diff --git a/src/cmd/compile/internal/ir/node.go b/src/cmd/compile/internal/ir/node.go
index 38f9123582..7bce0e985c 100644
--- a/src/cmd/compile/internal/ir/node.go
+++ b/src/cmd/compile/internal/ir/node.go
@@ -48,6 +48,12 @@ type Node interface {
 	SetEsc(x uint16)
 	Diag() bool
 	SetDiag(x bool)
+
+	// Typecheck values:
+	//  0 means the node is not typechecked
+	//  1 means the node is completely typechecked
+	//  2 means typechecking of the node is in progress
+	//  3 means the node has its type from types2, but may need transformation
 	Typecheck() uint8
 	SetTypecheck(x uint8)
 	NonNil() bool
diff --git a/src/cmd/compile/internal/noder/expr.go b/src/cmd/compile/internal/noder/expr.go
index 957295bdf0..effc63c09a 100644
--- a/src/cmd/compile/internal/noder/expr.go
+++ b/src/cmd/compile/internal/noder/expr.go
@@ -64,7 +64,7 @@ func (g *irgen) expr(expr syntax.Expr) ir.Node {
 	}
 
 	n := g.expr0(typ, expr)
-	if n.Typecheck() != 1 {
+	if n.Typecheck() != 1 && n.Typecheck() != 3 {
 		base.FatalfAt(g.pos(expr), "missed typecheck: %+v", n)
 	}
 	if !g.match(n.Type(), typ, tv.HasOk()) {
@@ -161,7 +161,7 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 		return g.selectorExpr(pos, typ, expr)
 
 	case *syntax.SliceExpr:
-		return Slice(pos, g.expr(expr.X), g.expr(expr.Index[0]), g.expr(expr.Index[1]), g.expr(expr.Index[2]))
+		return Slice(pos, g.typ(typ), g.expr(expr.X), g.expr(expr.Index[0]), g.expr(expr.Index[1]), g.expr(expr.Index[2]))
 
 	case *syntax.Operation:
 		if expr.Y == nil {
@@ -171,7 +171,7 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 		case ir.OEQ, ir.ONE, ir.OLT, ir.OLE, ir.OGT, ir.OGE:
 			return Compare(pos, g.typ(typ), op, g.expr(expr.X), g.expr(expr.Y))
 		default:
-			return Binary(pos, op, g.expr(expr.X), g.expr(expr.Y))
+			return Binary(pos, op, g.typ(typ), g.expr(expr.X), g.expr(expr.Y))
 		}
 
 	default:
diff --git a/src/cmd/compile/internal/noder/helpers.go b/src/cmd/compile/internal/noder/helpers.go
index 1210d4b58c..e4a1a54fe8 100644
--- a/src/cmd/compile/internal/noder/helpers.go
+++ b/src/cmd/compile/internal/noder/helpers.go
@@ -67,16 +67,47 @@ func Assert(pos src.XPos, x ir.Node, typ *types.Type) ir.Node {
 	return typed(typ, ir.NewTypeAssertExpr(pos, x, nil))
 }
 
-func Binary(pos src.XPos, op ir.Op, x, y ir.Node) ir.Node {
+// transformAdd transforms an addition operation (currently just addition of
+// strings). Equivalent to the "binary operators" case in typecheck.typecheck1.
+func transformAdd(n *ir.BinaryExpr) ir.Node {
+	l := n.X
+	if l.Type().IsString() {
+		var add *ir.AddStringExpr
+		if l.Op() == ir.OADDSTR {
+			add = l.(*ir.AddStringExpr)
+			add.SetPos(n.Pos())
+		} else {
+			add = ir.NewAddStringExpr(n.Pos(), []ir.Node{l})
+		}
+		r := n.Y
+		if r.Op() == ir.OADDSTR {
+			r := r.(*ir.AddStringExpr)
+			add.List.Append(r.List.Take()...)
+		} else {
+			add.List.Append(r)
+		}
+		add.SetType(l.Type())
+		return add
+	}
+	return n
+}
+
+func Binary(pos src.XPos, op ir.Op, typ *types.Type, x, y ir.Node) ir.Node {
 	switch op {
 	case ir.OANDAND, ir.OOROR:
 		return typed(x.Type(), ir.NewLogicalExpr(pos, op, x, y))
 	case ir.OADD:
-		if x.Type().IsString() {
-			// TODO(mdempsky): Construct OADDSTR directly.
-			return typecheck.Expr(ir.NewBinaryExpr(pos, op, x, y))
+		n := ir.NewBinaryExpr(pos, op, x, y)
+		if x.Type().HasTParam() || y.Type().HasTParam() {
+			// Delay transformAdd() if either arg has a type param,
+			// since it needs to know the exact types to decide whether
+			// to transform OADD to OADDSTR.
+			n.SetType(typ)
+			n.SetTypecheck(3)
+			return n
 		}
-		fallthrough
+		n1 := transformAdd(n)
+		return typed(typ, n1)
 	default:
 		return typed(x.Type(), ir.NewBinaryExpr(pos, op, x, y))
 	}
@@ -178,12 +209,56 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 	return n
 }
 
