[dev.typeparams] go/*: switch from ListExpr to MultiIndexExpr

When instantiating a generic type or function with multiple type
arguments, we need to represent an index expression with multiple
indexes in the AST. Previous to this CL this was done with a new
ast.ListExpr node, which allowed packing multiple expressions into a
single ast.Expr. This compositional pattern can be both inefficient and
cumbersome to work with, and introduces a new node type that only exists
to augment the meaning of an existing node type.

By comparison, other specializations of syntax are given distinct nodes
in go/ast, for example variations of switch or for statements, so the
use of ListExpr was also (arguably) inconsistent.

This CL removes ListExpr, and instead adds a MultiIndexExpr node, which
is exactly like IndexExpr but allows for multiple index arguments. This
requires special handling for this new node type, but a new wrapper in
the typeparams helper package largely mitigates this special handling.

Change-Id: I65eb29c025c599bae37501716284dc7eb953b2ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/327149
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/ast/ast.go

@@ -344,6 +344,15 @@ type (
 		Rbrack token.Pos // position of "]"
 	}
 
+	// A MultiIndexExpr node represents an expression followed by multiple
+	// indices.
+	MultiIndexExpr struct {
+		X       Expr      // expression
+		Lbrack  token.Pos // position of "["
+		Indices []Expr    // index expressions
+		Rbrack  token.Pos // position of "]"
+	}
+
 	// A SliceExpr node represents an expression followed by slice indices.
 	SliceExpr struct {
 		X      Expr      // expression
@@ -374,13 +383,6 @@ type (
 		Rparen   token.Pos // position of ")"
 	}
 
-	// A ListExpr node represents a list of expressions separated by commas.
-	// ListExpr nodes are used as index in IndexExpr nodes representing type
-	// or function instantiations with more than one type argument.
-	ListExpr struct {
-		ElemList []Expr
-	}
-
 	// A StarExpr node represents an expression of the form "*" Expression.
 	// Semantically it could be a unary "*" expression, or a pointer type.
 	//
@@ -494,15 +496,10 @@ func (x *CompositeLit) Pos() token.Pos {
 func (x *ParenExpr) Pos() token.Pos      { return x.Lparen }
 func (x *SelectorExpr) Pos() token.Pos   { return x.X.Pos() }
 func (x *IndexExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *MultiIndexExpr) Pos() token.Pos { return x.X.Pos() }
 func (x *SliceExpr) Pos() token.Pos      { return x.X.Pos() }
 func (x *TypeAssertExpr) Pos() token.Pos { return x.X.Pos() }
 func (x *CallExpr) Pos() token.Pos       { return x.Fun.Pos() }
-func (x *ListExpr) Pos() token.Pos {
-	if len(x.ElemList) > 0 {
-		return x.ElemList[0].Pos()
-	}
-	return token.NoPos
-}
 func (x *StarExpr) Pos() token.Pos       { return x.Star }
 func (x *UnaryExpr) Pos() token.Pos      { return x.OpPos }
 func (x *BinaryExpr) Pos() token.Pos     { return x.X.Pos() }
@@ -533,15 +530,10 @@ func (x *CompositeLit) End() token.Pos   { return x.Rbrace + 1 }
 func (x *ParenExpr) End() token.Pos      { return x.Rparen + 1 }
 func (x *SelectorExpr) End() token.Pos   { return x.Sel.End() }
 func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *MultiIndexExpr) End() token.Pos { return x.Rbrack + 1 }
 func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
 func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
 func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
-func (x *ListExpr) End() token.Pos {
-	if len(x.ElemList) > 0 {
-		return x.ElemList[len(x.ElemList)-1].End()
-	}
-	return token.NoPos
-}
 func (x *StarExpr) End() token.Pos       { return x.X.End() }
 func (x *UnaryExpr) End() token.Pos      { return x.X.End() }
 func (x *BinaryExpr) End() token.Pos     { return x.Y.End() }
@@ -570,10 +562,10 @@ func (*CompositeLit) exprNode()   {}
 func (*ParenExpr) exprNode()      {}
 func (*SelectorExpr) exprNode()   {}
 func (*IndexExpr) exprNode()      {}
+func (*MultiIndexExpr) exprNode() {}
 func (*SliceExpr) exprNode()      {}
 func (*TypeAssertExpr) exprNode() {}
 func (*CallExpr) exprNode()       {}
-func (*ListExpr) exprNode()       {}
 func (*StarExpr) exprNode()       {}
 func (*UnaryExpr) exprNode()      {}
 func (*BinaryExpr) exprNode()     {}

