cmd/compile: prevent irgen crashing for empty local declaration stmt

Updates #47631
Fixes #49611

Change-Id: Ib4a4466038e0d4a9aa9380d7909f29f7d15c6c69
Reviewed-on: https://go-review.googlesource.com/c/go/+/364314
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: cuonglm

src/cmd/compile/internal/noder/stmt.go

@@ -46,9 +46,11 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 		n.SetTypecheck(1)
 		return n
 	case *syntax.DeclStmt:
-		if _, ok := stmt.DeclList[0].(*syntax.TypeDecl); ok && g.topFuncIsGeneric {
-			// TODO: remove this restriction. See issue 47631.
-			base.ErrorfAt(g.pos(stmt), "type declarations inside generic functions are not currently supported")
+		if g.topFuncIsGeneric && len(stmt.DeclList) > 0 {
+			if _, ok := stmt.DeclList[0].(*syntax.TypeDecl); ok {
+				// TODO: remove this restriction. See issue 47631.
+				base.ErrorfAt(g.pos(stmt), "type declarations inside generic functions are not currently supported")
+			}
 		}
 		n := ir.NewBlockStmt(g.pos(stmt), nil)
 		g.decls(&n.List, stmt.DeclList)

test/fixedbugs/issue49611.go

@@ -0,0 +1,11 @@
+// compile
+
+// 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
+
+func f() {
+	var ()
+}

test/typeparam/issue49611.go

@@ -0,0 +1,11 @@
+// compile -G=3
+
+// 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
+
+func f[T any]() {
+	var ()
+}