cmd/compile: fix panic when refer to method of imported instantiated type

In case of reference to method call of an imported fully-instantiated
type, nameNode.Func will be nil causes checkFetchBody panic. To fix
this, make sure checkFetchBody is only called when Func is not nil.

Fixes #49246

Change-Id: I32e9208385a86d4600d8ebf6f5efd8fca571ea16
Reviewed-on: https://go-review.googlesource.com/c/go/+/360056
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

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

@@ -624,7 +624,7 @@ func (g *genInst) getDictOrSubdict(declInfo *instInfo, n ir.Node, nameNode *ir.N
 // yet. If so, it imports the body.
 func checkFetchBody(nameNode *ir.Name) {
 	if nameNode.Func.Body == nil && nameNode.Func.Inl != nil {
-		// If there is no body yet but Func.Inl exists, then we can can
+		// If there is no body yet but Func.Inl exists, then we can
 		// import the whole generic body.
 		assert(nameNode.Func.Inl.Cost == 1 && nameNode.Sym().Pkg != types.LocalPkg)
 		typecheck.ImportBody(nameNode.Func)
@@ -638,7 +638,18 @@ func checkFetchBody(nameNode *ir.Name) {
 // with the type arguments shapes. If the instantiated function is not already
 // cached, then it calls genericSubst to create the new instantiation.
 func (g *genInst) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMeth bool) *instInfo {
-	checkFetchBody(nameNode)
+	if nameNode.Func == nil {
+		// If nameNode.Func is nil, this must be a reference to a method of
+		// an imported instantiated type. We will have already called
+		// g.instantiateMethods() on the fully-instantiated type, so
+		// g.instInfoMap[sym] will be non-nil below.
+		rcvr := nameNode.Type().Recv()
+		if rcvr == nil || !deref(rcvr.Type).IsFullyInstantiated() {
+			base.FatalfAt(nameNode.Pos(), "Unexpected function instantiation %v with no body", nameNode)
+		}
+	} else {
+		checkFetchBody(nameNode)
+	}
 
 	// Convert any non-shape type arguments to their shape, so we can reduce the
 	// number of instantiations we have to generate. You can actually have a mix

test/typeparam/issue49246.dir/a.go

@@ -0,0 +1,20 @@
+// 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 R[T any] struct{ v T }
+
+func (r R[T]) Self() R[T] { return R[T]{} }
+
+type Fn[T any] func() R[T]
+
+func X() (r R[int]) { return r.Self() }
+
+func Y[T any](a Fn[T]) Fn[int] {
+	return func() (r R[int]) {
+		// No crash: return R[int]{}
+		return r.Self()
+	}
+}

test/typeparam/issue49246.dir/b.go

@@ -0,0 +1,9 @@
+// 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"
+
+func Crash() { a.Y(a.X)() }

test/typeparam/issue49246.go

@@ -0,0 +1,7 @@
+// compiledir -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