+// transformCompare transforms a compare operation (currently just equals/not
+// equals). Equivalent to the "comparison operators" case in
+// typecheck.typecheck1, including tcArith.
+func transformCompare(n *ir.BinaryExpr) {
+	if (n.Op() == ir.OEQ || n.Op() == ir.ONE) && !types.Identical(n.X.Type(), n.Y.Type()) {
+		// Comparison is okay as long as one side is assignable to the
+		// other. The only allowed case where the conversion is not CONVNOP is
+		// "concrete == interface". In that case, check comparability of
+		// the concrete type. The conversion allocates, so only do it if
+		// the concrete type is huge.
+		l, r := n.X, n.Y
+		lt, rt := l.Type(), r.Type()
+		converted := false
+		if rt.Kind() != types.TBLANK {
+			aop, _ := typecheck.Assignop(lt, rt)
+			if aop != ir.OXXX {
+				types.CalcSize(lt)
+				if rt.IsInterface() == lt.IsInterface() || lt.Width >= 1<<16 {
+					l = ir.NewConvExpr(base.Pos, aop, rt, l)
+					l.SetTypecheck(1)
+				}
+
+				converted = true
+			}
+		}
+
+		if !converted && lt.Kind() != types.TBLANK {
+			aop, _ := typecheck.Assignop(rt, lt)
+			if aop != ir.OXXX {
+				types.CalcSize(rt)
+				if rt.IsInterface() == lt.IsInterface() || rt.Width >= 1<<16 {
+					r = ir.NewConvExpr(base.Pos, aop, lt, r)
+					r.SetTypecheck(1)
+				}
+			}
+		}
+		n.X, n.Y = l, r
+	}
+}
+
 func Compare(pos src.XPos, typ *types.Type, op ir.Op, x, y ir.Node) ir.Node {
 	n := ir.NewBinaryExpr(pos, op, x, y)
-	if !types.Identical(x.Type(), y.Type()) {
-		// TODO(mdempsky): Handle subtleties of constructing mixed-typed comparisons.
-		n = typecheck.Expr(n).(*ir.BinaryExpr)
+	if x.Type().HasTParam() || y.Type().HasTParam() {
+		// Delay transformCompare() if either arg has a type param, since
+		// it needs to know the exact types to decide on any needed conversions.
+		n.SetType(typ)
+		n.SetTypecheck(3)
+		return n
 	}
+	transformCompare(n)
 	return typed(typ, n)
 }
 
@@ -267,13 +342,42 @@ func Index(pos src.XPos, typ *types.Type, x, index ir.Node) ir.Node {
 	return typecheck.Expr(n)
 }
 
-func Slice(pos src.XPos, x, low, high, max ir.Node) ir.Node {
+// transformSlice transforms a slice operation.  Equivalent to typecheck.tcSlice.
+func transformSlice(n *ir.SliceExpr) {
+	l := n.X
+	if l.Type().IsArray() {
+		addr := typecheck.NodAddr(n.X)
+		addr.SetImplicit(true)
+		typed(types.NewPtr(n.X.Type()), addr)
+		n.X = addr
+		l = addr
+	}
+	t := l.Type()
+	if t.IsString() {
+		n.SetOp(ir.OSLICESTR)
+	} else if t.IsPtr() && t.Elem().IsArray() {
+		if n.Op().IsSlice3() {
+			n.SetOp(ir.OSLICE3ARR)
+		} else {
+			n.SetOp(ir.OSLICEARR)
+		}
+	}
+}
+
+func Slice(pos src.XPos, typ *types.Type, x, low, high, max ir.Node) ir.Node {
 	op := ir.OSLICE
 	if max != nil {
 		op = ir.OSLICE3
 	}
-	// TODO(mdempsky): Avoid typecheck.Expr.
-	return typecheck.Expr(ir.NewSliceExpr(pos, op, x, low, high, max))
+	n := ir.NewSliceExpr(pos, op, x, low, high, max)
+	if x.Type().HasTParam() {
+		// transformSlice needs to know if x.Type() is a string or an array or a slice.
+		n.SetType(typ)
+		n.SetTypecheck(3)
+		return n
+	}
+	transformSlice(n)
+	return typed(typ, n)
 }
 
 func Unary(pos src.XPos, op ir.Op, x ir.Node) ir.Node {
diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 51ef46c7e7..1b76bb27c5 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -367,6 +367,23 @@ func (subst *subster) node(n ir.Node) ir.Node {
 		}
 		ir.EditChildren(m, edit)
 
