[dev.typeparams] cmd/compile: implement generic type switches

Add a new dynamicType node, which is used as a case entry when
the type being switched to is generic.

Change-Id: Ice77c6f224b8fdd3ff574fdf4a8ea5f6c7ddbe75
Reviewed-on: https://go-review.googlesource.com/c/go/+/339429
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

21 files changed, 342 insertions, 18 deletions

diff --git a/src/cmd/compile/internal/escape/expr.go b/src/cmd/compile/internal/escape/expr.go
index 4a6304d47a..62afb5b928 100644
--- a/src/cmd/compile/internal/escape/expr.go
+++ b/src/cmd/compile/internal/escape/expr.go
@@ -262,6 +262,9 @@ func (e *escape) exprSkipInit(k hole, n ir.Node) {
 		// Arguments of OADDSTR never escape;
 		// runtime.concatstrings makes sure of that.
 		e.discards(n.List)
+
+	case ir.ODYNAMICTYPE:
+		// Nothing to do - argument is a *runtime._type (+ maybe a *runtime.itab) pointing to static data section
 	}
 }
 
diff --git a/src/cmd/compile/internal/ir/expr.go b/src/cmd/compile/internal/ir/expr.go
index 9c5fbbc9aa..dc28483907 100644
--- a/src/cmd/compile/internal/ir/expr.go
+++ b/src/cmd/compile/internal/ir/expr.go
@@ -700,6 +700,15 @@ func NewDynamicTypeAssertExpr(pos src.XPos, op Op, x, t Node) *DynamicTypeAssert
 	return n
 }
 
+func (n *DynamicTypeAssertExpr) SetOp(op Op) {
+	switch op {
+	default:
+		panic(n.no("SetOp " + op.String()))
+	case ODYNAMICDOTTYPE, ODYNAMICDOTTYPE2:
+		n.op = op
+	}
+}
+
 // A UnaryExpr is a unary expression Op X,
 // or Op(X) for a builtin function that does not end up being a call.
 type UnaryExpr struct {
diff --git a/src/cmd/compile/internal/ir/node.go b/src/cmd/compile/internal/ir/node.go
index e5f0c38f86..f071cb78ce 100644
--- a/src/cmd/compile/internal/ir/node.go
+++ b/src/cmd/compile/internal/ir/node.go
@@ -258,7 +258,8 @@ const (
 	OBREAK // break [Label]
 	// OCASE:  case List: Body (List==nil means default)
 	//   For OTYPESW, List is a OTYPE node for the specified type (or OLITERAL
-	//   for nil), and, if a type-switch variable is specified, Rlist is an
+	//   for nil) or an ODYNAMICTYPE indicating a runtime type for generics.
+	//   If a type-switch variable is specified, Var is an
 	//   ONAME for the version of the type-switch variable with the specified
 	//   type.
 	OCASE
@@ -320,8 +321,9 @@ const (
 	OLINKSYMOFFSET // offset within a name
 
 	// opcodes for generics
-	ODYNAMICDOTTYPE
-	ODYNAMICDOTTYPE2
+	ODYNAMICDOTTYPE  // x = i.(T) where T is a type parameter (or derived from a type parameter)
+	ODYNAMICDOTTYPE2 // x, ok = i.(T) where T is a type parameter (or derived from a type parameter)
+	ODYNAMICTYPE     // a type node for type switches (represents a dynamic target type for a type switch)
 
 	// arch-specific opcodes
 	OTAILCALL    // tail call to another function
diff --git a/src/cmd/compile/internal/ir/node_gen.go b/src/cmd/compile/internal/ir/node_gen.go
index 56db6bb9cf..aa41c03beb 100644
--- a/src/cmd/compile/internal/ir/node_gen.go
+++ b/src/cmd/compile/internal/ir/node_gen.go
@@ -463,6 +463,34 @@ func (n *Decl) editChildren(edit func(Node) Node) {
 	}
 }
 
