go/types: print "nil" rather than "untyped nil"

This is a port of CL 366276 from types2 to go/types
with minor adjustments due to the slightly different
handling of nil in go/types.

It uses some more detailed error strings in stmt0.src;
the same changes are made to the corresponding types2
file.

For #48852.

Change-Id: I2cdf258799bcbe2d12bbadaf67b8b4504b356bd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/366277
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

4 files changed, 18 insertions, 13 deletions

diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 8cc5eda866..c35b1650be 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -105,6 +105,11 @@ func (x *operand) Pos() token.Pos {
 // cgofunc    <expr> (               <mode>       of type <typ>)
 //
 func operandString(x *operand, qf Qualifier) string {
+	// special-case nil
+	if x.mode == value && x.typ == Typ[UntypedNil] {
+		return "nil"
+	}
+
 	var buf bytes.Buffer
 
 	var expr string
diff --git a/src/go/types/testdata/check/stmt0.src b/src/go/types/testdata/check/stmt0.src
index 15df37703c..2cce0b59b2 100644
--- a/src/go/types/testdata/check/stmt0.src
+++ b/src/go/types/testdata/check/stmt0.src
@@ -69,10 +69,10 @@ func assignments1() {
 
 	// test cases for issue 5800
 	var (
-		_ int = nil /* ERROR "untyped nil value" */
-		_ [10]int = nil /* ERROR "untyped nil value" */
+		_ int = nil /* ERROR "cannot use nil as int value in variable declaration" */
+		_ [10]int = nil /* ERROR "cannot use nil as \[10\]int value in variable declaration" */
 		_ []byte = nil
-		_ struct{} = nil /* ERROR "untyped nil value" */
+		_ struct{} = nil /* ERROR "cannot use nil as struct{} value in variable declaration" */
 		_ func() = nil
 		_ map[int]string = nil
 		_ chan int = nil
diff --git a/src/go/types/testdata/fixedbugs/issue49296.go2 b/src/go/types/testdata/fixedbugs/issue49296.go2
index 8c6d0b678d..0ad71ef4b2 100644
--- a/src/go/types/testdata/fixedbugs/issue49296.go2
+++ b/src/go/types/testdata/fixedbugs/issue49296.go2
@@ -10,7 +10,7 @@ func _[
         T2 ~float64 | ~complex128 | chan int,
 ]() {
 	// TODO(rfindley): the types2 error here is clearer.
-        _ = T0(nil /* ERROR cannot convert nil \(untyped nil value\) 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 */ )
 }
diff --git a/src/go/types/testdata/spec/assignability.go2 b/src/go/types/testdata/spec/assignability.go2
index a6e71aac81..d5f6ab4419 100644
--- a/src/go/types/testdata/spec/assignability.go2
+++ b/src/go/types/testdata/spec/assignability.go2
@@ -155,28 +155,28 @@ func _[
 // TODO(rfindley) error messages about untyped nil diverge from types2 here.
 // Consider aligning them.
 func _[TP Interface](X TP) {
-	b = nil // ERROR cannot use.*untyped nil
-	a = nil // ERROR cannot use.*untyped nil
+	b = nil // ERROR cannot use 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
-	A = nil // ERROR cannot use.*untyped nil
+	B = nil // ERROR cannot use 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
-	X = nil // ERROR cannot use.*untyped nil
+	D = nil // ERROR cannot use nil
+	X = nil // ERROR cannot use nil
 }
 
 // "x is an untyped constant representable by a value of type T"