5 files changed, 303 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/reflectdata/reflect.go b/src/cmd/compile/internal/reflectdata/reflect.go
index 3576a23db9..d452d4f194 100644
--- a/src/cmd/compile/internal/reflectdata/reflect.go
+++ b/src/cmd/compile/internal/reflectdata/reflect.go
@@ -956,7 +956,7 @@ func writeType(t *types.Type) *obj.LSym {
 		// in the local package, even if they may be marked as part of
 		// another package (the package of their base generic type).
 		if tbase.Sym() != nil && tbase.Sym().Pkg != types.LocalPkg &&
-			len(tbase.RParams()) == 0 {
+			!tbase.IsInstantiated() {
 			if i := typecheck.BaseTypeIndex(t); i >= 0 {
 				lsym.Pkg = tbase.Sym().Pkg.Prefix
 				lsym.SymIdx = int32(i)
@@ -1777,9 +1777,11 @@ func methodWrapper(rcvr *types.Type, method *types.Field) *obj.LSym {
 		return lsym
 	}
 
-	// Only generate (*T).M wrappers for T.M in T's own package.
+	// Only generate (*T).M wrappers for T.M in T's own package, except for
+	// instantiated methods.
 	if rcvr.IsPtr() && rcvr.Elem() == method.Type.Recv().Type &&
-		rcvr.Elem().Sym() != nil && rcvr.Elem().Sym().Pkg != types.LocalPkg {
+		rcvr.Elem().Sym() != nil && rcvr.Elem().Sym().Pkg != types.LocalPkg &&
+		!rcvr.Elem().IsInstantiated() {
 		return lsym
 	}
 
diff --git a/src/cmd/compile/internal/types/type.go b/src/cmd/compile/internal/types/type.go
index e7831121bf..08855f518c 100644
--- a/src/cmd/compile/internal/types/type.go
+++ b/src/cmd/compile/internal/types/type.go
@@ -279,6 +279,13 @@ func (t *Type) SetRParams(rparams []*Type) {
 	}
 }
 