+func (n *DynamicType) Format(s fmt.State, verb rune) { fmtNode(n, s, verb) }
+func (n *DynamicType) copy() Node {
+	c := *n
+	c.init = copyNodes(c.init)
+	return &c
+}
+func (n *DynamicType) doChildren(do func(Node) bool) bool {
+	if doNodes(n.init, do) {
+		return true
+	}
+	if n.X != nil && do(n.X) {
+		return true
+	}
+	if n.ITab != nil && do(n.ITab) {
+		return true
+	}
+	return false
+}
+func (n *DynamicType) editChildren(edit func(Node) Node) {
+	editNodes(n.init, edit)
+	if n.X != nil {
+		n.X = edit(n.X).(Node)
+	}
+	if n.ITab != nil {
+		n.ITab = edit(n.ITab).(Node)
+	}
+}
+
 func (n *DynamicTypeAssertExpr) Format(s fmt.State, verb rune) { fmtNode(n, s, verb) }
 func (n *DynamicTypeAssertExpr) copy() Node {
 	c := *n
diff --git a/src/cmd/compile/internal/ir/op_string.go b/src/cmd/compile/internal/ir/op_string.go
index 7b08ee287a..b8cee71818 100644
--- a/src/cmd/compile/internal/ir/op_string.go
+++ b/src/cmd/compile/internal/ir/op_string.go
@@ -164,16 +164,17 @@ func _() {
 	_ = x[OLINKSYMOFFSET-153]
 	_ = x[ODYNAMICDOTTYPE-154]
 	_ = x[ODYNAMICDOTTYPE2-155]
-	_ = x[OTAILCALL-156]
-	_ = x[OGETG-157]
-	_ = x[OGETCALLERPC-158]
-	_ = x[OGETCALLERSP-159]
-	_ = x[OEND-160]
+	_ = x[ODYNAMICTYPE-156]
+	_ = x[OTAILCALL-157]
+	_ = x[OGETG-158]
+	_ = x[OGETCALLERPC-159]
+	_ = x[OGETCALLERSP-160]
+	_ = x[OEND-161]
 }
 
-const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESSLICE2ARRPTRASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVIDATACONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECOVERFPRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFUNSAFEADDUNSAFESLICEMETHEXPRMETHVALUEBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWFUNCINSTTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETDYNAMICDOTTYPEDYNAMICDOTTYPE2TAILCALLGETGGETCALLERPCGETCALLERSPEND"
+const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESSLICE2ARRPTRASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVIDATACONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECOVERFPRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFUNSAFEADDUNSAFESLICEMETHEXPRMETHVALUEBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWFUNCINSTTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETDYNAMICDOTTYPEDYNAMICDOTTYPE2DYNAMICTYPETAILCALLGETGGETCALLERPCGETCALLERSPEND"
 
-var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 136, 138, 141, 151, 158, 165, 172, 176, 180, 188, 196, 205, 208, 213, 220, 227, 233, 242, 250, 258, 264, 268, 277, 286, 293, 297, 300, 307, 315, 322, 328, 331, 337, 344, 352, 356, 363, 371, 373, 375, 377, 379, 381, 383, 388, 393, 401, 404, 413, 416, 420, 428, 435, 444, 457, 460, 463, 466, 469, 472, 475, 481, 484, 487, 493, 497, 500, 504, 509, 514, 520, 525, 529, 534, 542, 550, 556, 565, 576, 583, 592, 596, 603, 611, 615, 619, 623, 630, 637, 645, 651, 660, 671, 679, 688, 693, 698, 702, 710, 715, 719, 722, 730, 734, 736, 741, 743, 748, 754, 760, 766, 772, 780, 785, 789, 796, 802, 807, 813, 819, 826, 831, 835, 840, 844, 849, 857, 863, 870, 877, 883, 890, 903, 917, 932, 940, 944, 955, 966, 969}
+var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 136, 138, 141, 151, 158, 165, 172, 176, 180, 188, 196, 205, 208, 213, 220, 227, 233, 242, 250, 258, 264, 268, 277, 286, 293, 297, 300, 307, 315, 322, 328, 331, 337, 344, 352, 356, 363, 371, 373, 375, 377, 379, 381, 383, 388, 393, 401, 404, 413, 416, 420, 428, 435, 444, 457, 460, 463, 466, 469, 472, 475, 481, 484, 487, 493, 497, 500, 504, 509, 514, 520, 525, 529, 534, 542, 550, 556, 565, 576, 583, 592, 596, 603, 611, 615, 619, 623, 630, 637, 645, 651, 660, 671, 679, 688, 693, 698, 702, 710, 715, 719, 722, 730, 734, 736, 741, 743, 748, 754, 760, 766, 772, 780, 785, 789, 796, 802, 807, 813, 819, 826, 831, 835, 840, 844, 849, 857, 863, 870, 877, 883, 890, 903, 917, 932, 943, 951, 955, 966, 977, 980}
 