+		if x.Typecheck() == 3 {
+			// These are nodes whose transforms were delayed until
+			// their instantiated type was known.
+			if typecheck.IsCmp(x.Op()) {
+				transformCompare(m.(*ir.BinaryExpr))
+				m.SetTypecheck(1)
+			} else if x.Op() == ir.OSLICE || x.Op() == ir.OSLICE3 {
+				transformSlice(m.(*ir.SliceExpr))
+				m.SetTypecheck(1)
+			} else if x.Op() == ir.OADD {
+				m = transformAdd(m.(*ir.BinaryExpr))
+				m.SetTypecheck(1)
+			} else {
+				base.Fatalf("Unexpected node with Typecheck() == 3")
+			}
+		}
+
 		switch x.Op() {
 		case ir.OLITERAL:
 			t := m.Type()
diff --git a/src/cmd/compile/internal/typecheck/expr.go b/src/cmd/compile/internal/typecheck/expr.go
index 10a4c1b1dc..fb39709686 100644
--- a/src/cmd/compile/internal/typecheck/expr.go
+++ b/src/cmd/compile/internal/typecheck/expr.go
@@ -77,6 +77,10 @@ func tcShift(n, l, r ir.Node) (ir.Node, ir.Node, *types.Type) {
 	return l, r, t
 }
 
+func IsCmp(op ir.Op) bool {
+	return iscmp[op]
+}
+
 // tcArith typechecks operands of a binary arithmetic expression.
 // The result of tcArith MUST be assigned back to original operands,
 // t is the type of the expression, and should be set by the caller. e.g:
@@ -102,7 +106,7 @@ func tcArith(n ir.Node, op ir.Op, l, r ir.Node) (ir.Node, ir.Node, *types.Type)
 		// The conversion allocates, so only do it if the concrete type is huge.
 		converted := false
 		if r.Type().Kind() != types.TBLANK {
-			aop, _ = assignop(l.Type(), r.Type())
+			aop, _ = Assignop(l.Type(), r.Type())
 			if aop != ir.OXXX {
 				if r.Type().IsInterface() && !l.Type().IsInterface() && !types.IsComparable(l.Type()) {
 					base.Errorf("invalid operation: %v (operator %v not defined on %s)", n, op, typekind(l.Type()))
@@ -121,7 +125,7 @@ func tcArith(n ir.Node, op ir.Op, l, r ir.Node) (ir.Node, ir.Node, *types.Type)
 		}
 
 		if !converted && l.Type().Kind() != types.TBLANK {
-			aop, _ = assignop(r.Type(), l.Type())
+			aop, _ = Assignop(r.Type(), l.Type())
 			if aop != ir.OXXX {
 				if l.Type().IsInterface() && !r.Type().IsInterface() && !types.IsComparable(r.Type()) {
 					base.Errorf("invalid operation: %v (operator %v not defined on %s)", n, op, typekind(r.Type()))
diff --git a/src/cmd/compile/internal/typecheck/stmt.go b/src/cmd/compile/internal/typecheck/stmt.go
index 14ed175be9..175216f279 100644
--- a/src/cmd/compile/internal/typecheck/stmt.go
+++ b/src/cmd/compile/internal/typecheck/stmt.go
@@ -74,7 +74,7 @@ func typecheckrangeExpr(n *ir.RangeStmt) {
 			if ir.DeclaredBy(nn, n) {
 				nn.SetType(t)
 			} else if nn.Type() != nil {
-				if op, why := assignop(t, nn.Type()); op == ir.OXXX {
+				if op, why := Assignop(t, nn.Type()); op == ir.OXXX {
 					base.ErrorfAt(n.Pos(), "cannot assign type %v to %L in range%s", t, nn, why)
 				}
 			}
@@ -519,8 +519,8 @@ func tcSwitchExpr(n *ir.SwitchStmt) {
 			} else if t.IsInterface() && !n1.Type().IsInterface() && !types.IsComparable(n1.Type()) {
 				base.ErrorfAt(ncase.Pos(), "invalid case %L in switch (incomparable type)", n1)
 			} else {
-				op1, _ := assignop(n1.Type(), t)
-				op2, _ := assignop(t, n1.Type())
+				op1, _ := Assignop(n1.Type(), t)
+				op2, _ := Assignop(t, n1.Type())
 				if op1 == ir.OXXX && op2 == ir.OXXX {
 					if n.Tag != nil {
 						base.ErrorfAt(ncase.Pos(), "invalid case %v in switch on %v (mismatched types %v and %v)", n1, n.Tag, n1.Type(), t)
diff --git a/src/cmd/compile/internal/typecheck/subr.go b/src/cmd/compile/internal/typecheck/subr.go
index c40cfa2288..e58ef9fb05 100644
--- a/src/cmd/compile/internal/typecheck/subr.go
+++ b/src/cmd/compile/internal/typecheck/subr.go
@@ -317,7 +317,7 @@ func assignconvfn(n ir.Node, t *types.Type, context func() string) ir.Node {
 		return n
 	}
 
