cmd/compile: support field access for typeparam with structural constraint

In the compiler, we need to distinguish field and method access on a
type param. For field access, we avoid the dictionary access (to create
an interface bound) and just do the normal transformDot() (which will
create the field access on the shape type).

This field access works fine for non-pointer types, since the shape type
preserves the underlying type of all types in the shape. But we
generally merge all pointer types into a single shape, which means the
field will not be accessible via the shape type. So, we need to change
Shapify() so that a type which is a pointer type is mapped to its
underlying type, rather than being merged with other pointers.

Because we don't want to change the export format at this point in the
release, we need to compute StructuralType() directly in types1, rather
than relying on types2. That implementation is in types/type.go, along
with the helper specificTypes().

I enabled the compiler-related tests in issue50417.go, added an extra
test for unnamed pointer types, and added a bunch more tests for
interesting cases involving StructuralType(). I added a test
issue50417b.go similar to the original example, but also tests access to
an embedded field.

I also added a unit test in
cmd/compile/internal/types/structuraltype_test.go that tests a bunch of
unusual cases directly (some of which have no structural type).

Updates #50417

Change-Id: I77c55cbad98a2b95efbd4a02a026c07dfbb46caa
Reviewed-on: https://go-review.googlesource.com/c/go/+/376194
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

10 files changed, 512 insertions, 46 deletions

diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index e5f59d0286..66c73a9427 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -634,17 +634,38 @@ func (g *genInst) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMe
 		checkFetchBody(nameNode)
 	}
 
+	var tparams []*types.Type
+	if isMeth {
+		// Get the type params from the method receiver (after skipping
+		// over any pointer)
+		recvType := nameNode.Type().Recv().Type
+		recvType = deref(recvType)
+		tparams = recvType.RParams()
+	} else {
+		fields := nameNode.Type().TParams().Fields().Slice()
+		tparams = make([]*types.Type, len(fields))
+		for i, f := range fields {
+			tparams[i] = f.Type
+		}
+	}
+
 	// 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
 	// of shape and non-shape arguments, because of inferred or explicitly
 	// specified concrete type args.
 	s1 := make([]*types.Type, len(shapes))
 	for i, t := range shapes {
+		var tparam *types.Type
+		if tparams[i].Kind() == types.TTYPEPARAM {
+			// Shapes are grouped differently for structural types, so we
+			// pass the type param to Shapify(), so we can distinguish.
+			tparam = tparams[i]
+		}
 		if !t.IsShape() {
-			s1[i] = typecheck.Shapify(t, i)
+			s1[i] = typecheck.Shapify(t, i, tparam)
 		} else {
 			// Already a shape, but make sure it has the correct index.
-			s1[i] = typecheck.Shapify(shapes[i].Underlying(), i)
+			s1[i] = typecheck.Shapify(shapes[i].Underlying(), i, tparam)
 		}
 	}
 	shapes = s1
@@ -675,7 +696,7 @@ func (g *genInst) getInstantiation(nameNode *ir.Name, shapes []*types.Type, isMe
 		}
 
 		// genericSubst fills in info.dictParam and info.shapeToBound.
-		st := g.genericSubst(sym, nameNode, shapes, isMeth, info)
+		st := g.genericSubst(sym, nameNode, tparams, shapes, isMeth, info)
 		info.fun = st
 		g.instInfoMap[sym] = info
 
@@ -713,21 +734,7 @@ type subster struct {
 // function type where the receiver becomes the first parameter. For either a generic
 // method or function, a dictionary parameter is the added as the very first
 // parameter. genericSubst fills in info.dictParam and info.shapeToBound.
-func (g *genInst) genericSubst(newsym *types.Sym, nameNode *ir.Name, shapes []*types.Type, isMethod bool, info *instInfo) *ir.Func {
-	var tparams []*types.Type
-	if isMethod {
-		// Get the type params from the method receiver (after skipping
-		// over any pointer)
-		recvType := nameNode.Type().Recv().Type
-		recvType = deref(recvType)
-		tparams = recvType.RParams()
-	} else {
-		fields := nameNode.Type().TParams().Fields().Slice()
-		tparams = make([]*types.Type, len(fields))
-		for i, f := range fields {
-			tparams[i] = f.Type
-		}
-	}
+func (g *genInst) genericSubst(newsym *types.Sym, nameNode *ir.Name, tparams []*types.Type, shapes []*types.Type, isMethod bool, info *instInfo) *ir.Func {
 	gf := nameNode.Func
 	// Pos of the instantiated function is same as the generic function
 	newf := ir.NewFunc(gf.Pos())
@@ -1208,31 +1215,40 @@ func (g *genInst) dictPass(info *instInfo) {
 			ir.CurFunc = info.fun
 
