cmd/compile/internal/types2: print "nil" rather than "untyped nil"

When we have a typed nil, we already say so; thus it is sufficient
to use "nil" in all the other cases.

This is closer to (1.17) compiler behavior. In cases where the
1.17 compiler prints "untyped nil" (e.g., wrong uses of "copy"),
we already print a different message. We can do better in those
cases as well; will be addressed in a separate CL (see #49735).

Fixes #48852.

Change-Id: I9a7a72e0f99185b00f80040c5510a693b1ea80f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/366276
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -220,9 +220,6 @@ func (check *Checker) assignVar(lhs syntax.Expr, x *operand) Type {
 		return nil
 	case variable, mapindex:
 		// ok
-	case nilvalue:
-		check.error(&z, "cannot assign to nil") // default would print "untyped nil"
-		return nil
 	default:
 		if sel, ok := z.expr.(*syntax.SelectorExpr); ok {
 			var op operand

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

@@ -116,7 +116,7 @@ func operandString(x *operand, qf Qualifier) string {
 		case nil, Typ[Invalid]:
 			return "nil (with invalid type)"
 		case Typ[UntypedNil]:
-			return "untyped nil"
+			return "nil"
 		default:
 			return fmt.Sprintf("nil (of type %s)", TypeString(x.typ, qf))
 		}

src/cmd/compile/internal/types2/testdata/check/stmt0.src

@@ -69,10 +69,10 @@ func assignments1() {
 
 	// test cases for issue 5800
 	var (
-		_ int = nil /* ERROR "untyped nil" */
+		_ int = nil /* ERROR "nil" */
-		_ [10]int = nil /* ERROR "untyped nil" */
+		_ [10]int = nil /* ERROR "nil" */
 		_ []byte = nil
-		_ struct{} = nil /* ERROR "untyped nil" */
+		_ struct{} = nil /* ERROR "nil" */
 		_ func() = nil
 		_ map[int]string = nil
 		_ chan int = nil

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49296.go2

@@ -9,7 +9,7 @@ func _[
         T1 []int,
         T2 ~float64 | ~complex128 | chan int,
 ]() {
-        _ = T0(nil /* ERROR cannot convert untyped nil to T0 */ )
+        _ = T0(nil /* ERROR cannot convert nil to T0 */ )
         _ = T1(1 /* ERROR cannot convert 1 .* to T1 */ )
         _ = T2(2 /* ERROR cannot convert 2 .* to T2 */ )
 }

src/cmd/compile/internal/types2/testdata/spec/assignability.go2

@@ -153,28 +153,28 @@ func _[
 
 // "x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type"
 func _[TP Interface](X TP) {
-	b = nil // ERROR cannot use untyped nil
+	b = nil // ERROR cannot use nil
-	a = nil // ERROR cannot use untyped nil
+	a = nil // ERROR cannot use nil
 	l = nil
-	s = nil // ERROR cannot use untyped nil
+	s = nil // ERROR cannot use nil
 	p = nil
 	f = nil
 	i = nil
 	m = nil
 	c = nil
-	d = nil // ERROR cannot use untyped nil
+	d = nil // ERROR cannot use nil
 
-	B = nil // ERROR cannot use untyped nil
+	B = nil // ERROR cannot use nil
-	A = nil // ERROR cannot use untyped nil
+	A = nil // ERROR cannot use nil
 	L = nil
-	S = nil // ERROR cannot use untyped nil
+	S = nil // ERROR cannot use nil
 	P = nil
 	F = nil
 	I = nil
 	M = nil
 	C = nil
-	D = nil // ERROR cannot use untyped nil
+	D = nil // ERROR cannot use nil
-	X = nil // ERROR cannot use untyped nil
+	X = nil // ERROR cannot use nil
 }
 
 // "x is an untyped constant representable by a value of type T"

test/fixedbugs/issue6004.go

@@ -7,9 +7,8 @@
 package main
 
 func main() {
-	_ = nil // ERROR "use of untyped nil"
+	_ = nil                  // ERROR "use of untyped nil"
-	_, _ = nil, 1 // ERROR "use of untyped nil"
+	_, _ = nil, 1            // ERROR "use of untyped nil"
-	_, _ = 1, nil // ERROR "use of untyped nil"
+	_, _ = 1, nil            // ERROR "use of untyped nil"
-	_ = append(nil, 1, 2, 3) // ERROR "untyped nil"
+	_ = append(nil, 1, 2, 3) // ERROR "untyped nil|nil"
 }
-

test/fixedbugs/issue6402.go

@@ -9,5 +9,5 @@
 package p
 
 func f() uintptr {
-	return nil // ERROR "cannot use nil as type uintptr in return argument|incompatible type|cannot use untyped nil"
+	return nil // ERROR "cannot use nil as type uintptr in return argument|incompatible type|cannot use nil"
 }

test/fixedbugs/issue7223.go

@@ -7,14 +7,15 @@
 package main
 
 var bits1 uint = 10
+
 const bits2 uint = 10
 
 func main() {
 	_ = make([]byte, 1<<bits1)
 	_ = make([]byte, 1<<bits2)
-	_ = make([]byte, nil) // ERROR "non-integer.*len|untyped nil"
+	_ = make([]byte, nil)    // ERROR "non-integer.*len|nil"
-	_ = make([]byte, nil, 2) // ERROR "non-integer.*len|untyped nil"
+	_ = make([]byte, nil, 2) // ERROR "non-integer.*len|nil"
-	_ = make([]byte, 1, nil) // ERROR "non-integer.*cap|untyped nil"
+	_ = make([]byte, 1, nil) // ERROR "non-integer.*cap|nil"
-	_ = make([]byte, true) // ERROR "non-integer.*len|untyped bool"
+	_ = make([]byte, true)   // ERROR "non-integer.*len|untyped bool"
-	_ = make([]byte, "abc") // ERROR "non-integer.*len|untyped string"
+	_ = make([]byte, "abc")  // ERROR "non-integer.*len|untyped string"
 }