go/types, types2: better error for generic type decl. with missing constraint

If a generic type declaration is missing a constraint, syntactically
it is an array type declaration with an undefined array length.
Mention the possibility of a missing constraint in the error message
for the undefined array length.

For #56064.
For #55961.
For #51145.

Change-Id: Ic161aeda9ea44faa8aa3bf3e9d62b3b13a95d4c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/439559
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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

@@ -478,7 +478,7 @@ func (check *Checker) arrayLength(e syntax.Expr) int64 {
 	if name, _ := e.(*syntax.Name); name != nil {
 		obj := check.lookup(name.Value)
 		if obj == nil {
-			check.errorf(name, _InvalidArrayLen, "undefined %s for array length", name.Value)
+			check.errorf(name, _InvalidArrayLen, "undefined array length %s or missing type constraint", name.Value)
 			return -1
 		}
 		if _, ok := obj.(*Const); !ok {

src/go/types/typexpr.go

@@ -469,7 +469,7 @@ func (check *Checker) arrayLength(e ast.Expr) int64 {
 	if name, _ := e.(*ast.Ident); name != nil {
 		obj := check.lookup(name.Name)
 		if obj == nil {
-			check.errorf(name, _InvalidArrayLen, "undefined %s for array length", name.Name)
+			check.errorf(name, _InvalidArrayLen, "undefined array length %s or missing type constraint", name.Name)
 			return -1
 		}
 		if _, ok := obj.(*Const); !ok {

src/internal/types/testdata/fixedbugs/issue43527.go

@@ -8,7 +8,7 @@ const L = 10
 
 type (
 	_        [L]struct{}
-	_        [A /* ERROR undefined A for array length */ ]struct{}
+	_        [A /* ERROR undefined array length A or missing type constraint */ ]struct{}
 	_        [B /* ERROR invalid array length B */ ]struct{}
 	_[A any] struct{}