cmd/compile: fix stenciling of conversions between interfaces

Conversions need to allow nil through.
We do that using a CONVIFACE instead of a DOTTYPE.

Also reorganize how nonempty interfaces are handled.
For nonempty to empty, a simple CONVIFACE suffices.
For nonempty to nonempty, we need to call the runtime to get the
new itab. Use the entry from the dictionary to identify the
target interface type (if parameterized).

Fixes #47925

Change-Id: I4ffeed964318bb3e270b06f558e6ab9c5bfc7188
Reviewed-on: https://go-review.googlesource.com/c/go/+/344830
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/stencil.go

@@ -737,7 +737,7 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, shapes []*typ
 				return
 			}
 			c := n.(*ir.ConvExpr)
-			if c.X.Type().HasShape() {
+			if c.X.Type().HasShape() && !c.X.Type().IsInterface() {
 				ir.Dump("BAD FUNCTION", newf)
 				ir.Dump("BAD CONVERSION", c)
 				base.Fatalf("converting shape type to interface")
@@ -1135,7 +1135,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			x := x.(*ir.ConvExpr)
 			// Note: x's argument is still typed as a type parameter.
 			// m's argument now has an instantiated type.
-			if x.X.Type().HasTParam() {
+			if x.X.Type().HasTParam() || (x.X.Type().IsInterface() && x.Type().HasTParam()) {
 				m = convertUsingDictionary(subst.info, subst.info.dictParam, m.Pos(), m.(*ir.ConvExpr).X, x, m.Type(), x.X.Type())
 			}
 		case ir.ODOTTYPE, ir.ODOTTYPE2:
@@ -1231,9 +1231,52 @@ func findDictType(info *instInfo, t *types.Type) int {
 // CONVIFACE node or XDOT node (for a bound method call) that is causing the
 // conversion.
 func convertUsingDictionary(info *instInfo, dictParam *ir.Name, pos src.XPos, v ir.Node, gn ir.Node, dst, src *types.Type) ir.Node {
-	assert(src.HasTParam())
+	assert(src.HasTParam() || src.IsInterface() && gn.Type().HasTParam())
 	assert(dst.IsInterface())
 
+	if v.Type().IsInterface() {
+		// Converting from an interface. The shape-ness of the source doesn't really matter, as
+		// we'll be using the concrete type from the first interface word.
+		if dst.IsEmptyInterface() {
+			// Converting I2E. OCONVIFACE does that for us, and doesn't depend
+			// on what the empty interface was instantiated with. No dictionary entry needed.
+			v = ir.NewConvExpr(pos, ir.OCONVIFACE, dst, v)
+			v.SetTypecheck(1)
+			return v
+		}
+		gdst := gn.Type() // pre-stenciled destination type
+		if !gdst.HasTParam() {
+			// Regular OCONVIFACE works if the destination isn't parameterized.
+			v = ir.NewConvExpr(pos, ir.OCONVIFACE, dst, v)
+			v.SetTypecheck(1)
+			return v
+		}
+
+		// We get the destination interface type from the dictionary and the concrete
+		// type from the argument's itab. Call runtime.convI2I to get the new itab.
+		tmp := typecheck.Temp(v.Type())
+		as := ir.NewAssignStmt(pos, tmp, v)
+		as.SetTypecheck(1)
+		itab := ir.NewUnaryExpr(pos, ir.OITAB, tmp)
+		typed(types.Types[types.TUINTPTR].PtrTo(), itab)
+		idata := ir.NewUnaryExpr(pos, ir.OIDATA, tmp)
+		typed(types.Types[types.TUNSAFEPTR], idata)
+
+		fn := typecheck.LookupRuntime("convI2I")
+		fn.SetTypecheck(1)
+		types.CalcSize(fn.Type())
+		call := ir.NewCallExpr(pos, ir.OCALLFUNC, fn, nil)
+		typed(types.Types[types.TUINT8].PtrTo(), call)
+		ix := findDictType(info, gdst)
+		assert(ix >= 0)
+		inter := getDictionaryType(info, dictParam, pos, ix)
+		call.Args = []ir.Node{inter, itab}
+		i := ir.NewBinaryExpr(pos, ir.OEFACE, call, idata)
+		typed(dst, i)
+		i.PtrInit().Append(as)
+		return i
+	}
+
 	var rt ir.Node
 	if !dst.IsEmptyInterface() {
 		// We should have an itab entry in the dictionary. Using this itab
@@ -1248,11 +1291,6 @@ func convertUsingDictionary(info *instInfo, dictParam *ir.Name, pos src.XPos, v
 		}
 		assert(ix >= 0)
 		rt = getDictionaryEntry(pos, dictParam, ix, info.dictLen)
-	} else if v.Type().IsInterface() {
-		ta := ir.NewTypeAssertExpr(pos, v, nil)
-		ta.SetType(dst)
-		ta.SetTypecheck(1)
-		return ta
 	} else {
 		ix := findDictType(info, src)
 		assert(ix >= 0)
@@ -1261,19 +1299,13 @@ func convertUsingDictionary(info *instInfo, dictParam *ir.Name, pos src.XPos, v
 	}
 