+// IsInstantiated reports whether t is a fully instantiated generic type; i.e. an
+// instantiated generic type where all type arguments are non-generic or fully
+// instantiated generic types.
+func (t *Type) IsInstantiated() bool {
+	return len(t.RParams()) > 0 && !t.HasTParam()
+}
+
 // NoPkg is a nil *Pkg value for clarity.
 // It's intended for use when constructing types that aren't exported
 // and thus don't need to be associated with any package.
diff --git a/test/typeparam/setsimp.dir/a.go b/test/typeparam/setsimp.dir/a.go
new file mode 100644
index 0000000000..92449ce956
--- /dev/null
+++ b/test/typeparam/setsimp.dir/a.go
@@ -0,0 +1,128 @@
+// 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
+
+// SliceEqual reports whether two slices are equal: the same length and all
+// elements equal. All floating point NaNs are considered equal.
+func SliceEqual[Elem comparable](s1, s2 []Elem) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	for i, v1 := range s1 {
+		v2 := s2[i]
+		if v1 != v2 {
+			isNaN := func(f Elem) bool { return f != f }
+			if !isNaN(v1) || !isNaN(v2) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// A Set is a set of elements of some type.
+type Set[Elem comparable] struct {
+	m map[Elem]struct{}
+}
+
+// Make makes a new set.
+func Make[Elem comparable]() Set[Elem] {
+	return Set[Elem]{m: make(map[Elem]struct{})}
+}
+
+// Add adds an element to a set.
+func (s Set[Elem]) Add(v Elem) {
+	s.m[v] = struct{}{}
+}
+
+// Delete removes an element from a set. If the element is not present
+// in the set, this does nothing.
+func (s Set[Elem]) Delete(v Elem) {
+	delete(s.m, v)
+}
+
+// Contains reports whether v is in the set.
+func (s Set[Elem]) Contains(v Elem) bool {
+	_, ok := s.m[v]
+	return ok
+}
+
+// Len returns the number of elements in the set.
+func (s Set[Elem]) Len() int {
+	return len(s.m)
+}
+
+// Values returns the values in the set.
+// The values will be in an indeterminate order.
+func (s Set[Elem]) Values() []Elem {
+	r := make([]Elem, 0, len(s.m))
+	for v := range s.m {
+		r = append(r, v)
+	}
+	return r
+}
+
+// Equal reports whether two sets contain the same elements.
+func Equal[Elem comparable](s1, s2 Set[Elem]) bool {
+	if len(s1.m) != len(s2.m) {
+		return false
+	}
+	for v1 := range s1.m {
+		if !s2.Contains(v1) {
+			return false
+		}
+	}
+	return true
+}
+
+// Copy returns a copy of s.
+func (s Set[Elem]) Copy() Set[Elem] {
+	r := Set[Elem]{m: make(map[Elem]struct{}, len(s.m))}
+	for v := range s.m {
+		r.m[v] = struct{}{}
+	}
+	return r
+}
+
+// AddSet adds all the elements of s2 to s.
+func (s Set[Elem]) AddSet(s2 Set[Elem]) {
+	for v := range s2.m {
+		s.m[v] = struct{}{}
+	}
+}
+
+// SubSet removes all elements in s2 from s.
+// Values in s2 that are not in s are ignored.
+func (s Set[Elem]) SubSet(s2 Set[Elem]) {
+	for v := range s2.m {
+		delete(s.m, v)
+	}
+}
+
+// Intersect removes all elements from s that are not present in s2.
+// Values in s2 that are not in s are ignored.
+func (s Set[Elem]) Intersect(s2 Set[Elem]) {
+	for v := range s.m {
+		if !s2.Contains(v) {
+			delete(s.m, v)
+		}
+	}
+}
+
+// Iterate calls f on every element in the set.
+func (s Set[Elem]) Iterate(f func(Elem)) {
+	for v := range s.m {
+		f(v)
+	}
+}
+
+// Filter deletes any elements from s for which f returns false.
+func (s Set[Elem]) Filter(f func(Elem) bool) {
+	for v := range s.m {
+		if !f(v) {
+			delete(s.m, v)
+		}
+	}
+}
diff --git a/test/typeparam/setsimp.dir/main.go b/test/typeparam/setsimp.dir/main.go
new file mode 100644
index 0000000000..8fd1657143
--- /dev/null
+++ b/test/typeparam/setsimp.dir/main.go
@@ -0,0 +1,156 @@
+// 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 (
+	"a"
+	"fmt"
+	"sort"
+)
+
+func TestSet() {
+	s1 := a.Make[int]()
+	if got := s1.Len(); got != 0 {
+		panic(fmt.Sprintf("Len of empty set = %d, want 0", got))
+	}
+	s1.Add(1)
+	s1.Add(1)
+	s1.Add(1)
+	if got := s1.Len(); got != 1 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 1", s1, got))
+	}
+	s1.Add(2)
+	s1.Add(3)
+	s1.Add(4)
+	if got := s1.Len(); got != 4 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 4", s1, got))
+	}
+	if !s1.Contains(1) {
+		panic(fmt.Sprintf("(%v).Contains(1) == false, want true", s1))
+	}
+	if s1.Contains(5) {
+		panic(fmt.Sprintf("(%v).Contains(5) == true, want false", s1))
+	}
+	vals := s1.Values()
+	sort.Ints(vals)
+	w1 := []int{1, 2, 3, 4}
+	if !a.SliceEqual(vals, w1) {
+		panic(fmt.Sprintf("(%v).Values() == %v, want %v", s1, vals, w1))
+	}
+}
+
+func TestEqual() {
+	s1 := a.Make[string]()
+	s2 := a.Make[string]()
+	if !a.Equal(s1, s2) {
+		panic(fmt.Sprintf("a.Equal(%v, %v) = false, want true", s1, s2))
+	}
+	s1.Add("hello")
+	s1.Add("world")
+	if got := s1.Len(); got != 2 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 2", s1, got))
+	}
+	if a.Equal(s1, s2) {
+		panic(fmt.Sprintf("a.Equal(%v, %v) = true, want false", s1, s2))
+	}
+}
+
+func TestCopy() {
+	s1 := a.Make[float64]()
+	s1.Add(0)
+	s2 := s1.Copy()
+	if !a.Equal(s1, s2) {
+		panic(fmt.Sprintf("a.Equal(%v, %v) = false, want true", s1, s2))
+	}
+	s1.Add(1)
+	if a.Equal(s1, s2) {
+		panic(fmt.Sprintf("a.Equal(%v, %v) = true, want false", s1, s2))
+	}
+}
+
+func TestAddSet() {
+	s1 := a.Make[int]()
+	s1.Add(1)
+	s1.Add(2)
+	s2 := a.Make[int]()
+	s2.Add(2)
+	s2.Add(3)
+	s1.AddSet(s2)
+	if got := s1.Len(); got != 3 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 3", s1, got))
+	}
+	s2.Add(1)
+	if !a.Equal(s1, s2) {
+		panic(fmt.Sprintf("a.Equal(%v, %v) = false, want true", s1, s2))
+	}
+}
+
+func TestSubSet() {
+	s1 := a.Make[int]()
+	s1.Add(1)
+	s1.Add(2)
+	s2 := a.Make[int]()
+	s2.Add(2)
+	s2.Add(3)
+	s1.SubSet(s2)
+	if got := s1.Len(); got != 1 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 1", s1, got))
+	}
+	if vals, want := s1.Values(), []int{1}; !a.SliceEqual(vals, want) {
+		panic(fmt.Sprintf("after SubSet got %v, want %v", vals, want))
+	}
+}
+
+func TestIntersect() {
+	s1 := a.Make[int]()
+	s1.Add(1)
+	s1.Add(2)
+	s2 := a.Make[int]()
+	s2.Add(2)
+	s2.Add(3)
+	s1.Intersect(s2)
+	if got := s1.Len(); got != 1 {
+		panic(fmt.Sprintf("(%v).Len() == %d, want 1", s1, got))
+	}
+	if vals, want := s1.Values(), []int{2}; !a.SliceEqual(vals, want) {
+		panic(fmt.Sprintf("after Intersect got %v, want %v", vals, want))
+	}
+}
+
+func TestIterate() {
+	s1 := a.Make[int]()
+	s1.Add(1)
+	s1.Add(2)
+	s1.Add(3)
+	s1.Add(4)
+	tot := 0
+	s1.Iterate(func(i int) { tot += i })
+	if tot != 10 {
+		panic(fmt.Sprintf("total of %v == %d, want 10", s1, tot))
+	}
+}
+
+func TestFilter() {
+	s1 := a.Make[int]()
+	s1.Add(1)
+	s1.Add(2)
+	s1.Add(3)
+	s1.Filter(func(v int) bool { return v%2 == 0 })
+	if vals, want := s1.Values(), []int{2}; !a.SliceEqual(vals, want) {
+		panic(fmt.Sprintf("after Filter got %v, want %v", vals, want))
+	}
+
+}
+
+func main() {
+	TestSet()
+	TestEqual()
+	TestCopy()
+	TestAddSet()
+	TestSubSet()
+	TestIntersect()
+	TestIterate()
+	TestFilter()
+}
diff --git a/test/typeparam/setsimp.go b/test/typeparam/setsimp.go
new file mode 100644
index 0000000000..76930e5e4f
--- /dev/null
+++ b/test/typeparam/setsimp.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