cmd/compile/internal: support new(expr)

This CL adds compiler support for new(expr),
a feature of go1.26 that allows the user to specify
the initial value of the variable instead of its
type.

Also, a basic test of dynamic behavior.

See CL 704737 for spec change and CL 704935 for
type-checker changes.

For #45624

Change-Id: I65d27de1ee3aabb819b57cce8ea77f3073447757
Reviewed-on: https://go-review.googlesource.com/c/go/+/705157
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/ir/node.go

@@ -215,7 +215,7 @@ const (
 	ORSH              // X >> Y
 	OAND              // X & Y
 	OANDNOT           // X &^ Y
-	ONEW              // new(X); corresponds to calls to new in source code
+	ONEW              // new(X); corresponds to calls to new(T) in source code
 	ONOT              // !X
 	OBITNOT           // ^X
 	OPLUS             // +X

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

@@ -42,6 +42,10 @@ func TypeNode(t *types.Type) Node {
 
 // A DynamicType represents a type expression whose exact type must be
 // computed dynamically.
+//
+// TODO(adonovan): I think "dynamic" is a misnomer here; it's really a
+// type with free type parameters that needs to be instantiated to obtain
+// a ground type for which an rtype can exist.
 type DynamicType struct {
 	miniExpr
 

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

@@ -2431,8 +2431,16 @@ func (r *reader) expr() (res ir.Node) {
 
 	case exprNew:
 		pos := r.pos()
-		typ := r.exprType()
+		if r.Bool() {
-		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, typ))
+			// new(expr) -> tmp := expr; &tmp
+			x := r.expr()
+			var init ir.Nodes
+			addr := ir.NewAddrExpr(pos, r.tempCopy(pos, x, &init))
+			addr.SetInit(init)
+			return typecheck.Expr(addr)
+		}
+		// new(T)
+		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, r.exprType()))
 
 	case exprSizeof:
 		return ir.NewUintptr(r.pos(), r.typ().Size())
@@ -3239,6 +3247,7 @@ func (r *reader) exprType() ir.Node {
 	var rtype, itab ir.Node
 
 	if r.Bool() {
+		// non-empty interface
 		typ, rtype, _, _, itab = r.itab(pos)
 		if !typ.IsInterface() {
 			rtype = nil // TODO(mdempsky): Leave set?

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

@@ -2035,10 +2035,16 @@ func (w *writer) expr(expr syntax.Expr) {
 			case "new":
 				assert(len(expr.ArgList) == 1)
 				assert(!expr.HasDots)
+				arg := expr.ArgList[0]
 
 				w.Code(exprNew)
 				w.pos(expr)
-				w.exprType(nil, expr.ArgList[0])
+				tv := w.p.typeAndValue(arg)
+				if w.Bool(!tv.IsType()) {
+					w.expr(arg) // new(expr), go1.26
+				} else {
+					w.exprType(nil, arg) // new(T)
+				}
 				return
 
 			case "Sizeof":

test/newexpr.go

@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2025 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
+
+// Issue #45624 is the proposal to accept new(expr) in go1.26.
+// Here we test its run-time behavior.
+func main() {
+	{
+		p := new(123) // untyped constant expr
+		if *p != 123 {
+			panic("wrong value")
+		}
+	}
+	{
+		x := 42
+		p := new(x) // non-constant expr
+		if *p != x {
+			panic("wrong value")
+		}
+	}
+	{
+		x := [2]int{123, 456}
+		p := new(x) // composite value
+		if *p != x {
+			panic("wrong value")
+		}
+	}
+}

test/used.go

@@ -140,6 +140,5 @@ func _() {
 	_ = int                // ERROR "type int is not an expression|not an expression"
 	(x)                    // ERROR "x .* not used|not used"
 	_ = new(len)           // ERROR "len.*must be called"
-	// Disabled due to issue #43125.
+	_ = new(1 + 1)         // ok
-	// _ = new(1 + 1)         // DISABLED "1 \+ 1 is not a type"
 }