 		case ir.OXDOT:
+			// This is the case of a dot access on a type param. This is
+			// typically a bound call on the type param, but could be a
+			// field access, if the constraint has a single structural type.
 			mse := m.(*ir.SelectorExpr)
 			src := mse.X.Type()
 			assert(src.IsShape())
 
-			// The only dot on a shape type value are methods.
 			if mse.X.Op() == ir.OTYPE {
 				// Method expression T.M
 				m = g.buildClosure2(info, m)
 				// No need for transformDot - buildClosure2 has already
 				// transformed to OCALLINTER/ODOTINTER.
 			} else {
-				// Implement x.M as a conversion-to-bound-interface
-				//  1) convert x to the bound interface
-				//  2) call M on that interface
 				dst := info.dictInfo.shapeToBound[m.(*ir.SelectorExpr).X.Type()]
-				if src.IsInterface() {
-					// If type arg is an interface (unusual case),
-					// we do a type assert to the type bound.
-					mse.X = assertToBound(info, info.dictParam, m.Pos(), mse.X, dst)
-				} else {
-					mse.X = convertUsingDictionary(info, info.dictParam, m.Pos(), mse.X, m, dst, true)
-					// Note: we set nonEscaping==true, because we can assume the backing store for the
-					// interface conversion doesn't escape. The method call will immediately go to
-					// a wrapper function which copies all the data out of the interface value.
-					// (It only matters for non-pointer-shaped interface conversions. See issue 50182.)
+				// If we can't find the selected method in the
+				// AllMethods of the bound, then this must be an access
+				// to a field of a structural type. If so, we skip the
+				// dictionary lookups - transformDot() will convert to
+				// the desired direct field access.
+				if typecheck.Lookdot1(mse, mse.Sel, dst, dst.AllMethods(), 1) != nil {
+					// Implement x.M as a conversion-to-bound-interface
+					//  1) convert x to the bound interface
+					//  2) call M on that interface
+					if src.IsInterface() {
+						// If type arg is an interface (unusual case),
+						// we do a type assert to the type bound.
+						mse.X = assertToBound(info, info.dictParam, m.Pos(), mse.X, dst)
+					} else {
+						mse.X = convertUsingDictionary(info, info.dictParam, m.Pos(), mse.X, m, dst, true)
+						// Note: we set nonEscaping==true, because we can assume the backing store for the
+						// interface conversion doesn't escape. The method call will immediately go to
+						// a wrapper function which copies all the data out of the interface value.
+						// (It only matters for non-pointer-shaped interface conversions. See issue 50182.)
+					}
 				}
 				transformDot(mse, false)
 			}
diff --git a/src/cmd/compile/internal/reflectdata/reflect.go b/src/cmd/compile/internal/reflectdata/reflect.go
index eb1d7b0e07..42ea7bac46 100644
--- a/src/cmd/compile/internal/reflectdata/reflect.go
+++ b/src/cmd/compile/internal/reflectdata/reflect.go
@@ -1921,8 +1921,9 @@ func methodWrapper(rcvr *types.Type, method *types.Field, forItab bool) *obj.LSy
 
 			// Target method uses shaped names.
 			targs2 := make([]*types.Type, len(targs))
+			origRParams := deref(orig).OrigSym().Def.(*ir.Name).Type().RParams()
 			for i, t := range targs {
-				targs2[i] = typecheck.Shapify(t, i)
+				targs2[i] = typecheck.Shapify(t, i, origRParams[i])
 			}
 			targs = targs2
 
