oracle: describe: fix crash on bad ImportSpec

+ Test

Fixes issue #13380

Change-Id: I6d738756017f83a296bde6a04845cd985a351409
Reviewed-on: https://go-review.googlesource.com/18210
Reviewed-by: Michael Matloob <matloob@golang.org>

oracle/describe.go

@@ -513,11 +513,15 @@ func describePackage(qpos *queryPos, path []ast.Node) (*describePackageResult, e
 	var pkg *types.Package
 	switch n := path[0].(type) {
 	case *ast.ImportSpec:
-		var pkgname *types.PkgName
+		var obj types.Object
 		if n.Name != nil {
-			pkgname = qpos.info.Defs[n.Name].(*types.PkgName)
+			obj = qpos.info.Defs[n.Name]
-		} else if p := qpos.info.Implicits[n]; p != nil {
+		} else {
-			pkgname = p.(*types.PkgName)
+			obj = qpos.info.Implicits[n]
+		}
+		pkgname, _ := obj.(*types.PkgName)
+		if pkgname == nil {
+			return nil, fmt.Errorf("can't import package %s", n.Path.Value)
 		}
 		pkg = pkgname.Imported()
 		description = fmt.Sprintf("import of package %q", pkg.Path())

oracle/testdata/src/describe/main.go

@@ -6,6 +6,14 @@ package describe // @describe pkgdecl "describe"
 
 // TODO(adonovan): more coverage of the (extensive) logic.
 
+import (
+	"nosuchpkg"            // @describe badimport1 "nosuchpkg"
+	nosuchpkg2 "nosuchpkg" // @describe badimport2 "nosuchpkg2"
+)
+
+var _ nosuchpkg.T
+var _ nosuchpkg2.T
+
 type cake float64 // @describe type-ref-builtin "float64"
 
 const c = iota // @describe const-ref-iota "iota"

oracle/testdata/src/describe/main.golden

@@ -13,6 +13,12 @@ definition of package "describe"
 	const pi     untyped float = 3141/1000
 	const pie    cake = 1768225803696341/562949953421312
 
+-------- @describe badimport1 --------
+
+Error: can't import package "nosuchpkg"
+-------- @describe badimport2 --------
+
+Error: can't import package "nosuchpkg"
 -------- @describe type-ref-builtin --------
 reference to built-in type float64