src/go/ast/walk.go

@@ -116,6 +116,12 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.X)
 		Walk(v, n.Index)
 
+	case *MultiIndexExpr:
+		Walk(v, n.X)
+		for _, index := range n.Indices {
+			Walk(v, index)
+		}
+
 	case *SliceExpr:
 		Walk(v, n.X)
 		if n.Low != nil {
@@ -138,11 +144,6 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.Fun)
 		walkExprList(v, n.Args)
 
-	case *ListExpr:
-		for _, elem := range n.ElemList {
-			Walk(v, elem)
-		}
-
 	case *StarExpr:
 		Walk(v, n.X)
 

src/go/internal/typeparams/typeparams.go

@@ -7,42 +7,56 @@ package typeparams
 import (
 	"fmt"
 	"go/ast"
+	"go/token"
 )
 
 const Enabled = true
 
-func PackExpr(list []ast.Expr) ast.Expr {
+func PackIndexExpr(x ast.Expr, lbrack token.Pos, exprs []ast.Expr, rbrack token.Pos) ast.Expr {
-	switch len(list) {
+	switch len(exprs) {
 	case 0:
-		// Return an empty ListExpr here, rather than nil, as IndexExpr.Index must
+		panic("internal error: PackIndexExpr with empty expr slice")
-		// never be nil.
-		// TODO(rFindley) would a BadExpr be more appropriate here?
-		return &ast.ListExpr{}
 	case 1:
-		return list[0]
+		return &ast.IndexExpr{
+			X:      x,
+			Lbrack: lbrack,
+			Index:  exprs[0],
+			Rbrack: rbrack,
+		}
 	default:
-		return &ast.ListExpr{ElemList: list}
+		return &ast.MultiIndexExpr{
+			X:       x,
+			Lbrack:  lbrack,
+			Indices: exprs,
+			Rbrack:  rbrack,
+		}
 	}
 }
 
-// TODO(gri) Should find a more efficient solution that doesn't
+// IndexExpr wraps an ast.IndexExpr or ast.MultiIndexExpr into the
-//           require introduction of a new slice for simple
+// MultiIndexExpr interface.
-//           expressions.
+//
-func UnpackExpr(x ast.Expr) []ast.Expr {
+// Orig holds the original ast.Expr from which this IndexExpr was derived.
-	if x, _ := x.(*ast.ListExpr); x != nil {
+type IndexExpr struct {
-		return x.ElemList
+	Orig ast.Expr // the wrapped expr, which may be distinct from MultiIndexExpr below.
-	}
+	*ast.MultiIndexExpr
-	if x != nil {
+}
-		return []ast.Expr{x}
+
+func UnpackIndexExpr(n ast.Node) *IndexExpr {
+	switch e := n.(type) {
+	case *ast.IndexExpr:
+		return &IndexExpr{e, &ast.MultiIndexExpr{
+			X:       e.X,
+			Lbrack:  e.Lbrack,
+			Indices: []ast.Expr{e.Index},
+			Rbrack:  e.Rbrack,
+		}}
+	case *ast.MultiIndexExpr:
+		return &IndexExpr{e, e}
 	}
 	return nil
 }
 