diff --git a/src/cmd/compile/internal/typecheck/crawler.go b/src/cmd/compile/internal/typecheck/crawler.go
index a25c741488..4394c6e698 100644
--- a/src/cmd/compile/internal/typecheck/crawler.go
+++ b/src/cmd/compile/internal/typecheck/crawler.go
@@ -232,7 +232,7 @@ func (p *crawler) checkForFullyInst(t *types.Type) {
 		baseType := t.OrigSym().Def.(*ir.Name).Type()
 		shapes := make([]*types.Type, len(t.RParams()))
 		for i, t1 := range t.RParams() {
-			shapes[i] = Shapify(t1, i)
+			shapes[i] = Shapify(t1, i, baseType.RParams()[i])
 		}
 		for j := range t.Methods().Slice() {
 			baseNname := baseType.Methods().Slice()[j].Nname.(*ir.Name)
diff --git a/src/cmd/compile/internal/typecheck/subr.go b/src/cmd/compile/internal/typecheck/subr.go
index 04a4ed392f..9f6966233d 100644
--- a/src/cmd/compile/internal/typecheck/subr.go
+++ b/src/cmd/compile/internal/typecheck/subr.go
@@ -1430,11 +1430,15 @@ func genericTypeName(sym *types.Sym) string {
 // For now, we only consider two types to have the same shape, if they have exactly
 // the same underlying type or they are both pointer types.
 //
+//  tparam is the associated typeparam. If there is a structural type for
+//  the associated type param (not common), then a pointer type t is mapped to its
+//  underlying type, rather than being merged with other pointers.
+//
 //  Shape types are also distinguished by the index of the type in a type param/arg
 //  list. We need to do this so we can distinguish and substitute properly for two
 //  type params in the same function that have the same shape for a particular
 //  instantiation.
-func Shapify(t *types.Type, index int) *types.Type {
+func Shapify(t *types.Type, index int, tparam *types.Type) *types.Type {
 	assert(!t.IsShape())
 	// Map all types with the same underlying type to the same shape.
 	u := t.Underlying()
@@ -1443,7 +1447,8 @@ func Shapify(t *types.Type, index int) *types.Type {
 	// TODO: Make unsafe.Pointer the same shape as normal pointers.
 	// Note: pointers to arrays are special because of slice-to-array-pointer
 	// conversions. See issue 49295.
-	if u.Kind() == types.TPTR && u.Elem().Kind() != types.TARRAY {
+	if u.Kind() == types.TPTR && u.Elem().Kind() != types.TARRAY &&
+		tparam.Bound().StructuralType() == nil {
 		u = types.Types[types.TUINT8].PtrTo()
 	}
 
