cmd/cgo: update to handle ast.IndexListExpr

Allows cgo to work with generics.

Updates #47781.

Change-Id: Id1a5d1a0a8193c5b157e3e671b1490d687d10384
Reviewed-on: https://go-review.googlesource.com/c/go/+/353882
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/typeparam.go

@@ -0,0 +1,17 @@
+// 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 cgotest
+
+// #include <stddef.h>
+import "C"
+
+func generic[T, U any](t T, u U) {}
+
+func useGeneric() {
+	const zero C.size_t = 0
+
+	generic(zero, zero)
+	generic[C.size_t, C.size_t](0, 0)
+}

misc/go.mod

@@ -8,4 +8,4 @@
 // directory.)
 module misc
 
-go 1.12
+go 1.18

src/cmd/cgo/ast.go

@@ -338,8 +338,7 @@ func (f *File) walk(x interface{}, context astContext, visit func(*File, interfa
 
 	// everything else just recurs
 	default:
-		error_(token.NoPos, "unexpected type %T in walk", x)
-		panic("unexpected type")
+		f.walkUnexpected(x, context, visit)
 
 	case nil:
 

src/cmd/cgo/ast_go1.go

@@ -0,0 +1,17 @@
+// 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.
+
+//go:build !go1.18
+// +build !go1.18
+
+package main
+
+import (
+	"go/token"
+)
+
+func (f *File) walkUnexpected(x interface{}, context astContext, visit func(*File, interface{}, astContext)) {
+	error_(token.NoPos, "unexpected type %T in walk", x)
+	panic("unexpected type")
+}

src/cmd/cgo/ast_go118.go

@@ -0,0 +1,25 @@
+// 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.
+
+//go:build go1.18
+// +build go1.18
+
+package main
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+func (f *File) walkUnexpected(x interface{}, context astContext, visit func(*File, interface{}, astContext)) {
+	switch n := x.(type) {
+	default:
+		error_(token.NoPos, "unexpected type %T in walk", x)
+		panic("unexpected type")
+
+	case *ast.IndexListExpr:
+		f.walk(&n.X, ctxExpr, visit)
+		f.walk(n.Indices, ctxExpr, visit)
+	}
+}

src/cmd/cgo/gcc.go

@@ -1506,7 +1506,7 @@ func (p *Package) rewriteName(f *File, r *Ref, addPosition bool) ast.Expr {
 				Args: []ast.Expr{getNewIdent(name.Mangle)},
 			}
 		case "type":
-			// Okay - might be new(T)
+			// Okay - might be new(T), T(x), Generic[T], etc.
 			if r.Name.Type == nil {
 				error_(r.Pos(), "expression C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
 			}