-	op, why := assignop(n.Type(), t)
+	op, why := Assignop(n.Type(), t)
 	if op == ir.OXXX {
 		base.Errorf("cannot use %L as type %v in %s%s", n, t, context(), why)
 		op = ir.OCONV
@@ -333,7 +333,7 @@ func assignconvfn(n ir.Node, t *types.Type, context func() string) ir.Node {
 // If so, return op code to use in conversion.
 // If not, return OXXX. In this case, the string return parameter may
 // hold a reason why. In all other cases, it'll be the empty string.
-func assignop(src, dst *types.Type) (ir.Op, string) {
+func Assignop(src, dst *types.Type) (ir.Op, string) {
 	if src == dst {
 		return ir.OCONVNOP, ""
 	}
@@ -483,7 +483,7 @@ func convertop(srcConstant bool, src, dst *types.Type) (ir.Op, string) {
 	}
 
 	// 1. src can be assigned to dst.
-	op, why := assignop(src, dst)
+	op, why := Assignop(src, dst)
 	if op != ir.OXXX {
 		return op, why
 	}
diff --git a/src/cmd/compile/internal/typecheck/typecheck.go b/src/cmd/compile/internal/typecheck/typecheck.go
index 30632ac18b..f06a8623d0 100644
--- a/src/cmd/compile/internal/typecheck/typecheck.go
+++ b/src/cmd/compile/internal/typecheck/typecheck.go
@@ -297,7 +297,7 @@ func typecheck(n ir.Node, top int) (res ir.Node) {
 
 	// Skip typecheck if already done.
 	// But re-typecheck ONAME/OTYPE/OLITERAL/OPACK node in case context has changed.
-	if n.Typecheck() == 1 {
+	if n.Typecheck() == 1 || n.Typecheck() == 3 {
 		switch n.Op() {
 		case ir.ONAME, ir.OTYPE, ir.OLITERAL, ir.OPACK:
 			break
@@ -1640,7 +1640,7 @@ func checkassignto(src *types.Type, dst ir.Node) {
 		return
 	}
 
