go/types, types2: first argument to append must never be be nil

The current implementation followed the spec faithfully for
the special case for append. Per the spec:

As a special case, append also accepts a first argument assignable to
type []byte with a second argument of string type followed by ... .

As it happens, nil is assignable to []byte, so append(nil, ""...)
didn't get an error message but a subsequent assertion failed.

This CL ensures that the first argument to append is never nil and
always a slice. We should make the spec more precise (separate CL).

Fixes #76220.

Change-Id: I581d11827a75afbb257077814beea813d4fe2441
Reviewed-on: https://go-review.googlesource.com/c/go/+/718860
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Brett Howell <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: Robert Griesemer

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

@@ -91,6 +91,17 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		// to type []byte with a second argument of string type followed by ... .
 		// This form appends the bytes of the string."
 
+		// In either case, the first argument must be a slice; in particular it
+		// cannot be the predeclared nil value. Note that nil is not excluded by
+		// the assignability requirement alone for the special case (go.dev/issue/76220).
+		// spec: "If S is a type parameter, all types in its type set
+		// must have the same underlying slice type []E."
+		E, err := sliceElem(x)
+		if err != nil {
+			check.errorf(x, InvalidAppend, "invalid append: %s", err.format(check))
+			return
+		}
+
 		// Handle append(bytes, y...) special case, where
 		// the type set of y is {string} or {string, []byte}.
 		var sig *Signature
@@ -119,13 +130,6 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 
 		// general case
 		if sig == nil {
-			// spec: "If S is a type parameter, all types in its type set
-			// must have the same underlying slice type []E."
-			E, err := sliceElem(x)
-			if err != nil {
-				check.errorf(x, InvalidAppend, "invalid append: %s", err.format(check))
-				return
-			}
 			// check arguments by creating custom signature
 			sig = makeSig(x.typ, x.typ, NewSlice(E)) // []E required for variadic signature
 			sig.variadic = true

src/go/types/builtins.go

@@ -0,0 +1,17 @@
+// Copyright 2025 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
+
+func _() {
+	append(nil /* ERROR "argument must be a slice; have untyped nil" */, ""...)
+}
+
+// test case from issue
+
+func main() {
+	s := "hello"
+	msg := append(nil /* ERROR "argument must be a slice; have untyped nil" */, s...)
+	print(msg)
+}