixedbugs/issue16133.go

+// errorcheckdir -s
+
+// Copyright 2016 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 error messages referring to multiple different
+// packages with the same package name.
+
+package ignored