[dev.typeparams] cmd/compile: handle calling a method on a type param in stenciling

- Have to delay the extra transformation on methods invoked on a type
   param, since the actual transformation (including path through
   embedded fields) will depend on the instantiated type. I am currently
   doing the transformation during the stencil substitution phase. We
   probably should have a separate pass after noder2 and stenciling,
   which drives the extra transformations that were in the old
   typechecker.

 - We handle method values (that are not called) and method calls. We
   don't currently handle method expressions.

 - Handle type substitution in function types, which is needed for
   function args in generic functions.

 - Added stringer.go and map.go tests, testing the above changes
   (including constraints with embedded interfaces).

Change-Id: I3831a937d2b8814150f75bebf9f23ab10b93fa00
Reviewed-on: https://go-review.googlesource.com/c/go/+/290550
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -87,11 +87,13 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 
 	case *syntax.CompositeLit:
 		return g.compLit(typ, expr)
+
 	case *syntax.FuncLit:
 		return g.funcLit(typ, expr)
 
 	case *syntax.AssertExpr:
 		return Assert(pos, g.expr(expr.X), g.typeExpr(expr.Type))
+
 	case *syntax.CallExpr:
 		fun := g.expr(expr.Fun)
 		if inferred, ok := g.info.Inferred[expr]; ok && len(inferred.Targs) > 0 {
@@ -114,6 +116,7 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 
 		}
 		return Call(pos, g.typ(typ), fun, g.exprs(expr.ArgList), expr.HasDots)
+
 	case *syntax.IndexExpr:
 		var targs []ir.Node
 		if _, ok := expr.Index.(*syntax.ListExpr); ok {
@@ -139,6 +142,7 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 
 	case *syntax.ParenExpr:
 		return g.expr(expr.X) // skip parens; unneeded after parse+typecheck
+
 	case *syntax.SelectorExpr:
 		// Qualified identifier.
 		if name, ok := expr.X.(*syntax.Name); ok {
@@ -147,8 +151,8 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 				return typecheck.Expr(g.use(expr.Sel))
 			}
 		}
+		return g.selectorExpr(pos, typ, expr)
 
-		return g.selectorExpr(pos, 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]))
 
@@ -172,15 +176,22 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 // selectorExpr resolves the choice of ODOT, ODOTPTR, OCALLPART (eventually
 // ODOTMETH & ODOTINTER), and OMETHEXPR and deals with embedded fields here rather
 // than in typecheck.go.
-func (g *irgen) selectorExpr(pos src.XPos, expr *syntax.SelectorExpr) ir.Node {
-	selinfo := g.info.Selections[expr]
+func (g *irgen) selectorExpr(pos src.XPos, typ types2.Type, expr *syntax.SelectorExpr) ir.Node {
+	x := g.expr(expr.X)
+	if x.Type().Kind() == types.TTYPEPARAM {
+		// Leave a method call on a type param as an OXDOT, since it can
+		// only be fully transformed once it has an instantiated type.
+		n := ir.NewSelectorExpr(pos, ir.OXDOT, x, typecheck.Lookup(expr.Sel.Value))
+		typed(g.typ(typ), n)
+		return n
+	}
 
+	selinfo := g.info.Selections[expr]
 	// Everything up to the last selection is an implicit embedded field access,
 	// and the last selection is determined by selinfo.Kind().
 	index := selinfo.Index()
 	embeds, last := index[:len(index)-1], index[len(index)-1]
 
-	x := g.expr(expr.X)
 	origx := x
 	for _, ix := range embeds {
 		x = Implicit(DotField(pos, x, ix))

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

@@ -119,6 +119,16 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 	n := ir.NewCallExpr(pos, ir.OCALL, fun, args)
 	n.IsDDD = dots
 
+	if fun.Op() == ir.OXDOT {
+		if fun.(*ir.SelectorExpr).X.Type().Kind() != types.TTYPEPARAM {
+			base.FatalfAt(pos, "Expecting type param receiver in %v", fun)
+		}
+		// For methods called in a generic function, don't do any extra
+		// transformations. We will do those later when we create the
+		// instantiated function and have the correct receiver type.
+		typed(typ, n)
+		return n
+	}
 	if fun.Op() != ir.OFUNCINST {
 		// If no type params, still do normal typechecking, since we're
 		// still missing some things done by tcCall below (mainly

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

@@ -173,6 +173,33 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			m.SetType(subst.typ(x.Type()))
 		}
 		ir.EditChildren(m, edit)
+
+		// A method value/call via a type param will have been left as an
+		// OXDOT. When we see this during stenciling, finish the
+		// typechecking, now that we have the instantiated receiver type.
+		// We need to do this now, since the access/selection to the
+		// method for the real type is very different from the selection
+		// for the type param.
+		if x.Op() == ir.OXDOT {
+			// Will transform to an OCALLPART
+			m.SetTypecheck(0)
+			typecheck.Expr(m)
+		}
+		if x.Op() == ir.OCALL {
+			call := m.(*ir.CallExpr)
+			if call.X.Op() != ir.OCALLPART {
+				base.FatalfAt(call.Pos(), "Expecting OXDOT with CALL")
+			}
+			// Redo the typechecking, now that we know the method
+			// value is being called
+			call.X.(*ir.SelectorExpr).SetOp(ir.OXDOT)
+			call.X.SetTypecheck(0)
+			call.X.SetType(nil)
+			typecheck.Callee(call.X)
+			m.SetTypecheck(0)
+			typecheck.Call(m.(*ir.CallExpr))
+		}
+
 		if x.Op() == ir.OCLOSURE {
 			x := x.(*ir.ClosureExpr)
 			// Need to save/duplicate x.Func.Nname,
@@ -206,6 +233,31 @@ func (subst *subster) list(l []ir.Node) []ir.Node {
 	return s
 }
 