diff --git a/src/cmd/compile/internal/types/structuraltype.go b/src/cmd/compile/internal/types/structuraltype.go
new file mode 100644
index 0000000000..2d49e77aae
--- /dev/null
+++ b/src/cmd/compile/internal/types/structuraltype.go
@@ -0,0 +1,187 @@
+package types
+
+// Implementation of structural type computation for types.
+
+// TODO: we would like to depend only on the types2 computation of structural type,
+// but we can only do that the next time we change the export format and export
+// structural type info along with each constraint type, since the compiler imports
+// types directly into types1 format.
+
+// A term describes elementary type sets:
+//
+// term{false, T}  set of type T
+// term{true, T}   set of types with underlying type t
+// term{}          empty set (we specifically check for typ == nil)
+type term struct {
+	tilde bool
+	typ   *Type
+}
+
+// StructuralType returns the structural type of an interface, or nil if it has no
+// structural type.
+func (t *Type) StructuralType() *Type {
+	sts, _ := specificTypes(t)
+	var su *Type
+	for _, st := range sts {
+		u := st.typ.Underlying()
+		if su != nil {
+			u = match(su, u)
+			if u == nil {
+				return nil
+			}
+		}
+		// su == nil || match(su, u) != nil
+		su = u
+	}
+	return su
+}
+
+// If x and y are identical, match returns x.
+// If x and y are identical channels but for their direction
+// and one of them is unrestricted, match returns the channel
+// with the restricted direction.
+// In all other cases, match returns nil.
+// x and y are assumed to be underlying types, hence are not named types.
+func match(x, y *Type) *Type {
+	if IdenticalStrict(x, y) {
+		return x
+	}
+
+	if x.IsChan() && y.IsChan() && IdenticalStrict(x.Elem(), y.Elem()) {
+		// We have channels that differ in direction only.
+		// If there's an unrestricted channel, select the restricted one.
+		// If both have the same direction, return x (either is fine).
+		switch {
+		case x.ChanDir().CanSend() && x.ChanDir().CanRecv():
+			return y
+		case y.ChanDir().CanSend() && y.ChanDir().CanRecv():
+			return x
+		}
+	}
+	return nil
+}
+
+// specificTypes returns the list of specific types of an interface type or nil if
+// there are none. It also returns a flag that indicates, for an empty term list
+// result, whether it represents the empty set, or the infinite set of all types (in
+// both cases, there are no specific types).
+func specificTypes(t *Type) (list []term, inf bool) {
+	t.wantEtype(TINTER)
+
+	// We have infinite term list before processing any type elements
+	// (or if there are no type elements).
+	inf = true
+	for _, m := range t.Methods().Slice() {
+		var r2 []term
+		inf2 := false
+
+		switch {
+		case m.IsMethod():
+			inf2 = true
+
+		case m.Type.IsUnion():
+			nt := m.Type.NumTerms()
+			for i := 0; i < nt; i++ {
+				t, tilde := m.Type.Term(i)
+				if t.IsInterface() {
+					r3, r3inf := specificTypes(t)
+					if r3inf {
+						// Union with an infinite set of types is
+						// infinite, so skip remaining terms.
+						r2 = nil
+						inf2 = true
+						break
+					}
+					// Add the elements of r3 to r2.
+					for _, r3e := range r3 {
+						r2 = insertType(r2, r3e)
+					}
+				} else {
+					r2 = insertType(r2, term{tilde, t})
+				}
+			}
+
+		case m.Type.IsInterface():
+			r2, inf2 = specificTypes(m.Type)
+
+		default:
+			// m.Type is a single non-interface type, so r2 is just a
+			// one-element list, inf2 is false.
+			r2 = []term{term{false, m.Type}}
+		}
+
+		if inf2 {
+			// If the current type element has infinite types,
+			// its intersection with r is just r, so skip this type element.
+			continue
+		}
+
+		if inf {
+			// If r is infinite, then the intersection of r and r2 is just r2.
+			list = r2
+			inf = false
+			continue
+		}
+
+		// r and r2 are finite, so intersect r and r2.
+		var r3 []term
+		for _, re := range list {
+			for _, r2e := range r2 {
+				if tm := intersect(re, r2e); tm.typ != nil {
+					r3 = append(r3, tm)
+				}
+			}
+		}
+		list = r3
+	}
+	return
+}
+
+// insertType adds t to the returned list if it is not already in list.
+func insertType(list []term, tm term) []term {
+	for i, elt := range list {
+		if new := union(elt, tm); new.typ != nil {
+			// Replace existing elt with the union of elt and new.
+			list[i] = new
+			return list
+		}
+	}
+	return append(list, tm)
+}
+
+// If x and y are disjoint, return term with nil typ (which means the union should
