go.tools/go/types: use qualified names when printing types in errors

Also: Require that clean package paths are not ".".

R=adonovan, r
CC=golang-dev
https://golang.org/cl/10368047

go/types/api.go

@@ -138,6 +138,8 @@ type Importer func(imports map[string]*Package, path string) (pkg *Package, err
 // entire package is checked. If there are errors, the package may be
 // only partially type-checked, and the resulting package may be incomplete
 // (missing objects, imports, etc.).
+// The provided package path must not resolve to ".", otherwise Check
+// returns immediately with a corresponding error message.
 func (ctxt *Context) Check(path string, fset *token.FileSet, files ...*ast.File) (*Package, error) {
 	return check(ctxt, path, fset, files...)
 }

go/types/check.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"go/ast"
 	"go/token"
+	"path"
 
 	"code.google.com/p/go.tools/go/exact"
 )
@@ -83,9 +84,9 @@ func (check *checker) later(f *Func, sig *Signature, body *ast.BlockStmt) {
 // A bailout panic is raised to indicate early termination.
 type bailout struct{}
 
-func check(ctxt *Context, path string, fset *token.FileSet, files ...*ast.File) (pkg *Package, err error) {
+func check(ctxt *Context, pkgPath string, fset *token.FileSet, files ...*ast.File) (pkg *Package, err error) {
 	pkg = &Package{
-		path:    path,
+		path:    pkgPath,
 		scope:   NewScope(Universe),
 		imports: make(map[string]*Package),
 	}
@@ -117,6 +118,12 @@ func check(ctxt *Context, path string, fset *token.FileSet, files ...*ast.File)
 		}
 	}()
 
+	// we need a reasonable path to continue
+	if path.Clean(pkgPath) == "." {
+		check.errorf(token.NoPos, "Check invoked with invalid package path: %q", pkgPath)
+		return
+	}
+
 	// determine package name and files
 	i := 0
 	for _, file := range files {

go/types/errors.go

@@ -320,9 +320,13 @@ func writeType(buf *bytes.Buffer, typ Type) {
 	case *Named:
 		s := "<Named w/o object>"
 		if obj := t.obj; obj != nil {
-			if obj.pkg != nil && obj.pkg.path != "" {
-				buf.WriteString(obj.pkg.path)
-				buf.WriteString(".")
+			if obj.pkg != nil {
+				// TODO(gri) Ideally we only want the qualification
+				// if we are referring to a type that was imported;
+				// but not when we are at the "top". We don't have
+				// this information easily available here.
+				buf.WriteString(obj.pkg.name)
+				buf.WriteByte('.')
 			}
 			s = t.obj.name
 		}

go/types/types_test.go

@@ -20,7 +20,8 @@ func makePkg(t *testing.T, src string) (*Package, error) {
 	if err != nil {
 		return nil, err
 	}
-	pkg, err := Check("", fset, file)
+	// use the package name as package path
+	pkg, err := Check(file.Name.Name, fset, file)
 	return pkg, err
 }
 
@@ -56,7 +57,7 @@ var testTypes = []testEntry{
 	{`struct {
 		string
 		elems []T
-	}`, `struct{string; elems []T}`},
+	}`, `struct{string; elems []p.T}`},
 
 	// pointers
 	dup("*int"),