+// tstruct substitutes type params in a structure type
+func (subst *subster) tstruct(t *types.Type) *types.Type {
+	if t.NumFields() == 0 {
+		return t
+	}
+	var newfields []*types.Field
+	for i, f := range t.Fields().Slice() {
+		t2 := subst.typ(f.Type)
+		if t2 != f.Type && newfields == nil {
+			newfields = make([]*types.Field, t.NumFields())
+			for j := 0; j < i; j++ {
+				newfields[j] = t.Field(j)
+			}
+		}
+		if newfields != nil {
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+		}
+	}
+	if newfields != nil {
+		return types.NewStruct(t.Pkg(), newfields)
+	}
+	return t
+
+}
+
 // typ substitutes any type parameter found with the corresponding type argument.
 func (subst *subster) typ(t *types.Type) *types.Type {
 	for i, tp := range subst.tparams.Slice() {
@@ -237,20 +289,23 @@ func (subst *subster) typ(t *types.Type) *types.Type {
 		}
 
 	case types.TSTRUCT:
-		newfields := make([]*types.Field, t.NumFields())
-		change := false
-		for i, f := range t.Fields().Slice() {
-			t2 := subst.typ(f.Type)
-			if t2 != f.Type {
-				change = true
-			}
-			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
-		}
-		if change {
-			return types.NewStruct(t.Pkg(), newfields)
+		newt := subst.tstruct(t)
+		if newt != t {
+			return newt
+		}
+
+	case types.TFUNC:
+		newrecvs := subst.tstruct(t.Recvs())
+		newparams := subst.tstruct(t.Params())
+		newresults := subst.tstruct(t.Results())
+		if newrecvs != t.Recvs() || newparams != t.Params() || newresults != t.Results() {
+			var newrecv *types.Field
+			if newrecvs.NumFields() > 0 {
+				newrecv = newrecvs.Field(0)
+			}
+			return types.NewSignature(t.Pkg(), newrecv, nil, newparams.FieldSlice(), newresults.FieldSlice())
 		}
 
-		// TODO: case TFUNC
 		// TODO: case TCHAN
 		// TODO: case TMAP
 		// TODO: case TINTER

src/cmd/compile/internal/types/type.go

@@ -1657,6 +1657,7 @@ func NewInterface(pkg *Pkg, methods []*Field) *Type {
 // not really be needed except for the type checker).
 func NewTypeParam(pkg *Pkg, constraint *Type) *Type {
 	t := New(TTYPEPARAM)
+	constraint.wantEtype(TINTER)
 	t.methods = constraint.methods
 	t.Extra.(*Interface).pkg = pkg
 	return t

test/typeparam/map.go

@@ -0,0 +1,39 @@
+// 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"
+	"reflect"
+	"strconv"
+)
+
+// Map calls the function f on every element of the slice s,
+// returning a new slice of the results.
+func mapper[F, T any](s []F, f func(F) T) []T {
+	r := make([]T, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+func main() {
+	got := mapper([]int{1, 2, 3}, strconv.Itoa)
+	want := []string{"1", "2", "3"}
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("Got %s, want %s", got, want))
+	}
+
+	fgot := mapper([]float64{2.5, 2.3, 3.5}, func(f float64) string {
+		return strconv.FormatFloat(f, 'f', -1, 64)
+	})
+	fwant := []string{"2.5", "2.3", "3.5"}
+	if !reflect.DeepEqual(fgot, fwant) {
+		panic(fmt.Sprintf("Got %s, want %s", fgot, fwant))
+	}
+}

test/typeparam/stringer.go

@@ -0,0 +1,88 @@
+// 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.
+
+// Test method calls on type parameters
+
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+// Simple constraint
+type Stringer interface {
+	String() string
+}
+
+func stringify[T Stringer](s []T) (ret []string) {
+	for _, v := range s {
+		ret = append(ret, v.String())
+	}
+	return ret
+}
+
+type myint int
+
+func (i myint) String() string {
+	return strconv.Itoa(int(i))
+}
+
+// Constraint with an embedded interface, but still only requires String()
+type Stringer2 interface {
+	CanBeStringer2() int
+	SubStringer2
+}
+
+type SubStringer2 interface {
+	CanBeSubStringer2() int
+	String() string
+}
+
+func stringify2[T Stringer2](s []T) (ret []string) {
+	for _, v := range s {
+		ret = append(ret, v.String())
+	}
+	return ret
+}
+
+func (myint) CanBeStringer2() int {
+	return 0
+}
+
+func (myint) CanBeSubStringer2() int {
+	return 0
+}
+
+// Test use of method values that are not called
+func stringify3[T Stringer](s []T) (ret []string) {
+	for _, v := range s {
+		f := v.String
+		ret = append(ret, f())
+	}
+	return ret
+}
+
+func main() {
+	x := []myint{myint(1), myint(2), myint(3)}
+
+	got := stringify(x)
+	want := []string{"1", "2", "3"}
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("Got %s, want %s", got, want))
+	}
+
+	got = stringify2(x)
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("Got %s, want %s", got, want))
+	}
+
+	got = stringify3(x)
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("Got %s, want %s", got, want))
+	}
+}