+// include both types). If x and y are not disjoint, return the single type which is
+// the union of x and y.
+func union(x, y term) term {
+	if disjoint(x, y) {
+		return term{false, nil}
+	}
+	if x.tilde || !y.tilde {
+		return x
+	}
+	return y
+}
+
+// intersect returns the intersection x ∩ y.
+func intersect(x, y term) term {
+	if disjoint(x, y) {
+		return term{false, nil}
+	}
+	if !x.tilde || y.tilde {
+		return x
+	}
+	return y
+}
+
+// disjoint reports whether x ∩ y == ∅.
+func disjoint(x, y term) bool {
+	ux := x.typ
+	if y.tilde {
+		ux = ux.Underlying()
+	}
+	uy := y.typ
+	if x.tilde {
+		uy = uy.Underlying()
+	}
+	return !IdenticalStrict(ux, uy)
+}
diff --git a/src/cmd/compile/internal/types/structuraltype_test.go b/src/cmd/compile/internal/types/structuraltype_test.go
new file mode 100644
index 0000000000..fc34458338
--- /dev/null
+++ b/src/cmd/compile/internal/types/structuraltype_test.go
@@ -0,0 +1,135 @@
+// Copyright 2022 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.
+
+// Test that StructuralType() calculates the correct value of structural type for
+// unusual cases.
+
+package types
+
+import (
+	"cmd/internal/src"
+	"testing"
+)
+
+type test struct {
+	typ            *Type
+	structuralType *Type
+}
+
+func TestStructuralType(t *testing.T) {
+	// These are the few constants that need to be initialized in order to use
+	// the types package without using the typecheck package by calling
+	// typecheck.InitUniverse() (the normal way to initialize the types package).
+	PtrSize = 8
+	RegSize = 8
+	MaxWidth = 1 << 50
+
+	// type intType = int
+	intType := newType(TINT)
+	// type structf = struct { f int }
+	structf := NewStruct(nil, []*Field{
+		NewField(src.NoXPos, LocalPkg.Lookup("f"), intType),
+	})
+
+	// type Sf structf
+	Sf := newType(TFORW)
+	Sf.sym = LocalPkg.Lookup("Sf")
+	Sf.SetUnderlying(structf)
+
+	// type A int
+	A := newType(TFORW)
+	A.sym = LocalPkg.Lookup("A")
+	A.SetUnderlying(intType)
+
+	// type B int
+	B := newType(TFORW)
+	B.sym = LocalPkg.Lookup("B")
+	B.SetUnderlying(intType)
+
+	emptyInterface := NewInterface(BuiltinPkg, []*Field{}, false)
+	any := newType(TFORW)
+	any.sym = LocalPkg.Lookup("any")
+	any.SetUnderlying(emptyInterface)
+
+	// The tests marked NONE have no structural type; all the others have a
+	// structural type of structf - "struct { f int }"
+	tests := []*test{
+		{
+			// interface { struct { f int } }
+			embed(structf),
+			structf,
+		},
+		{
+			// interface { struct { f int }; any }
+			embed(structf, any),
+			structf,
+		},
+		{
+			// interface { Sf }
+			embed(Sf),
+			structf,
+		},
+		{
+			// interface { any | Sf }
+			embed(any, Sf),
+			structf,
+		},
+		{
+			// interface { struct { f int }; Sf } - NONE
+			embed(structf, Sf),
+			nil,
+		},
+		{
+			// interface { struct { f int } | ~struct { f int } }
+			embed(NewUnion([]*Type{structf, structf}, []bool{false, true})),
+			structf,
+		},
+		{
+			// interface { ~struct { f int } ; Sf }
+			embed(NewUnion([]*Type{structf}, []bool{true}), Sf),
+			structf,
+		},
+		{
+			// interface { struct { f int } ; Sf } - NONE
+			embed(NewUnion([]*Type{structf}, []bool{false}), Sf),
+			nil,
+		},
+		{
+			// interface { Sf | A; B | Sf}
+			embed(NewUnion([]*Type{Sf, A}, []bool{false, false}),
+				NewUnion([]*Type{B, Sf}, []bool{false, false})),
+			structf,
+		},
+		{
+			// interface { Sf | A; A | Sf } - NONE
+			embed(NewUnion([]*Type{Sf, A}, []bool{false, false}),
+				NewUnion([]*Type{A, Sf}, []bool{false, false})),
+			nil,
+		},
+		{
+			// interface { Sf | any } - NONE
+			embed(NewUnion([]*Type{Sf, any}, []bool{false, false})),
+			nil,
+		},
+		{
+			// interface { Sf | any; Sf }
+			embed(NewUnion([]*Type{Sf, any}, []bool{false, false}), Sf),
+			structf,
+		},
+	}
+	for _, tst := range tests {
+		if got, want := tst.typ.StructuralType(), tst.structuralType; got != want {
+			t.Errorf("StructuralType(%v) = %v, wanted %v",
+				tst.typ, got, want)
+		}
+	}
+}
+
+func embed(types ...*Type) *Type {
+	fields := make([]*Field, len(types))
+	for i, t := range types {
+		fields[i] = NewField(src.NoXPos, nil, t)
+	}
+	return NewInterface(LocalPkg, fields, false)
+}
diff --git a/src/go/internal/gcimporter/gcimporter_test.go b/src/go/internal/gcimporter/gcimporter_test.go
index 15a7b176bb..c9c5946d9f 100644
--- a/src/go/internal/gcimporter/gcimporter_test.go
+++ b/src/go/internal/gcimporter/gcimporter_test.go
@@ -169,8 +169,9 @@ func TestImportTypeparamTests(t *testing.T) {
 	}
 
