go/types,types2: allow new(expr)

For golang#45624

Change-Id: I6d77a2a1d6095cac0edc36060cbf98c72b749404
Reviewed-on: https://go-review.googlesource.com/c/go/+/704935
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

src/cmd/compile/internal/types2/builtins.go

@@ -636,11 +636,30 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		}
 
 	case _New:
-		// new(T)
+		// new(T) or new(expr)
 		// (no argument evaluated yet)
-		T := check.varType(argList[0])
-		if !isValid(T) {
-			return
+		arg := argList[0]
+		check.exprOrType(x, arg, true)
+		var T Type
+		switch x.mode {
+		case builtin:
+			check.errorf(x, UncalledBuiltin, "%s must be called", x)
+			x.mode = invalid
+		case typexpr:
+			// new(T)
+			T = x.typ
+			if !isValid(T) {
+				return
+			}
+		default:
+			// new(expr)
+			check.verifyVersionf(call.Fun, go1_26, "new(expr)")
+			T = Default(x.typ)
+			if T != x.typ {
+				// untyped constant: check for overflow.
+				check.assignment(x, T, "argument to new")
+			}
+			check.validVarType(arg, T)
 		}
 
 		x.mode = value

src/cmd/compile/internal/types2/version.go

@@ -43,6 +43,7 @@ var (
 	go1_21 = asGoVersion("go1.21")
 	go1_22 = asGoVersion("go1.22")
 	go1_23 = asGoVersion("go1.23")
+	go1_26 = asGoVersion("go1.26")
 
 	// current (deployed) Go version
 	go_current = asGoVersion(fmt.Sprintf("go1.%d", goversion.Version))

src/go/types/builtins.go

@@ -43,6 +43,7 @@ var (
 	go1_21 = asGoVersion("go1.21")
 	go1_22 = asGoVersion("go1.22")
 	go1_23 = asGoVersion("go1.23")
+	go1_26 = asGoVersion("go1.26")
 
 	// current (deployed) Go version
 	go_current = asGoVersion(fmt.Sprintf("go1.%d", goversion.Version))

src/go/types/version.go

@@ -609,24 +609,48 @@ func min2() {
 	)
 }
 
-func new1() {
-	_ = new() // ERROR "not enough arguments"
+func newInvalid() {
+	f2 := func() (x, y int) { return }
+
+	_ = new()     // ERROR "not enough arguments"
 	_ = new(1, 2) // ERROR "too many arguments"
-	_ = new("foo" /* ERROR "not a type" */)
-	p := new(float64)
+	new /* ERROR "not used" */ (int)
+	_ = &new /* ERROR "cannot take address" */ (int)
+	_ = new(int... /* ERROR "invalid use of ..." */)
+	_ = new(f0 /* ERROR "f0() (no value) used as value or type" */ ())
+	_ = new(len /* ERROR "len (built-in) must be called" */)
+	_ = new(1 /* ERROR "argument to new (overflows)" */ << 70)
+	_ = new(f2 /* ERRORx "multiple-value.*in single-value context" */ ())
+}
+
+// new(T)
+func newType() {
 	_ = new(struct{ x, y int })
+
+	p := new(float64)
 	q := new(*float64)
 	_ = *p == **q
-	new /* ERROR "not used" */ (int)
-        _ = &new /* ERROR "cannot take address" */ (int)
-
-	_ = new(int... /* ERROR "invalid use of ..." */ )
 }
 
-func new2() {
+// new(expr), added in go1.26
+func newExpr() {
 	f1 := func() (x []int) { return }
-	_ = new(f0 /* ERROR "not a type" */ ())
-	_ = new(f1 /* ERROR "not a type" */ ())
+	var (
+		_ *[]int        = new(f1())
+		_ *func() []int = new(f1)
+		_ *bool         = new(false)
+		_ *int          = new(123)
+		_ *float64      = new(1.0)
+		_ *uint         = new(uint(3))
+		_ *rune         = new('a')
+		_ *string       = new("A")
+		_ *struct{}     = new(struct{}{})
+		_ *any          = new(any)
+
+		// from issue 43125
+		_ = new(-1)
+		_ = new(1 + 1)
+	)
 }
 
 func panic1() {

src/internal/types/testdata/check/builtins0.go

@@ -0,0 +1,13 @@
+// -lang=go1.25
+
+// 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.
+
+// Check Go language version-specific errors.
+
+//go:build go1.25
+
+package p
+
+var _ = new /* ERROR "new(expr) requires go1.26 or later" */ (123)

src/internal/types/testdata/check/go1_25.go

@@ -139,7 +139,7 @@ func _() {
 	unsafe.Sizeof(t)       // ERROR "unsafe.Sizeof\(t\) .* not used"
 	_ = int                // ERROR "type int is not an expression|not an expression"
 	(x)                    // ERROR "x .* not used|not used"
-	_ = new(x2)            // ERROR "x2 is not a type|not a type"
+	_ = new(len)           // ERROR "len.*must be called"
 	// Disabled due to issue #43125.
 	// _ = new(1 + 1)         // DISABLED "1 \+ 1 is not a type"
 }