 func (i Op) String() string {
 	if i >= Op(len(_Op_index)-1) {
diff --git a/src/cmd/compile/internal/ir/type.go b/src/cmd/compile/internal/ir/type.go
index 431468375a..63dd673dcd 100644
--- a/src/cmd/compile/internal/ir/type.go
+++ b/src/cmd/compile/internal/ir/type.go
@@ -319,3 +319,17 @@ func TypeNodeAt(pos src.XPos, t *types.Type) Ntype {
 	}
 	return newTypeNode(pos, t)
 }
+
+// A DynamicType represents the target type in a type switch.
+type DynamicType struct {
+	miniExpr
+	X    Node // a *runtime._type for the targeted type
+	ITab Node // for type switches from nonempty interfaces to non-interfaces, this is the itab for that pair.
+}
+
+func NewDynamicType(pos src.XPos, x Node) *DynamicType {
+	n := &DynamicType{X: x}
+	n.pos = pos
+	n.op = ODYNAMICTYPE
+	return n
+}
diff --git a/src/cmd/compile/internal/noder/irgen.go b/src/cmd/compile/internal/noder/irgen.go
index 571e294416..7bc8a6bcc3 100644
--- a/src/cmd/compile/internal/noder/irgen.go
+++ b/src/cmd/compile/internal/noder/irgen.go
@@ -107,6 +107,10 @@ type gfInfo struct {
 	// Nodes in generic functions that are a conversion from a typeparam/derived
 	// type to a specific interface.
 	itabConvs []ir.Node
+	// For type switches on nonempty interfaces, a map from OTYPE entries of
+	// HasTParam type, to the interface type we're switching from.
+	// TODO: what if the type we're switching from is a shape type?
+	type2switchType map[ir.Node]*types.Type
 }
 
 // instInfo is information gathered on an gcshape (or fully concrete)
diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index b37f76dcee..5f2250d2f4 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -1140,6 +1140,38 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			m = ir.NewDynamicTypeAssertExpr(dt.Pos(), op, dt.X, rt)
 			m.SetType(dt.Type())
 			m.SetTypecheck(1)
+		case ir.OCASE:
+			if _, ok := x.(*ir.CommClause); ok {
+				// This is not a type switch. TODO: Should we use an OSWITCH case here instead of OCASE?
+				break
+			}
+			x := x.(*ir.CaseClause)
+			m := m.(*ir.CaseClause)
+			for i, c := range x.List {
+				if c.Op() == ir.OTYPE && c.Type().HasTParam() {
+					// Use a *runtime._type for the dynamic type.
+					ix := findDictType(subst.info, c.Type())
+					assert(ix >= 0)
+					dt := ir.NewDynamicType(c.Pos(), getDictionaryEntry(c.Pos(), subst.info.dictParam, ix, subst.info.dictLen))
+
+					// For type switch from nonemoty interfaces to non-interfaces, we need an itab as well.
+					if _, ok := subst.info.gfInfo.type2switchType[c]; ok {
+						// Type switch from nonempty interface. We need a *runtime.itab
+						// for the dynamic type.
+						ix := -1
+						for i, ic := range subst.info.gfInfo.itabConvs {
+							if ic == c {
+								ix = subst.info.startItabConv + i
+								break
+							}
+						}
+						assert(ix >= 0)
+						dt.ITab = getDictionaryEntry(c.Pos(), subst.info.dictParam, ix, subst.info.dictLen)
+					}
+					typed(m.List[i].Type(), dt)
+					m.List[i] = dt
+				}
+			}
 		}
 		return m
 	}
@@ -1483,6 +1515,9 @@ func (g *irgen) finalizeSyms() {
 			case ir.OCONVIFACE:
 				srctype = subst.Typ(n.(*ir.ConvExpr).X.Type())
 				dsttype = subst.Typ(n.Type())
+			case ir.OTYPE:
+				srctype = subst.Typ(n.Type())
+				dsttype = subst.Typ(info.type2switchType[n])
 			default:
 				base.Fatalf("itab entry with unknown op %s", n.Op())
 			}
@@ -1652,6 +1687,21 @@ func (g *irgen) getGfInfo(gn *ir.Name) *gfInfo {
 				ir.Visit(n1, visitFunc)
 			}
 		}
+		if n.Op() == ir.OSWITCH && n.(*ir.SwitchStmt).Tag != nil && n.(*ir.SwitchStmt).Tag.Op() == ir.OTYPESW && !n.(*ir.SwitchStmt).Tag.(*ir.TypeSwitchGuard).X.Type().IsEmptyInterface() {
+			for _, cc := range n.(*ir.SwitchStmt).Cases {
+				for _, c := range cc.List {
+					if c.Op() == ir.OTYPE && c.Type().HasTParam() {
+						// Type switch from a non-empty interface to a noninterface.
+						infoPrint("  Itab for type switch: %v\n", c)
+						info.itabConvs = append(info.itabConvs, c)
+						if info.type2switchType == nil {
+							info.type2switchType = map[ir.Node]*types.Type{}
+						}
+						info.type2switchType[c] = n.(*ir.SwitchStmt).Tag.(*ir.TypeSwitchGuard).X.Type()
+					}
+				}
+			}
+		}
 		addType(&info, n, n.Type())
 	}
 
diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index 61af92b62a..ff113877df 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -313,6 +313,10 @@ assignOK:
 			r := r.(*ir.TypeAssertExpr)
 			stmt.SetOp(ir.OAS2DOTTYPE)
 			r.SetOp(ir.ODOTTYPE2)
+		case ir.ODYNAMICDOTTYPE:
+			r := r.(*ir.DynamicTypeAssertExpr)
+			stmt.SetOp(ir.OAS2DOTTYPE)
+			r.SetOp(ir.ODYNAMICDOTTYPE2)
 		default:
 			break assignOK
 		}
diff --git a/src/cmd/compile/internal/typecheck/stmt.go b/src/cmd/compile/internal/typecheck/stmt.go
index 0143411822..c322d490e5 100644
--- a/src/cmd/compile/internal/typecheck/stmt.go
+++ b/src/cmd/compile/internal/typecheck/stmt.go
@@ -172,6 +172,10 @@ assignOK:
 			r := r.(*ir.TypeAssertExpr)
 			stmt.SetOp(ir.OAS2DOTTYPE)
 			r.SetOp(ir.ODOTTYPE2)
+		case ir.ODYNAMICDOTTYPE:
+			r := r.(*ir.DynamicTypeAssertExpr)
+			stmt.SetOp(ir.OAS2DOTTYPE)
+			r.SetOp(ir.ODYNAMICDOTTYPE2)
 		default:
 			break assignOK
 		}
diff --git a/src/cmd/compile/internal/walk/switch.go b/src/cmd/compile/internal/walk/switch.go
index 162de018f6..3705c5b192 100644
--- a/src/cmd/compile/internal/walk/switch.go
+++ b/src/cmd/compile/internal/walk/switch.go
@@ -360,10 +360,10 @@ func walkSwitchType(sw *ir.SwitchStmt) {
 			}
 
 			if singleType != nil && singleType.IsInterface() {
-				s.Add(ncase.Pos(), n1.Type(), caseVar, jmp)
+				s.Add(ncase.Pos(), n1, caseVar, jmp)
 				caseVarInitialized = true
 			} else {
-				s.Add(ncase.Pos(), n1.Type(), nil, jmp)
+				s.Add(ncase.Pos(), n1, nil, jmp)
 			}
 		}
 
@@ -377,6 +377,17 @@ func walkSwitchType(sw *ir.SwitchStmt) {
 				}
 				val = ifaceData(ncase.Pos(), s.facename, singleType)
 			}
+			if len(ncase.List) == 1 && ncase.List[0].Op() == ir.ODYNAMICTYPE {
+				dt := ncase.List[0].(*ir.DynamicType)
+				x := ir.NewDynamicTypeAssertExpr(ncase.Pos(), ir.ODYNAMICDOTTYPE, val, dt.X)
+				if dt.ITab != nil {
+					// TODO: make ITab a separate field in DynamicTypeAssertExpr?
+					x.T = dt.ITab
+				}
+				x.SetType(caseVar.Type())
+				x.SetTypecheck(1)
+				val = x
+			}
 			l := []ir.Node{
 				ir.NewDecl(ncase.Pos(), ir.ODCL, caseVar),
 				ir.NewAssignStmt(ncase.Pos(), caseVar, val),
@@ -446,7 +457,8 @@ type typeClause struct {
 	body ir.Nodes
 }
 