-	if op, why := assignop(src, dst.Type()); op == ir.OXXX {
+	if op, why := Assignop(src, dst.Type()); op == ir.OXXX {
 		base.Errorf("cannot assign %v to %L in multiple assignment%s", src, dst, why)
 		return
 	}
diff --git a/test/typeparam/adder.go b/test/typeparam/adder.go
new file mode 100644
index 0000000000..0c25ad4ef2
--- /dev/null
+++ b/test/typeparam/adder.go
@@ -0,0 +1,29 @@
+// 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"
+)
+
+type AddType interface {
+	type int, int64, string
+}
+
+// _Add can add numbers or strings
+func _Add[T AddType](a, b T) T {
+	return a + b
+}
+
+func main() {
+	if got, want := _Add(5, 3), 8; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := _Add("ab", "cd"), "abcd"; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+}
diff --git a/test/typeparam/list2.go b/test/typeparam/list2.go
new file mode 100644
index 0000000000..385193d876
--- /dev/null
+++ b/test/typeparam/list2.go
@@ -0,0 +1,601 @@
+// 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 list provides a doubly linked list of some element type
+// (generic form of the "container/list" package).
+
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+// Element is an element of a linked list.
+type _Element[T any] struct {
+	// Next and previous pointers in the doubly-linked list of elements.
+	// To simplify the implementation, internally a list l is implemented
+	// as a ring, such that &l.root is both the next element of the last
+	// list element (l.Back()) and the previous element of the first list
+	// element (l.Front()).
+	next, prev *_Element[T]
+
+	// The list to which this element belongs.
+	list *_List[T]
+
+	// The value stored with this element.
+	Value T
+}
+
+// Next returns the next list element or nil.
+func (e *_Element[T]) Next() *_Element[T] {
+	if p := e.next; e.list != nil && p != &e.list.root {
+		return p
+	}
+	return nil
+}
+
+// Prev returns the previous list element or nil.
+func (e *_Element[T]) Prev() *_Element[T] {
+	if p := e.prev; e.list != nil && p != &e.list.root {
+		return p
+	}
+	return nil
+}
+
+// _List represents a doubly linked list.
+// The zero value for _List is an empty list ready to use.
+type _List[T any] struct {
+	root _Element[T] // sentinel list element, only &root, root.prev, and root.next are used
+	len  int     // current list length excluding (this) sentinel element
+}
+
+// Init initializes or clears list l.
+func (l *_List[T]) Init() *_List[T] {
+	l.root.next = &l.root
+	l.root.prev = &l.root
+	l.len = 0
+	return l
+}
+
+// New returns an initialized list.
+func _New[T any]() *_List[T] { return new(_List[T]).Init() }
+
+// Len returns the number of elements of list l.
+// The complexity is O(1).
+func (l *_List[_]) Len() int { return l.len }
+
+// Front returns the first element of list l or nil if the list is empty.
+func (l *_List[T]) Front() *_Element[T] {
+	if l.len == 0 {
+		return nil
+	}
+	return l.root.next
+}
+
+// Back returns the last element of list l or nil if the list is empty.
+func (l *_List[T]) Back() *_Element[T] {
+	if l.len == 0 {
+		return nil
+	}
+	return l.root.prev
+}
+
+// lazyInit lazily initializes a zero _List value.
+func (l *_List[_]) lazyInit() {
+	if l.root.next == nil {
+		l.Init()
+	}
+}
+
+// insert inserts e after at, increments l.len, and returns e.
+func (l *_List[T]) insert(e, at *_Element[T]) *_Element[T] {
+	e.prev = at
+	e.next = at.next
+	e.prev.next = e
+	e.next.prev = e
+	e.list = l
+	l.len++
+	return e
+}
+
+// insertValue is a convenience wrapper for insert(&_Element[T]{Value: v}, at).
+func (l *_List[T]) insertValue(v T, at *_Element[T]) *_Element[T] {
+	return l.insert(&_Element[T]{Value: v}, at)
+}
+
+// remove removes e from its list, decrements l.len, and returns e.
+func (l *_List[T]) remove(e *_Element[T]) *_Element[T] {
+	e.prev.next = e.next
+	e.next.prev = e.prev
+	e.next = nil // avoid memory leaks
+	e.prev = nil // avoid memory leaks
+	e.list = nil
+	l.len--
+	return e
+}
+
+// move moves e to next to at and returns e.
+func (l *_List[T]) move(e, at *_Element[T]) *_Element[T] {
+	if e == at {
+		return e
+	}
+	e.prev.next = e.next
+	e.next.prev = e.prev
+
+	e.prev = at
+	e.next = at.next
+	e.prev.next = e
+	e.next.prev = e