-func IsListExpr(n ast.Node) bool {
-	_, ok := n.(*ast.ListExpr)
-	return ok
-}
-
 func Get(n ast.Node) *ast.FieldList {
 	switch n := n.(type) {
 	case *ast.TypeSpec:

src/go/parser/parser.go

@@ -600,7 +600,7 @@ func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Ex
 	}
 
 	// x[P], x[P1, P2], ...
-	return nil, &ast.IndexExpr{X: x, Lbrack: lbrack, Index: typeparams.PackExpr(args), Rbrack: rbrack}
+	return nil, typeparams.PackIndexExpr(x, lbrack, args, rbrack)
 }
 
 func (p *parser) parseFieldDecl() *ast.Field {
@@ -991,7 +991,7 @@ func (p *parser) parseMethodSpec() *ast.Field {
 					p.exprLev--
 				}
 				rbrack := p.expectClosing(token.RBRACK, "type argument list")
-				typ = &ast.IndexExpr{X: ident, Lbrack: lbrack, Index: typeparams.PackExpr(list), Rbrack: rbrack}
+				typ = typeparams.PackIndexExpr(ident, lbrack, list, rbrack)
 			}
 		case p.tok == token.LPAREN:
 			// ordinary method
@@ -1178,7 +1178,6 @@ func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
 	}
 
 	opening := p.expect(token.LBRACK)
-
 	p.exprLev++
 	var list []ast.Expr
 	for p.tok != token.RBRACK && p.tok != token.EOF {
@@ -1192,7 +1191,17 @@ func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
 
 	closing := p.expectClosing(token.RBRACK, "type argument list")
 
-	return &ast.IndexExpr{X: typ, Lbrack: opening, Index: typeparams.PackExpr(list), Rbrack: closing}
+	if len(list) == 0 {
+		p.errorExpected(closing, "type argument list")
+		return &ast.IndexExpr{
+			X:      typ,
+			Lbrack: opening,
+			Index:  &ast.BadExpr{From: opening + 1, To: closing},
+			Rbrack: closing,
+		}
+	}
+
+	return typeparams.PackIndexExpr(typ, opening, list, closing)
 }
 
 func (p *parser) tryIdentOrType() ast.Expr {
@@ -1455,7 +1464,7 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 	}
 
 	// instance expression
-	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: typeparams.PackExpr(args), Rbrack: rbrack}
+	return typeparams.PackIndexExpr(x, lbrack, args, rbrack)
 }
 
 func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
@@ -1557,6 +1566,7 @@ func (p *parser) checkExpr(x ast.Expr) ast.Expr {
 		panic("unreachable")
 	case *ast.SelectorExpr:
 	case *ast.IndexExpr:
+	case *ast.MultiIndexExpr:
 	case *ast.SliceExpr:
 	case *ast.TypeAssertExpr:
 		// If t.Type == nil we have a type assertion of the form
@@ -1646,7 +1656,7 @@ func (p *parser) parsePrimaryExpr() (x ast.Expr) {
 					return
 				}
 				// x is possibly a composite literal type
-			case *ast.IndexExpr:
+			case *ast.IndexExpr, *ast.MultiIndexExpr:
 				if p.exprLev < 0 {
 					return
 				}

src/go/printer/nodes.go

@@ -871,17 +871,15 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		// TODO(gri): should treat[] like parentheses and undo one level of depth
 		p.expr1(x.X, token.HighestPrec, 1)
 		p.print(x.Lbrack, token.LBRACK)
-		// Note: we're a bit defensive here to handle the case of a ListExpr of
-		// length 1.
-		if list := typeparams.UnpackExpr(x.Index); len(list) > 0 {
-			if len(list) > 1 {
-				p.exprList(x.Lbrack, list, depth+1, commaTerm, x.Rbrack, false)
-			} else {
-				p.expr0(list[0], depth+1)
-			}
-		} else {
 		p.expr0(x.Index, depth+1)
-		}
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.MultiIndexExpr:
+		// TODO(gri): as for IndexExpr, should treat [] like parentheses and undo
+		// one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		p.exprList(x.Lbrack, x.Indices, depth+1, commaTerm, x.Rbrack, false)
 		p.print(x.Rbrack, token.RBRACK)
 