-func (s *typeSwitch) Add(pos src.XPos, typ *types.Type, caseVar *ir.Name, jmp ir.Node) {
+func (s *typeSwitch) Add(pos src.XPos, n1 ir.Node, caseVar *ir.Name, jmp ir.Node) {
+	typ := n1.Type()
 	var body ir.Nodes
 	if caseVar != nil {
 		l := []ir.Node{
@@ -462,9 +474,25 @@ func (s *typeSwitch) Add(pos src.XPos, typ *types.Type, caseVar *ir.Name, jmp ir
 	// cv, ok = iface.(type)
 	as := ir.NewAssignListStmt(pos, ir.OAS2, nil, nil)
 	as.Lhs = []ir.Node{caseVar, s.okname} // cv, ok =
-	dot := ir.NewTypeAssertExpr(pos, s.facename, nil)
-	dot.SetType(typ) // iface.(type)
-	as.Rhs = []ir.Node{dot}
+	switch n1.Op() {
+	case ir.OTYPE:
+		// Static type assertion (non-generic)
+		dot := ir.NewTypeAssertExpr(pos, s.facename, nil)
+		dot.SetType(typ) // iface.(type)
+		as.Rhs = []ir.Node{dot}
+	case ir.ODYNAMICTYPE:
+		// Dynamic type assertion (generic)
+		dt := n1.(*ir.DynamicType)
+		dot := ir.NewDynamicTypeAssertExpr(pos, ir.ODYNAMICDOTTYPE, s.facename, dt.X)
+		if dt.ITab != nil {
+			dot.T = dt.ITab
+		}
+		dot.SetType(typ)
+		dot.SetTypecheck(1)
+		as.Rhs = []ir.Node{dot}
+	default:
+		base.Fatalf("unhandled type case %s", n1.Op())
+	}
 	appendWalkStmt(&body, as)
 
 	// if ok { goto label }
@@ -473,9 +501,10 @@ func (s *typeSwitch) Add(pos src.XPos, typ *types.Type, caseVar *ir.Name, jmp ir
 	nif.Body = []ir.Node{jmp}
 	body.Append(nif)
 
