[dev.typeparams] cmd/compile: use existing findpkg algorithm when importing through types2

Change-Id: I9044de7829d22addb5bc570401508082e3f007eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/269057
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -95,6 +95,13 @@ func parseFiles(filenames []string, allowGenerics bool) (lines uint) {
 			},
 			Importer: &gcimports{
 				packages: make(map[string]*types2.Package),
+				lookup: func(path string) (io.ReadCloser, error) {
+					file, ok := findpkg(path)
+					if !ok {
+						return nil, fmt.Errorf("can't find import: %q", path)
+					}
+					return os.Open(file)
+				},
 			},
 		}
 		conf.Check(Ctxt.Pkgpath, files, nil)

test/typeparam/importtest.go

@@ -0,0 +1,16 @@
+// compile -G
+
+// Copyright 2020 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.
+
+// This file checks that basic importing works in -G mode.
+
+package p
+
+import "fmt"
+import "math"
+
+func f(x float64) {
+	fmt.Println(math.Sin(x))
+}