 	case *ast.SliceExpr:

src/go/types/call.go

@@ -16,23 +16,22 @@ import (
 
 // funcInst type-checks a function instantiation inst and returns the result in x.
 // The operand x must be the evaluation of inst.X and its type must be a signature.
-func (check *Checker) funcInst(x *operand, inst *ast.IndexExpr) {
+func (check *Checker) funcInst(x *operand, ix *typeparams.IndexExpr) {
-	xlist := typeparams.UnpackExpr(inst.Index)
+	targs := check.typeList(ix.Indices)
-	targs := check.typeList(xlist)
 	if targs == nil {
 		x.mode = invalid
-		x.expr = inst
+		x.expr = ix.Orig
 		return
 	}
-	assert(len(targs) == len(xlist))
+	assert(len(targs) == len(ix.Indices))
 
 	// check number of type arguments (got) vs number of type parameters (want)
 	sig := x.typ.(*Signature)
 	got, want := len(targs), len(sig.tparams)
 	if got > want {
-		check.errorf(xlist[got-1], _Todo, "got %d type arguments but want %d", got, want)
+		check.errorf(ix.Indices[got-1], _Todo, "got %d type arguments but want %d", got, want)
 		x.mode = invalid
-		x.expr = inst
+		x.expr = ix.Orig
 		return
 	}
 
@@ -40,11 +39,11 @@ func (check *Checker) funcInst(x *operand, inst *ast.IndexExpr) {
 	inferred := false
 
 	if got < want {
-		targs = check.infer(inst, sig.tparams, targs, nil, nil, true)
+		targs = check.infer(ix.Orig, sig.tparams, targs, nil, nil, true)
 		if targs == nil {
 			// error was already reported
 			x.mode = invalid
-			x.expr = inst
+			x.expr = ix.Orig
 			return
 		}
 		got = len(targs)
@@ -55,8 +54,8 @@ func (check *Checker) funcInst(x *operand, inst *ast.IndexExpr) {
 	// determine argument positions (for error reporting)
 	// TODO(rFindley) use a positioner here? instantiate would need to be
 	//                updated accordingly.
-	poslist := make([]token.Pos, len(xlist))
+	poslist := make([]token.Pos, len(ix.Indices))
-	for i, x := range xlist {
+	for i, x := range ix.Indices {
 		poslist[i] = x.Pos()
 	}
 
@@ -64,25 +63,27 @@ func (check *Checker) funcInst(x *operand, inst *ast.IndexExpr) {
 	res := check.instantiate(x.Pos(), sig, targs, poslist).(*Signature)
 	assert(res.tparams == nil) // signature is not generic anymore
 	if inferred {
-		check.recordInferred(inst, targs, res)
+		check.recordInferred(ix.Orig, targs, res)
 	}
 	x.typ = res
 	x.mode = value
-	x.expr = inst
+	x.expr = ix.Orig
 }
 
 func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
-	var inst *ast.IndexExpr
+	ix := typeparams.UnpackIndexExpr(call.Fun)
-	if iexpr, _ := call.Fun.(*ast.IndexExpr); iexpr != nil {
+	if ix != nil {
-		if check.indexExpr(x, iexpr) {
+		if check.indexExpr(x, ix) {
 			// Delay function instantiation to argument checking,
 			// where we combine type and value arguments for type
 			// inference.
 			assert(x.mode == value)
-			inst = iexpr
+		} else {
+			ix = nil
 		}
-		x.expr = iexpr
+		x.expr = call.Fun
 		check.record(x)
+
 	} else {
 		check.exprOrType(x, call.Fun)
 	}
@@ -149,21 +150,20 @@ func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
 