-	if !typ.IsInterface() {
+	if n1.Op() == ir.OTYPE && !typ.IsInterface() {
+		// Defer static, noninterface cases so they can be binary searched by hash.
 		s.clauses = append(s.clauses, typeClause{
-			hash: types.TypeHash(typ),
+			hash: types.TypeHash(n1.Type()),
 			body: body,
 		})
 		return
diff --git a/test/typeparam/typeswitch1.go b/test/typeparam/typeswitch1.go
new file mode 100644
index 0000000000..27161b3db8
--- /dev/null
+++ b/test/typeparam/typeswitch1.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
+
+func f[T any](i interface{}) {
+	switch i.(type) {
+	case T:
+		println("T")
+	case int:
+		println("int")
+	case int32, int16:
+		println("int32/int16")
+	case struct { a, b T }:
+		println("struct{T,T}")
+	default:
+		println("other")
+	}
+}
+func main() {
+	f[float64](float64(6))
+	f[float64](int(7))
+	f[float64](int32(8))
+	f[float64](struct{a, b float64}{a:1, b:2})
+	f[float64](int8(9))
+}
diff --git a/test/typeparam/typeswitch1.out b/test/typeparam/typeswitch1.out
new file mode 100644
index 0000000000..4bdbccfddb
--- /dev/null
+++ b/test/typeparam/typeswitch1.out
@@ -0,0 +1,5 @@
+T
+int
+int32/int16
+struct{T,T}
+other
diff --git a/test/typeparam/typeswitch2.go b/test/typeparam/typeswitch2.go
new file mode 100644
index 0000000000..913c56321c
--- /dev/null
+++ b/test/typeparam/typeswitch2.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 "reflect"
+
+func f[T any](i interface{}) {
+	switch x := i.(type) {
+	case T:
+		println("T", x)
+	case int:
+		println("int", x)
+	case int32, int16:
+		println("int32/int16", reflect.ValueOf(x).Int())
+	case struct { a, b T }:
+		println("struct{T,T}", x.a, x.b)
+	default:
+		println("other", reflect.ValueOf(x).Int())
+	}
+}
+func main() {
+	f[float64](float64(6))
+	f[float64](int(7))
+	f[float64](int32(8))
+	f[float64](struct{a, b float64}{a:1, b:2})
+	f[float64](int8(9))
+}
diff --git a/test/typeparam/typeswitch2.out b/test/typeparam/typeswitch2.out
new file mode 100644
index 0000000000..944cc04cc6
--- /dev/null
+++ b/test/typeparam/typeswitch2.out
@@ -0,0 +1,5 @@
+T +6.000000e+000
+int 7
+int32/int16 8
+struct{T,T} +1.000000e+000 +2.000000e+000
+other 9
diff --git a/test/typeparam/typeswitch3.go b/test/typeparam/typeswitch3.go
new file mode 100644
index 0000000000..6ab0301140
--- /dev/null
+++ b/test/typeparam/typeswitch3.go
@@ -0,0 +1,35 @@
+// 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 I interface { foo() int }
+
+type myint int
+
+func (x myint) foo() int { return int(x) }
+
+type myfloat float64
+func (x myfloat) foo() int { return int(x) }
+
+type myint32 int32
+func (x myint32) foo() int { return int(x) }
+
+func f[T I](i I) {
+	switch x := i.(type) {
+	case T:
+		println("T", x.foo())
+	case myint:
+		println("myint", x.foo())
+	default:
+		println("other", x.foo())
+	}
+}
+func main() {
+	f[myfloat](myint(6))
+	f[myfloat](myfloat(7))
+	f[myfloat](myint32(8))
+}
diff --git a/test/typeparam/typeswitch3.out b/test/typeparam/typeswitch3.out
new file mode 100644
index 0000000000..2c69c72c30
--- /dev/null
+++ b/test/typeparam/typeswitch3.out
@@ -0,0 +1,3 @@
+myint 6
+T 7
+other 8
diff --git a/test/typeparam/typeswitch4.go b/test/typeparam/typeswitch4.go
new file mode 100644
index 0000000000..6113026b65
--- /dev/null
+++ b/test/typeparam/typeswitch4.go
@@ -0,0 +1,33 @@
+// 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 I interface { foo() int }
+
+type myint int
+
+func (x myint) foo() int {return int(x)}
+
+type myfloat float64
+func (x myfloat) foo() int {return int(x)}
+
+type myint32 int32
+func (x myint32) foo() int { return int(x) }
+
+func f[T I](i I) {
+	switch x := i.(type) {
+	case T, myint32:
+		println("T/myint32", x.foo())
+	default:
+		println("other", x.foo())
+	}
+}
+func main() {
+	f[myfloat](myint(6))
+	f[myfloat](myfloat(7))
+	f[myfloat](myint32(8))
+}
diff --git a/test/typeparam/typeswitch4.out b/test/typeparam/typeswitch4.out
new file mode 100644
index 0000000000..b0d54077c9
--- /dev/null
+++ b/test/typeparam/typeswitch4.out
@@ -0,0 +1,3 @@
+other 6
+T/myint32 7
+T/myint32 8
diff --git a/test/typeparam/typeswitch5.go b/test/typeparam/typeswitch5.go
new file mode 100644
index 0000000000..1fc6e0a14e
--- /dev/null
+++ b/test/typeparam/typeswitch5.go
@@ -0,0 +1,28 @@
+// 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 myint int
+func (x myint) foo() int {return int(x)}
+
+type myfloat float64
+func (x myfloat) foo() float64 {return float64(x) }
+
+func f[T any](i interface{}) {
+	switch x := i.(type) {
+	case interface { foo() T }:
+		println("fooer", x.foo())
+	default:
+		println("other")
+	}
+}
+func main() {
+	f[int](myint(6))
+	f[int](myfloat(7))
+	f[float64](myint(8))
+	f[float64](myfloat(9))
+}
diff --git a/test/typeparam/typeswitch5.out b/test/typeparam/typeswitch5.out
new file mode 100644
index 0000000000..6b4cb4416f
--- /dev/null
+++ b/test/typeparam/typeswitch5.out
@@ -0,0 +1,4 @@
+fooer 6
+other
+other
+fooer +9.000000e+000