cmd/compile/internal/types2: better error message for invalid array decls

Fixes #43527.

Change-Id: I988a4d49f2f54b4b1741688fb52a55bf313d39e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/348731
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/testdata/fixedbugs/issue43527.go2

@@ -0,0 +1,16 @@
+// Copyright 2021 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 p
+
+const L = 10
+
+type (
+	_        [L]struct{}
+	_        [A /* ERROR undeclared name A for array length */ ]struct{}
+	_        [B /* ERROR not an expression */ ]struct{}
+	_[A any] struct{}
+
+	B int
+)

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

@@ -428,6 +428,14 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 // and returns the constant length >= 0, or a value < 0
 // to indicate an error (and thus an unknown length).
 func (check *Checker) arrayLength(e syntax.Expr) int64 {
+	// If e is an undeclared identifier, the array declaration might be an
+	// attempt at a parameterized type declaration with missing constraint.
+	// Provide a better error message than just "undeclared name: X".
+	if name, _ := e.(*syntax.Name); name != nil && check.lookup(name.Value) == nil {
+		check.errorf(name, "undeclared name %s for array length", name.Value)
+		return -1
+	}
+
 	var x operand
 	check.expr(&x, e)
 	if x.mode != constant_ {
@@ -436,6 +444,7 @@ func (check *Checker) arrayLength(e syntax.Expr) int64 {
 		}
 		return -1
 	}
+
 	if isUntyped(x.typ) || isInteger(x.typ) {
 		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
 			if representableConst(val, check, Typ[Int], nil) {
@@ -447,6 +456,7 @@ func (check *Checker) arrayLength(e syntax.Expr) int64 {
 			}
 		}
 	}
+
 	check.errorf(&x, "array length %s must be integer", &x)
 	return -1
 }

src/go/types/testdata/fixedbugs/issue43527.go2

@@ -0,0 +1,16 @@
+// Copyright 2021 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 p
+
+const L = 10
+
+type (
+	_        [L]struct{}
+	_        [A /* ERROR undeclared name A for array length */ ]struct{}
+	_        [B /* ERROR not an expression */ ]struct{}
+	_[A any] struct{}
+
+	B int
+)

src/go/types/typexpr.go

@@ -412,6 +412,14 @@ func (check *Checker) instantiatedType(x ast.Expr, targsx []ast.Expr, def *Named
 // and returns the constant length >= 0, or a value < 0
 // to indicate an error (and thus an unknown length).
 func (check *Checker) arrayLength(e ast.Expr) int64 {
+	// If e is an undeclared identifier, the array declaration might be an
+	// attempt at a parameterized type declaration with missing constraint.
+	// Provide a better error message than just "undeclared name: X".
+	if name, _ := e.(*ast.Ident); name != nil && check.lookup(name.Name) == nil {
+		check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
+		return -1
+	}
+
 	var x operand
 	check.expr(&x, e)
 	if x.mode != constant_ {
@@ -420,6 +428,7 @@ func (check *Checker) arrayLength(e ast.Expr) int64 {
 		}
 		return -1
 	}
+
 	if isUntyped(x.typ) || isInteger(x.typ) {
 		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
 			if representableConst(val, check, Typ[Int], nil) {
@@ -431,6 +440,7 @@ func (check *Checker) arrayLength(e ast.Expr) int64 {
 			}
 		}
 	}
+
 	check.errorf(&x, _InvalidArrayLen, "array length %s must be integer", &x)
 	return -1
 }