 	// evaluate type arguments, if any
 	var targs []Type
-	if inst != nil {
+	if ix != nil {
-		xlist := typeparams.UnpackExpr(inst.Index)
+		targs = check.typeList(ix.Indices)
-		targs = check.typeList(xlist)
 		if targs == nil {
 			check.use(call.Args...)
 			x.mode = invalid
 			x.expr = call
 			return statement
 		}
-		assert(len(targs) == len(xlist))
+		assert(len(targs) == len(ix.Indices))
 
 		// check number of type arguments (got) vs number of type parameters (want)
 		got, want := len(targs), len(sig.tparams)
 		if got > want {
-			check.errorf(xlist[want], _Todo, "got %d type arguments but want %d", got, want)
+			check.errorf(ix.Indices[want], _Todo, "got %d type arguments but want %d", got, want)
 			check.use(call.Args...)
 			x.mode = invalid
 			x.expr = call

src/go/types/expr.go

@@ -1331,9 +1331,10 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 	case *ast.SelectorExpr:
 		check.selector(x, e)
 
-	case *ast.IndexExpr:
+	case *ast.IndexExpr, *ast.MultiIndexExpr:
-		if check.indexExpr(x, e) {
+		ix := typeparams.UnpackIndexExpr(e)
-			check.funcInst(x, e)
+		if check.indexExpr(x, ix) {
+			check.funcInst(x, ix)
 		}
 		if x.mode == invalid {
 			goto Error
@@ -1423,13 +1424,8 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		// types, which are comparatively rare.
 
 	default:
-		if typeparams.IsListExpr(e) {
-			// catch-all for unexpected expression lists
-			check.errorf(e, _Todo, "unexpected list of expressions")
-		} else {
 		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
 	}
-	}
 
 	// everything went well
 	x.expr = e

src/go/types/exprstring.go

@@ -67,11 +67,11 @@ func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
 		buf.WriteByte('.')
 		buf.WriteString(x.Sel.Name)
 
-	case *ast.IndexExpr:
+	case *ast.IndexExpr, *ast.MultiIndexExpr:
-		WriteExpr(buf, x.X)
+		ix := typeparams.UnpackIndexExpr(x)
+		WriteExpr(buf, ix.X)
 		buf.WriteByte('[')
-		exprs := typeparams.UnpackExpr(x.Index)
+		for i, e := range ix.Indices {
-		for i, e := range exprs {
 			if i > 0 {
 				buf.WriteString(", ")
 			}

src/go/types/index.go

@@ -15,18 +15,18 @@ import (
 // If e is a valid function instantiation, indexExpr returns true.
 // In that case x represents the uninstantiated function value and
 // it is the caller's responsibility to instantiate the function.
-func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool) {
+func (check *Checker) indexExpr(x *operand, expr *typeparams.IndexExpr) (isFuncInst bool) {
-	check.exprOrType(x, e.X)
+	check.exprOrType(x, expr.X)
 
 	switch x.mode {
 	case invalid:
-		check.use(typeparams.UnpackExpr(e.Index)...)
+		check.use(expr.Indices...)
 		return false
 
 	case typexpr:
 		// type instantiation
 		x.mode = invalid
-		x.typ = check.varType(e)
+		x.typ = check.varType(expr.Orig)
 		if x.typ != Typ[Invalid] {
 			x.mode = typexpr
 		}
@@ -77,7 +77,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		x.typ = typ.elem
 
 	case *Map:
-		index := check.singleIndex(e)
+		index := check.singleIndex(expr)
 		if index == nil {
 			x.mode = invalid
 			return
@@ -88,7 +88,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		// ok to continue even if indexing failed - map element type is known
 		x.mode = mapindex
 		x.typ = typ.elem
-		x.expr = e
+		x.expr = expr.Orig
 		return
 
 	case *Union:
@@ -137,7 +137,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 			// If there are maps, the index expression must be assignable
 			// to the map key type (as for simple map index expressions).
 			if nmaps > 0 {
-				index := check.singleIndex(e)
+				index := check.singleIndex(expr)
 				if index == nil {
 					x.mode = invalid
 					return
@@ -151,7 +151,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 				if nmaps == typ.NumTerms() {
 					x.mode = mapindex
 					x.typ = telem
-					x.expr = e
+					x.expr = expr.Orig
 					return
 				}
 