 	// Figure out what the data field of the interface will be.
-	var data ir.Node
-	if v.Type().IsInterface() {
-		data = ir.NewUnaryExpr(pos, ir.OIDATA, v)
-	} else {
-		data = ir.NewConvExpr(pos, ir.OCONVIDATA, nil, v)
-	}
+	data := ir.NewConvExpr(pos, ir.OCONVIDATA, nil, v)
 	typed(types.Types[types.TUNSAFEPTR], data)
 
 	// Build an interface from the type and data parts.
 	var i ir.Node = ir.NewBinaryExpr(pos, ir.OEFACE, rt, data)
 	typed(dst, i)
 	return i
-
 }
 
 func (subst *subster) namelist(l []*ir.Name) []*ir.Name {
@@ -1557,7 +1589,7 @@ func (g *irgen) finalizeSyms() {
 			default:
 				base.Fatalf("itab entry with unknown op %s", n.Op())
 			}
-			if srctype.IsInterface() {
+			if srctype.IsInterface() || dsttype.IsEmptyInterface() {
 				// No itab is wanted if src type is an interface. We
 				// will use a type assert instead.
 				d.off = objw.Uintptr(lsym, d.off, 0)

test/typeparam/eface.go

@@ -53,19 +53,15 @@ func main() {
 		println("test 4 failed")
 	}
 	if h[int](myint(5)) != myint(5) {
-		// TODO: disabled
-		//println("test 5 failed")
+		println("test 5 failed")
 	}
 	if h[int](myint(6)) != interface{ foo() }(myint(6)) {
-		// TODO: disabled
-		//println("test 6 failed")
+		println("test 6 failed")
 	}
 	if i[int](myint(7)) != myint(7) {
-		// TODO: This happens to work, but not for the right reasons.
 		println("test 7 failed")
 	}
 	if i[int](myint(8)) != I[int](myint(8)) {
-		// TODO: disabled
-		//println("test 8 failed")
+		println("test 8 failed")
 	}
 }

test/typeparam/issue47925.go

@@ -0,0 +1,20 @@
+// 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 myifacer[T any] interface{ do(T) error }
+
+type stuff[T any] struct{}
+
+func (s stuff[T]) run() interface{} {
+	var i myifacer[T]
+	return i
+}
+
+func main() {
+	stuff[int]{}.run()
+}

test/typeparam/issue47925b.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[T any] interface {
+	foo()
+}
+
+type E[T any] interface {
+}
+
+//go:noinline
+func f[T I[T]](x T) E[T] {
+	// contains a cast from nonempty to empty interface
+	return E[T](I[T](x))
+}
+
+type S struct {
+	x int
+}
+
+func (s *S) foo() {}
+
+func main() {
+	i := f(&S{x: 7})
+	if i.(*S).x != 7 {
+		panic("bad")
+	}
+}

test/typeparam/issue47925c.go

@@ -0,0 +1,36 @@
+// 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[T any] interface {
+	foo()
+}
+
+type J[T any] interface {
+	foo()
+	bar()
+}
+
+//go:noinline
+func f[T J[T]](x T) I[T] {
+	// contains a cast between two nonempty interfaces
+	return I[T](J[T](x))
+}
+
+type S struct {
+	x int
+}
+
+func (s *S) foo() {}
+func (s *S) bar() {}
+
+func main() {
+	i := f(&S{x: 7})
+	if i.(*S).x != 7 {
+		panic("bad")
+	}
+}

test/typeparam/issue47925d.go

@@ -0,0 +1,47 @@
+// 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[T any] interface {
+	foo()
+}
+
+type J[T any] interface {
+	foo()
+	bar()
+}
+
+//go:noinline
+func f[T J[T]](x T, g func(T) T) I[T] {
+	// contains a cast between two nonempty interfaces
+	// Also make sure we don't evaluate g(x) twice.
+	return I[T](J[T](g(x)))
+}
+
+type S struct {
+	x int
+}
+
+func (s *S) foo() {}
+func (s *S) bar() {}
+
+var cnt int
+
+func inc(s *S) *S {
+	cnt++
+	return s
+}
+
+func main() {
+	i := f(&S{x: 7}, inc)
+	if i.(*S).x != 7 {
+		panic("bad")
+	}
+	if cnt != 1 {
+		panic("multiple calls")
+	}
+}