+
+	return e
+}
+
+// Remove removes e from l if e is an element of list l.
+// It returns the element value e.Value.
+// The element must not be nil.
+func (l *_List[T]) Remove(e *_Element[T]) T {
+	if e.list == l {
+		// if e.list == l, l must have been initialized when e was inserted
+		// in l or l == nil (e is a zero _Element) and l.remove will crash
+		l.remove(e)
+	}
+	return e.Value
+}
+
+// PushFront inserts a new element e with value v at the front of list l and returns e.
+func (l *_List[T]) PushFront(v T) *_Element[T] {
+	l.lazyInit()
+	return l.insertValue(v, &l.root)
+}
+
+// PushBack inserts a new element e with value v at the back of list l and returns e.
+func (l *_List[T]) PushBack(v T) *_Element[T] {
+	l.lazyInit()
+	return l.insertValue(v, l.root.prev)
+}
+
+// InsertBefore inserts a new element e with value v immediately before mark and returns e.
+// If mark is not an element of l, the list is not modified.
+// The mark must not be nil.
+func (l *_List[T]) InsertBefore(v T, mark *_Element[T]) *_Element[T] {
+	if mark.list != l {
+		return nil
+	}
+	// see comment in _List.Remove about initialization of l
+	return l.insertValue(v, mark.prev)
+}
+
+// InsertAfter inserts a new element e with value v immediately after mark and returns e.
+// If mark is not an element of l, the list is not modified.
+// The mark must not be nil.
+func (l *_List[T]) InsertAfter(v T, mark *_Element[T]) *_Element[T] {
+	if mark.list != l {
+		return nil
+	}
+	// see comment in _List.Remove about initialization of l
+	return l.insertValue(v, mark)
+}
+
+// MoveToFront moves element e to the front of list l.
+// If e is not an element of l, the list is not modified.
+// The element must not be nil.
+func (l *_List[T]) MoveToFront(e *_Element[T]) {
+	if e.list != l || l.root.next == e {
+		return
+	}
+	// see comment in _List.Remove about initialization of l
+	l.move(e, &l.root)
+}
+
+// MoveToBack moves element e to the back of list l.
+// If e is not an element of l, the list is not modified.
+// The element must not be nil.
+func (l *_List[T]) MoveToBack(e *_Element[T]) {
+	if e.list != l || l.root.prev == e {
+		return
+	}
+	// see comment in _List.Remove about initialization of l
+	l.move(e, l.root.prev)
+}
+
+// MoveBefore moves element e to its new position before mark.
+// If e or mark is not an element of l, or e == mark, the list is not modified.
+// The element and mark must not be nil.
+func (l *_List[T]) MoveBefore(e, mark *_Element[T]) {
+	if e.list != l || e == mark || mark.list != l {
+		return
+	}
+	l.move(e, mark.prev)
+}
+
+// MoveAfter moves element e to its new position after mark.
+// If e or mark is not an element of l, or e == mark, the list is not modified.
+// The element and mark must not be nil.
+func (l *_List[T]) MoveAfter(e, mark *_Element[T]) {
+	if e.list != l || e == mark || mark.list != l {
+		return
+	}
+	l.move(e, mark)
+}
+
+// PushBackList inserts a copy of an other list at the back of list l.
+// The lists l and other may be the same. They must not be nil.
+func (l *_List[T]) PushBackList(other *_List[T]) {
+	l.lazyInit()
+	for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
+		l.insertValue(e.Value, l.root.prev)
+	}
+}
+
+// PushFrontList inserts a copy of an other list at the front of list l.
+// The lists l and other may be the same. They must not be nil.
+func (l *_List[T]) PushFrontList(other *_List[T]) {
+	l.lazyInit()
+	for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
+		l.insertValue(e.Value, &l.root)
+	}
+}
+
+// Transform runs a transform function on a list returning a new list.
+func _Transform[TElem1, TElem2 any](lst *_List[TElem1], f func(TElem1) TElem2) *_List[TElem2] {
+	ret := _New[TElem2]()
+	for p := lst.Front(); p != nil; p = p.Next() {
+		ret.PushBack(f(p.Value))
+	}
+	return ret
+}
+
+func checkListLen[T any](l *_List[T], len int) bool {
+	if n := l.Len(); n != len {
+		panic(fmt.Sprintf("l.Len() = %d, want %d", n, len))
+		return false
+	}
+	return true
+}
+
+func checkListPointers[T any](l *_List[T], es []*_Element[T]) {
+	root := &l.root
+
+	if !checkListLen(l, len(es)) {
+		return
+	}
+
+	// zero length lists must be the zero value or properly initialized (sentinel circle)
