go/types: add test for imported constraints pre-1.18

This is a port of CL 363834 from types2 to go/types.

Change-Id: I32583ead4bce626e0761f4c327678050404a15c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/364937
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/check_test.go

@@ -31,6 +31,7 @@ import (
 	"go/parser"
 	"go/scanner"
 	"go/token"
+	"internal/buildcfg"
 	"internal/testenv"
 	"os"
 	"path/filepath"
@@ -199,11 +200,27 @@ func asGoVersion(s string) string {
 	return ""
 }
 
+// excludedForUnifiedBuild lists files that cannot be tested
+// when using the unified build's export data.
+// TODO(gri) enable as soon as the unified build supports this.
+var excludedForUnifiedBuild = map[string]bool{
+	"issue47818.go2": true,
+}
+
 func testFiles(t *testing.T, sizes Sizes, filenames []string, srcs [][]byte, manual bool, imp Importer) {
 	if len(filenames) == 0 {
 		t.Fatal("no source files")
 	}
 
+	if buildcfg.Experiment.Unified {
+		for _, f := range filenames {
+			if excludedForUnifiedBuild[filepath.Base(f)] {
+				t.Logf("%s cannot be tested with unified build - skipped", f)
+				return
+			}
+		}
+	}
+
 	if strings.HasSuffix(filenames[0], ".go1") {
 		// TODO(rfindley): re-enable this test by using GoVersion.
 		t.Skip("type params are enabled")

src/go/types/testdata/fixedbugs/issue47818.go2

@@ -8,6 +8,8 @@
 
 package go1_17
 
+import "constraints"
+
 type T[P /* ERROR type parameters require go1\.18 or later */ any /* ERROR undeclared name: any \(requires version go1\.18 or later\) */ ] struct{}
 
 // for init (and main, but we're not in package main) we should only get one error
@@ -56,4 +58,4 @@ type (
 	_ = C2
 )
 
-// TODO(gri) need test cases for imported constraint types (see also issue #47967)
+type Ordered constraints /* ERROR using type constraint constraints\.Ordered requires go1\.18 or later */ .Ordered