diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index e49702c04c..2bc1756b8d 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -610,21 +610,16 @@ func (g *irgen) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMeth
 	// number of instantiations we have to generate. You can actually have a mix
 	// of shape and non-shape arguments, because of inferred or explicitly
 	// specified concrete type args.
-	var s1 []*types.Type
+	s1 := make([]*types.Type, len(shapes))
 	for i, t := range shapes {
 		if !t.IsShape() {
-			if s1 == nil {
-				s1 = make([]*types.Type, len(shapes))
-				copy(s1[0:i], shapes[0:i])
-			}
 			s1[i] = typecheck.Shapify(t, i)
-		} else if s1 != nil {
-			s1[i] = shapes[i]
+		} else {
+			// Already a shape, but make sure it has the correct index.
+			s1[i] = typecheck.Shapify(shapes[i].Underlying(), i)
 		}
 	}
-	if s1 != nil {
-		shapes = s1
-	}
+	shapes = s1
 
 	sym := typecheck.MakeFuncInstSym(nameNode.Sym(), shapes, false, isMeth)
 	info := g.instInfoMap[sym]
diff --git a/test/typeparam/issue48645a.go b/test/typeparam/issue48645a.go
new file mode 100644
index 0000000000..8d5aac94c6
--- /dev/null
+++ b/test/typeparam/issue48645a.go
@@ -0,0 +1,31 @@
+// run -gcflags=-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 main
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type Stream[T any] struct {
+}
+
+func (s Stream[T]) DropWhile() Stream[T] {
+	return Pipe[T, T](s)
+}
+
+func Pipe[T, R any](s Stream[T]) Stream[R] {
+	it := func(fn func(R) bool) {
+	}
+	fmt.Println(reflect.TypeOf(it).String())
+	return Stream[R]{}
+}
+
+func main() {
+	s := Stream[int]{}
+	s = s.DropWhile()
+}
diff --git a/test/typeparam/issue48645a.out b/test/typeparam/issue48645a.out
new file mode 100644
index 0000000000..5093d0f0ff
--- /dev/null
+++ b/test/typeparam/issue48645a.out
@@ -0,0 +1 @@
+func(func(int) bool)
diff --git a/test/typeparam/issue48645b.go b/test/typeparam/issue48645b.go
new file mode 100644
index 0000000000..0f3a7f230a
--- /dev/null
+++ b/test/typeparam/issue48645b.go
@@ -0,0 +1,81 @@
+// run -gcflags=-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 main
+
+type Iterator[T any] interface {
+	Iterate(fn func(T) bool)
+}
+
+type IteratorFunc[T any] func(fn func(T) bool)
+
+func (f IteratorFunc[T]) Iterate(fn func(T) bool) {
+	f(fn)
+}
+
+type Stream[T any] struct {
+	it Iterator[T]
+}
+
+func (s Stream[T]) Iterate(fn func(T) bool) {
+	if s.it == nil {
+		return
+	}
+	s.it.Iterate(fn)
+}
+
+func FromIterator[T any](it Iterator[T]) Stream[T] {
+	return Stream[T]{it: it}
+}
+
+func (s Stream[T]) DropWhile(fn func(T) bool) Stream[T] {
+	return Pipe[T, T](s, func(t T) (T, bool) {
+		return t, true
+	})
+}
+
+func Pipe[T, R any](s Stream[T], op func(d T) (R, bool)) Stream[R] {
+	it := func(fn func(R) bool) {
+		// XXX Not getting the closure right when converting to interface.
+		// s.it.Iterate(func(t T) bool {
+		// 	r, ok := op(t)
+		// 	if !ok {
+		// 		return true
+		// 	}
+
+		// 	return fn(r)
+		// })
+	}
+
+	return FromIterator[R](IteratorFunc[R](it))
+}
+
+func Reduce[T, U any](s Stream[T], identity U, acc func(U, T) U) (r U) {
+	r = identity
+	s.Iterate(func(t T) bool {
+		r = acc(r, t)
+		return true
+	})
+
+	return r
+}
+
+type myIterator struct {
+}
+
+func (myIterator) Iterate(fn func(int) bool) {
+}
+
+func main() {
+	s := Stream[int]{}
+	s.it = myIterator{}
+	s = s.DropWhile(func(i int) bool {
+		return false
+	})
+	Reduce(s, nil, func(acc []int, e int) []int {
+		return append(acc, e)
+	})
+}