+	if len(es) == 0 {
+		if l.root.next != nil && l.root.next != root || l.root.prev != nil && l.root.prev != root {
+			panic(fmt.Sprintf("l.root.next = %p, l.root.prev = %p; both should both be nil or %p", l.root.next, l.root.prev, root))
+		}
+		return
+	}
+	// len(es) > 0
+
+	// check internal and external prev/next connections
+	for i, e := range es {
+		prev := root
+		Prev := (*_Element[T])(nil)
+		if i > 0 {
+			prev = es[i-1]
+			Prev = prev
+		}
+		if p := e.prev; p != prev {
+			panic(fmt.Sprintf("elt[%d](%p).prev = %p, want %p", i, e, p, prev))
+		}
+		if p := e.Prev(); p != Prev {
+			panic(fmt.Sprintf("elt[%d](%p).Prev() = %p, want %p", i, e, p, Prev))
+		}
+
+		next := root
+		Next := (*_Element[T])(nil)
+		if i < len(es)-1 {
+			next = es[i+1]
+			Next = next
+		}
+		if n := e.next; n != next {
+			panic(fmt.Sprintf("elt[%d](%p).next = %p, want %p", i, e, n, next))
+		}
+		if n := e.Next(); n != Next {
+			panic(fmt.Sprintf("elt[%d](%p).Next() = %p, want %p", i, e, n, Next))
+		}
+	}
+}
+
+func TestList() {
+	l := _New[string]()
+	checkListPointers(l, []*(_Element[string]){})
+
+	// Single element list
+	e := l.PushFront("a")
+	checkListPointers(l, []*(_Element[string]){e})
+	l.MoveToFront(e)
+	checkListPointers(l, []*(_Element[string]){e})
+	l.MoveToBack(e)
+	checkListPointers(l, []*(_Element[string]){e})
+	l.Remove(e)
+	checkListPointers(l, []*(_Element[string]){})
+
+	// Bigger list
+	l2 := _New[int]()
+	e2 := l2.PushFront(2)
+	e1 := l2.PushFront(1)
+	e3 := l2.PushBack(3)
+	e4 := l2.PushBack(600)
+	checkListPointers(l2, []*(_Element[int]){e1, e2, e3, e4})
+
+	l2.Remove(e2)
+	checkListPointers(l2, []*(_Element[int]){e1, e3, e4})
+
+	l2.MoveToFront(e3) // move from middle
+	checkListPointers(l2, []*(_Element[int]){e3, e1, e4})
+
+	l2.MoveToFront(e1)
+	l2.MoveToBack(e3) // move from middle
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e3})
+
+	l2.MoveToFront(e3) // move from back
+	checkListPointers(l2, []*(_Element[int]){e3, e1, e4})
+	l2.MoveToFront(e3) // should be no-op
+	checkListPointers(l2, []*(_Element[int]){e3, e1, e4})
+
+	l2.MoveToBack(e3) // move from front
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e3})
+	l2.MoveToBack(e3) // should be no-op
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e3})
+
+	e2 = l2.InsertBefore(2, e1) // insert before front
+	checkListPointers(l2, []*(_Element[int]){e2, e1, e4, e3})
+	l2.Remove(e2)
+	e2 = l2.InsertBefore(2, e4) // insert before middle
+	checkListPointers(l2, []*(_Element[int]){e1, e2, e4, e3})
+	l2.Remove(e2)
+	e2 = l2.InsertBefore(2, e3) // insert before back
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e2, e3})
+	l2.Remove(e2)
+
+	e2 = l2.InsertAfter(2, e1) // insert after front
+	checkListPointers(l2, []*(_Element[int]){e1, e2, e4, e3})
+	l2.Remove(e2)
+	e2 = l2.InsertAfter(2, e4) // insert after middle
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e2, e3})
+	l2.Remove(e2)
+	e2 = l2.InsertAfter(2, e3) // insert after back
+	checkListPointers(l2, []*(_Element[int]){e1, e4, e3, e2})
+	l2.Remove(e2)
+
+	// Check standard iteration.
+	sum := 0
+	for e := l2.Front(); e != nil; e = e.Next() {
+		sum += e.Value
+	}
+	if sum != 604 {
+		panic(fmt.Sprintf("sum over l = %d, want 604", sum))
+	}
+
+	// Clear all elements by iterating
+	var next *_Element[int]
+	for e := l2.Front(); e != nil; e = next {
+		next = e.Next()
+		l2.Remove(e)
+	}
+	checkListPointers(l2, []*(_Element[int]){})
+}
+
+func checkList[T comparable](l *_List[T], es []interface{}) {
+	if !checkListLen(l, len(es)) {
+		return
+	}
+
+	i := 0
+	for e := l.Front(); e != nil; e = e.Next() {
+		le := e.Value
+		// Comparison between a generically-typed variable le and an interface.
+		if le != es[i] {
+			panic(fmt.Sprintf("elt[%d].Value = %v, want %v", i, le, es[i]))
+		}
+		i++
+	}
+}
+
+func TestExtending() {
+	l1 := _New[int]()
+	l2 := _New[int]()
+
+	l1.PushBack(1)
+	l1.PushBack(2)
+	l1.PushBack(3)
+
+	l2.PushBack(4)
+	l2.PushBack(5)
+
+	l3 := _New[int]()
+	l3.PushBackList(l1)
+	checkList(l3, []interface{}{1, 2, 3})
+	l3.PushBackList(l2)
+	checkList(l3, []interface{}{1, 2, 3, 4, 5})
+
+	l3 = _New[int]()
+	l3.PushFrontList(l2)
+	checkList(l3, []interface{}{4, 5})