@@ -180,7 +180,7 @@ func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) (isFuncInst bool)
 		return
 	}
 
-	index := check.singleIndex(e)
+	index := check.singleIndex(expr)
 	if index == nil {
 		x.mode = invalid
 		return
@@ -311,23 +311,16 @@ L:
 // singleIndex returns the (single) index from the index expression e.
 // If the index is missing, or if there are multiple indices, an error
 // is reported and the result is nil.
-func (check *Checker) singleIndex(e *ast.IndexExpr) ast.Expr {
+func (check *Checker) singleIndex(expr *typeparams.IndexExpr) ast.Expr {
-	index := e.Index
+	if len(expr.Indices) == 0 {
-	if index == nil {
+		check.invalidAST(expr.Orig, "index expression %v with 0 indices", expr)
-		check.invalidAST(e, "missing index for %s", e)
 		return nil
 	}
-
+	if len(expr.Indices) > 1 {
-	indexes := typeparams.UnpackExpr(index)
-	if len(indexes) == 0 {
-		check.invalidAST(index, "index expression %v with 0 indices", index)
-		return nil
-	}
-	if len(indexes) > 1 {
 		// TODO(rFindley) should this get a distinct error code?
-		check.invalidOp(indexes[1], _InvalidIndex, "more than one index")
+		check.invalidOp(expr.Indices[1], _InvalidIndex, "more than one index")
 	}
-	return indexes[0]
+	return expr.Indices[0]
 }
 
 // index checks an index expression for validity.

src/go/types/resolver.go

@@ -499,10 +499,12 @@ L: // unpack receiver type
 	}
 
 	// unpack type parameters, if any
-	if ptyp, _ := rtyp.(*ast.IndexExpr); ptyp != nil {
+	switch rtyp.(type) {
-		rtyp = ptyp.X
+	case *ast.IndexExpr, *ast.MultiIndexExpr:
+		ix := typeparams.UnpackIndexExpr(rtyp)
+		rtyp = ix.X
 		if unpackParams {
-			for _, arg := range typeparams.UnpackExpr(ptyp.Index) {
+			for _, arg := range ix.Indices {
 				var par *ast.Ident
 				switch arg := arg.(type) {
 				case *ast.Ident:
@@ -510,7 +512,7 @@ L: // unpack receiver type
 				case *ast.BadExpr:
 					// ignore - error already reported by parser
 				case nil:
-					check.invalidAST(ptyp, "parameterized receiver contains nil parameters")
+					check.invalidAST(ix.Orig, "parameterized receiver contains nil parameters")
 				default:
 					check.errorf(arg, _Todo, "receiver type parameter %s must be an identifier", arg)
 				}

src/go/types/signature.go

@@ -244,24 +244,21 @@ func isubst(x ast.Expr, smap map[*ast.Ident]*ast.Ident) ast.Expr {
 			new.X = X
 			return &new
 		}
-	case *ast.IndexExpr:
+	case *ast.IndexExpr, *ast.MultiIndexExpr:
-		elems := typeparams.UnpackExpr(n.Index)
+		ix := typeparams.UnpackIndexExpr(x)
-		var newElems []ast.Expr
+		var newIndexes []ast.Expr
-		for i, elem := range elems {
+		for i, index := range ix.Indices {
-			new := isubst(elem, smap)
+			new := isubst(index, smap)
-			if new != elem {
+			if new != index {
-				if newElems == nil {
+				if newIndexes == nil {
-					newElems = make([]ast.Expr, len(elems))
+					newIndexes = make([]ast.Expr, len(ix.Indices))
-					copy(newElems, elems)
+					copy(newIndexes, ix.Indices)
 				}
-				newElems[i] = new
+				newIndexes[i] = new
 			}
 		}
-		if newElems != nil {
+		if newIndexes != nil {
-			index := typeparams.PackExpr(newElems)
+			return typeparams.PackIndexExpr(ix.X, ix.Lbrack, newIndexes, ix.Rbrack)
-			new := *n
-			new.Index = index
-			return &new
 		}
 	case *ast.ParenExpr:
 		return isubst(n.X, smap) // no need to keep parentheses