 	skip := map[string]string{
-		"equal.go":  "inconsistent embedded sorting", // TODO(rfindley): investigate this.
-		"nested.go": "fails to compile",              // TODO(rfindley): investigate this.
+		"equal.go":      "inconsistent embedded sorting", // TODO(rfindley): investigate this.
+		"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
+		"issue50417.go": "inconsistent interface member sorting",
 	}
 
 	for _, entry := range list {
diff --git a/test/run.go b/test/run.go
index 75073993b8..2a7f080f9d 100644
--- a/test/run.go
+++ b/test/run.go
@@ -2178,6 +2178,7 @@ var unifiedFailures = setOf(
 	"typeparam/typeswitch2.go", // duplicate case failure due to stenciling
 	"typeparam/typeswitch3.go", // duplicate case failure due to stenciling
 	"typeparam/typeswitch4.go", // duplicate case failure due to stenciling
+	"typeparam/issue50417b.go", // Need to handle field access on a type param
 	"typeparam/issue50552.go",  // gives missing method for instantiated type
 )
 
diff --git a/test/typeparam/issue50417.go b/test/typeparam/issue50417.go
index f6cf73b18f..cd46f3feab 100644
--- a/test/typeparam/issue50417.go
+++ b/test/typeparam/issue50417.go
@@ -22,12 +22,11 @@ func f0t[P ~struct{ f int }](p P) {
 	p.f = 0
 }
 
-// TODO(danscales) enable once the compiler is fixed
-// var _ = f0[Sf]
-// var _ = f0t[Sf]
+var _ = f0[Sf]
+var _ = f0t[Sf]
 
 func f1[P interface {
-	Sf
+	~struct{ f int }
 	m()
 }](p P) {
 	_ = p.f
@@ -35,6 +34,8 @@ func f1[P interface {
 	p.m()
 }
 
+var _ = f1[Sfm]
+
 type Sm struct{}
 
 func (Sm) m() {}
@@ -54,8 +55,7 @@ func f2[P interface {
 	p.m()
 }
 
-// TODO(danscales) enable once the compiler is fixed
-// var _ = f2[Sfm]
+var _ = f2[Sfm]
 
 // special case: structural type is a named pointer type
 
@@ -66,5 +66,75 @@ func f3[P interface{ PSfm }](p P) {
 	p.f = 0
 }
 
-// TODO(danscales) enable once the compiler is fixed
-// var _ = f3[PSfm]
+var _ = f3[PSfm]
+
+// special case: structural type is an unnamed pointer type
+
+func f4[P interface{ *Sfm }](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+var _ = f4[*Sfm]
+
+type A int
+type B int
+type C float64
+
+type Int interface {
+	*Sf | A
+	*Sf | B
+}
+
+func f5[P Int](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+var _ = f5[*Sf]
+
+type Int2 interface {
+	*Sf | A
+	any
+	*Sf | C
+}
+
+func f6[P Int2](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+var _ = f6[*Sf]
+
+type Int3 interface {
+	Sf
+	~struct{ f int }
+}
+
+func f7[P Int3](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+var _ = f7[Sf]
+
+type Em1 interface {
+	*Sf | A
+}
+
+type Em2 interface {
+	*Sf | B
+}
+
+type Int4 interface {
+	Em1
+	Em2
+	any
+}
+
+func f8[P Int4](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+var _ = f8[*Sf]
diff --git a/test/typeparam/issue50417b.go b/test/typeparam/issue50417b.go
new file mode 100644
index 0000000000..e6b205cb37
--- /dev/null
+++ b/test/typeparam/issue50417b.go
@@ -0,0 +1,50 @@
+// run -gcflags=-G=3
+
+// Copyright 2022 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 MyStruct struct {
+	b1, b2 string
+	E
+}
+
+type E struct {
+	val int
+}
+
+type C interface {
+	~struct {
+		b1, b2 string
+		E
+	}
+}
+
+func f[T C]() T {
+	var x T = T{
+		b1: "a",
+		b2: "b",
+	}
+
+	if got, want := x.b2, "b"; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	x.b1 = "y"
+	x.val = 5
+
+	return x
+}
+
+func main() {
+	x := f[MyStruct]()
+	if got, want := x.b1, "y"; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+	if got, want := x.val, 5; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+}