cmd/compile: ensure generic function is loaded when it needs to be re-exported

In the case where we need to re-export a generic function/method from
another package in the export data of the current package, make sure it
is loaded before trying to write it out.

Fixed #49667

Change-Id: I177754bb762689f34cf5c8ad246d43f1cdbbf195
Reviewed-on: https://go-review.googlesource.com/c/go/+/365837
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/typecheck/iexport.go

@@ -1418,6 +1418,12 @@ func (w *exportWriter) funcExt(n *ir.Name) {
 		w.uint64(1 + uint64(n.Func.Inl.Cost))
 		w.bool(n.Func.Inl.CanDelayResults)
 		if n.Func.ExportInline() || n.Type().HasTParam() {
+			if n.Type().HasTParam() {
+				// If this generic function/method is from another
+				// package, but we didn't use for instantiation in
+				// this package, we may not yet have imported it.
+				ImportedBody(n.Func)
+			}
 			w.p.doInline(n)
 		}
 

test/typeparam/issue49667.dir/a.go

@@ -0,0 +1,12 @@
+// Copyright 2021 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.
+
+package a
+
+type A[T any] struct {
+}
+
+func (a A[T]) F() {
+	_ = a
+}

test/typeparam/issue49667.dir/b.go

@@ -0,0 +1,11 @@
+// Copyright 2021 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.
+
+package b
+
+import "a"
+
+type B[T any] struct {
+	_ a.A[T]
+}

test/typeparam/issue49667.dir/main.go

@@ -0,0 +1,11 @@
+// Copyright 2021 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.
+
+package main
+
+import "b"
+
+func main() {
+	var _ b.B[int]
+}

test/typeparam/issue49667.go

@@ -0,0 +1,7 @@
+// rundir -G=3
+
+// Copyright 2021 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.
+
+package ignored