diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue43527.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue43527.go2
index e4bcee51fe5..2955c261f95 100644
--- a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue43527.go2
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue43527.go2
@@ -9,7 +9,7 @@ const L = 10
 type (
 	_        [L]struct{}
 	_        [A /* ERROR undeclared name A for array length */ ]struct{}
-	_        [B /* ERROR not an expression */ ]struct{}
+	_        [B /* ERROR invalid array length B */ ]struct{}
 	_[A any] struct{}
 
 	B int
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51145.go b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51145.go
new file mode 100644
index 00000000000..b84391df197
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue51145.go
@@ -0,0 +1,18 @@
+// Copyright 2022 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
+
+import "fmt"
+
+type (
+	_ [fmt /* ERROR invalid array length fmt */ ]int
+	_ [float64 /* ERROR invalid array length float64 */ ]int
+	_ [f /* ERROR invalid array length f */ ]int
+	_ [nil /* ERROR invalid array length nil */ ]int
+)
+
+func f()
+
+var _ fmt.Stringer // use fmt
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index de778fb010b..149bd5b0b3d 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -502,12 +502,20 @@ func (check *Checker) instantiatedType(x syntax.Expr, xlist []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
+	// If e is an identifier, the array declaration might be an
+	// attempt at a parameterized type declaration with missing
+	// constraint. Provide an error message that mentions array
+	// length.
+	if name, _ := e.(*syntax.Name); name != nil {
+		obj := check.lookup(name.Value)
+		if obj == nil {
+			check.errorf(name, "undeclared name %s for array length", name.Value)
+			return -1
+		}
+		if _, ok := obj.(*Const); !ok {
+			check.errorf(name, "invalid array length %s", name.Value)
+			return -1
+		}
 	}
 
 	var x operand
diff --git a/src/go/types/errorcodes.go b/src/go/types/errorcodes.go
index 51f091a9cbe..a7514b317a0 100644
--- a/src/go/types/errorcodes.go
+++ b/src/go/types/errorcodes.go
@@ -98,13 +98,10 @@ const (
 	// _InvalidDeclCycle occurs when a declaration cycle is not valid.
 	//
 	// Example:
-	//  import "unsafe"
-	//
-	//  type T struct {
-	//  	a [n]int
+	//  type S struct {
+	//  	S
 	//  }
 	//
-	//  var n = unsafe.Sizeof(T{})
 	_InvalidDeclCycle
 
 	// _InvalidTypeCycle occurs when a cycle in type definitions results in a
diff --git a/src/go/types/testdata/fixedbugs/issue43527.go2 b/src/go/types/testdata/fixedbugs/issue43527.go2
index e4bcee51fe5..2955c261f95 100644
--- a/src/go/types/testdata/fixedbugs/issue43527.go2
+++ b/src/go/types/testdata/fixedbugs/issue43527.go2
@@ -9,7 +9,7 @@ const L = 10
 type (
 	_        [L]struct{}
 	_        [A /* ERROR undeclared name A for array length */ ]struct{}
-	_        [B /* ERROR not an expression */ ]struct{}
+	_        [B /* ERROR invalid array length B */ ]struct{}
 	_[A any] struct{}
 
 	B int
diff --git a/src/go/types/testdata/fixedbugs/issue51145.go b/src/go/types/testdata/fixedbugs/issue51145.go
new file mode 100644
index 00000000000..b84391df197
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue51145.go
@@ -0,0 +1,18 @@
+// Copyright 2022 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
+
+import "fmt"
+
+type (
+	_ [fmt /* ERROR invalid array length fmt */ ]int
+	_ [float64 /* ERROR invalid array length float64 */ ]int
+	_ [f /* ERROR invalid array length f */ ]int
+	_ [nil /* ERROR invalid array length nil */ ]int
+)
+
+func f()
+
+var _ fmt.Stringer // use fmt
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index 00c250b5b63..db6a904aaa5 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -487,12 +487,20 @@ func (check *Checker) instantiatedType(ix *typeparams.IndexExpr, def *Named) (re
 // 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
+	// If e is an identifier, the array declaration might be an
+	// attempt at a parameterized type declaration with missing
+	// constraint. Provide an error message that mentions array
+	// length.
+	if name, _ := e.(*ast.Ident); name != nil {
+		obj := check.lookup(name.Name)
+		if obj == nil {
+			check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
+			return -1
+		}
+		if _, ok := obj.(*Const); !ok {
+			check.errorf(name, _InvalidArrayLen, "invalid array length %s", name.Name)
+			return -1
+		}
 	}
 
 	var x operand
diff --git a/test/fixedbugs/bug255.go b/test/fixedbugs/bug255.go
index 38df7813c9d..184ff2d378b 100644
--- a/test/fixedbugs/bug255.go
+++ b/test/fixedbugs/bug255.go
@@ -6,12 +6,13 @@
 
 package main
 
-var a [10]int      // ok
-var b [1e1]int     // ok
-var c [1.5]int     // ERROR "truncated|must be integer"
-var d ["abc"]int   // ERROR "invalid array bound|not numeric|must be integer"
-var e [nil]int     // ERROR "use of untyped nil|invalid array bound|not numeric|must be constant"
-var f [e]int       // ok: error already reported for e
+var a [10]int    // ok
+var b [1e1]int   // ok
+var c [1.5]int   // ERROR "truncated|must be integer"
+var d ["abc"]int // ERROR "invalid array bound|not numeric|must be integer"
+var e [nil]int   // ERROR "use of untyped nil|invalid array (bound|length)|not numeric|must be constant"
+// var f [e]int  // ok with Go 1.17 because an error was reported for e; leads to an error for Go 1.18
+var f [ee]int      // ERROR "undefined|undeclared"
 var g [1 << 65]int // ERROR "array bound is too large|overflows|must be integer"
 var h [len(a)]int  // ok