src/go/types/testdata/check/typeinst.go2

@@ -33,11 +33,11 @@ var _ A3
 var x int
 type _ x /* ERROR not a type */ [int]
 
-type _ int /* ERROR not a generic type */ []
+type _ int /* ERROR not a generic type */ [] // ERROR expected type argument list
-type _ myInt /* ERROR not a generic type */ []
+type _ myInt /* ERROR not a generic type */ [] // ERROR expected type argument list
 
 // TODO(gri) better error messages
-type _ T1 /* ERROR got 0 arguments but 1 type parameters */ []
+type _ T1[] // ERROR expected type argument list
 type _ T1[x /* ERROR not a type */ ]
 type _ T1 /* ERROR got 2 arguments but 1 type parameters */ [int, float32]
 

src/go/types/testdata/fixedbugs/issue45635.go2

@@ -10,7 +10,7 @@ func main() {
 
 type N[T any] struct{}
 
-var _ N /* ERROR "0 arguments but 1 type parameters" */ []
+var _ N [] // ERROR expected type argument list
 
 type I interface {
 	~map[int]int | ~[]int

src/go/types/typexpr.go

@@ -261,13 +261,13 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 			check.errorf(&x, _NotAType, "%s is not a type", &x)
 		}
 
-	case *ast.IndexExpr:
+	case *ast.IndexExpr, *ast.MultiIndexExpr:
+		ix := typeparams.UnpackIndexExpr(e)
 		if typeparams.Enabled {
-			exprs := typeparams.UnpackExpr(e.Index)
+			return check.instantiatedType(ix, def)
-			return check.instantiatedType(e.X, exprs, def)
 		}
 		check.errorf(e0, _NotAType, "%s is not a type", e0)
-		check.use(e.X)
+		check.use(ix.X)
 
 	case *ast.ParenExpr:
 		// Generic types must be instantiated before they can be used in any form.
@@ -403,8 +403,8 @@ func (check *Checker) typeOrNil(e ast.Expr) Type {
 	return Typ[Invalid]
 }
 
-func (check *Checker) instantiatedType(x ast.Expr, targs []ast.Expr, def *Named) Type {
+func (check *Checker) instantiatedType(ix *typeparams.IndexExpr, def *Named) Type {
-	b := check.genericType(x, true) // TODO(gri) what about cycles?
+	b := check.genericType(ix.X, true) // TODO(gri) what about cycles?
 	if b == Typ[Invalid] {
 		return b // error already reported
 	}
@@ -420,19 +420,19 @@ func (check *Checker) instantiatedType(x ast.Expr, targs []ast.Expr, def *Named)
 	def.setUnderlying(typ)
 
 	typ.check = check
-	typ.pos = x.Pos()
+	typ.pos = ix.X.Pos()
 	typ.base = base
 
 	// evaluate arguments (always)
-	typ.targs = check.typeList(targs)
+	typ.targs = check.typeList(ix.Indices)
 	if typ.targs == nil {
 		def.setUnderlying(Typ[Invalid]) // avoid later errors due to lazy instantiation
 		return Typ[Invalid]
 	}
 
 	// determine argument positions (for error reporting)
-	typ.poslist = make([]token.Pos, len(targs))
+	typ.poslist = make([]token.Pos, len(ix.Indices))
-	for i, arg := range targs {
+	for i, arg := range ix.Indices {
 		typ.poslist[i] = arg.Pos()
 	}