+	l3.PushFrontList(l1)
+	checkList(l3, []interface{}{1, 2, 3, 4, 5})
+
+	checkList(l1, []interface{}{1, 2, 3})
+	checkList(l2, []interface{}{4, 5})
+
+	l3 = _New[int]()
+	l3.PushBackList(l1)
+	checkList(l3, []interface{}{1, 2, 3})
+	l3.PushBackList(l3)
+	checkList(l3, []interface{}{1, 2, 3, 1, 2, 3})
+
+	l3 = _New[int]()
+	l3.PushFrontList(l1)
+	checkList(l3, []interface{}{1, 2, 3})
+	l3.PushFrontList(l3)
+	checkList(l3, []interface{}{1, 2, 3, 1, 2, 3})
+
+	l3 = _New[int]()
+	l1.PushBackList(l3)
+	checkList(l1, []interface{}{1, 2, 3})
+	l1.PushFrontList(l3)
+	checkList(l1, []interface{}{1, 2, 3})
+}
+
+func TestRemove() {
+	l := _New[int]()
+	e1 := l.PushBack(1)
+	e2 := l.PushBack(2)
+	checkListPointers(l, []*(_Element[int]){e1, e2})
+	e := l.Front()
+	l.Remove(e)
+	checkListPointers(l, []*(_Element[int]){e2})
+	l.Remove(e)
+	checkListPointers(l, []*(_Element[int]){e2})
+}
+
+func TestIssue4103() {
+	l1 := _New[int]()
+	l1.PushBack(1)
+	l1.PushBack(2)
+
+	l2 := _New[int]()
+	l2.PushBack(3)
+	l2.PushBack(4)
+
+	e := l1.Front()
+	l2.Remove(e) // l2 should not change because e is not an element of l2
+	if n := l2.Len(); n != 2 {
+		panic(fmt.Sprintf("l2.Len() = %d, want 2", n))
+	}
+
+	l1.InsertBefore(8, e)
+	if n := l1.Len(); n != 3 {
+		panic(fmt.Sprintf("l1.Len() = %d, want 3", n))
+	}
+}
+
+func TestIssue6349() {
+	l := _New[int]()
+	l.PushBack(1)
+	l.PushBack(2)
+
+	e := l.Front()
+	l.Remove(e)
+	if e.Value != 1 {
+		panic(fmt.Sprintf("e.value = %d, want 1", e.Value))
+	}
+	if e.Next() != nil {
+		panic(fmt.Sprintf("e.Next() != nil"))
+	}
+	if e.Prev() != nil {
+		panic(fmt.Sprintf("e.Prev() != nil"))
+	}
+}
+
+func TestMove() {
+	l := _New[int]()
+	e1 := l.PushBack(1)
+	e2 := l.PushBack(2)
+	e3 := l.PushBack(3)
+	e4 := l.PushBack(4)
+
+	l.MoveAfter(e3, e3)
+	checkListPointers(l, []*(_Element[int]){e1, e2, e3, e4})
+	l.MoveBefore(e2, e2)
+	checkListPointers(l, []*(_Element[int]){e1, e2, e3, e4})
+
+	l.MoveAfter(e3, e2)
+	checkListPointers(l, []*(_Element[int]){e1, e2, e3, e4})
+	l.MoveBefore(e2, e3)
+	checkListPointers(l, []*(_Element[int]){e1, e2, e3, e4})
+
+	l.MoveBefore(e2, e4)
+	checkListPointers(l, []*(_Element[int]){e1, e3, e2, e4})
+	e2, e3 = e3, e2
+
+	l.MoveBefore(e4, e1)
+	checkListPointers(l, []*(_Element[int]){e4, e1, e2, e3})
+	e1, e2, e3, e4 = e4, e1, e2, e3
+
+	l.MoveAfter(e4, e1)
+	checkListPointers(l, []*(_Element[int]){e1, e4, e2, e3})
+	e2, e3, e4 = e4, e2, e3
+
+	l.MoveAfter(e2, e3)
+	checkListPointers(l, []*(_Element[int]){e1, e3, e2, e4})
+	e2, e3 = e3, e2
+}
+
+// Test PushFront, PushBack, PushFrontList, PushBackList with uninitialized _List
+func TestZeroList() {
+	var l1 = new(_List[int])
+	l1.PushFront(1)
+	checkList(l1, []interface{}{1})
+
+	var l2 = new(_List[int])
+	l2.PushBack(1)
+	checkList(l2, []interface{}{1})
+
+	var l3 = new(_List[int])
+	l3.PushFrontList(l1)
+	checkList(l3, []interface{}{1})
+
+	var l4 = new(_List[int])
+	l4.PushBackList(l2)
+	checkList(l4, []interface{}{1})
+}
+
+// Test that a list l is not modified when calling InsertBefore with a mark that is not an element of l.
+func TestInsertBeforeUnknownMark() {
+	var l _List[int]
+	l.PushBack(1)
+	l.PushBack(2)
+	l.PushBack(3)
+	l.InsertBefore(1, new(_Element[int]))
+	checkList(&l, []interface{}{1, 2, 3})
+}
+
+// Test that a list l is not modified when calling InsertAfter with a mark that is not an element of l.
+func TestInsertAfterUnknownMark() {
+	var l _List[int]
+	l.PushBack(1)
+	l.PushBack(2)
+	l.PushBack(3)
+	l.InsertAfter(1, new(_Element[int]))
+	checkList(&l, []interface{}{1, 2, 3})
+}
+
+// Test that a list l is not modified when calling MoveAfter or MoveBefore with a mark that is not an element of l.
+func TestMoveUnknownMark() {
+	var l1 _List[int]
+	e1 := l1.PushBack(1)
+
+	var l2 _List[int]
+	e2 := l2.PushBack(2)
+
+	l1.MoveAfter(e1, e2)
+	checkList(&l1, []interface{}{1})
+	checkList(&l2, []interface{}{2})
+
+	l1.MoveBefore(e1, e2)
+	checkList(&l1, []interface{}{1})
+	checkList(&l2, []interface{}{2})
+}
+
+// Test the Transform function.
+func TestTransform() {
+	l1 := _New[int]()
+	l1.PushBack(1)
+	l1.PushBack(2)
+	l2 := _Transform(l1, strconv.Itoa)
+	checkList(l2, []interface{}{"1", "2"})
+}
+
+
+func main() {
+	TestList()
+}
+