[dev.regabi] test: add another closure test case

When deciding whether a captured variable can be passed by value, the
compiler is sensitive to the order that the OCLOSURE node is
typechecked relative to the order that the variable is passed to
"checkassign". Today, for an assignment like:

    q, g = 2, func() int { return q }

we get this right because we always typecheck the full RHS expression
list before calling checkassign on any LHS expression.

But I nearly made a change that would interleave this ordering,
causing us to call checkassign on q before typechecking the function
literal. And alarmingly, there weren't any tests that caught this.

So this commit adds one.

Change-Id: I66cacd61066c7a229070861a7d973bcc434904cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/280998
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/test/closure2.go b/test/closure2.go
index e4db05d884..812d41f8ce 100644
--- a/test/closure2.go
+++ b/test/closure2.go
@@ -9,6 +9,8 @@
 
 package main
 
+var never bool
+
 func main() {
 	{
 		type X struct {
@@ -115,4 +117,16 @@ func main() {
 			panic("g() != 2")
 		}
 	}
+
+	{
+		var g func() int
+		q := 0
+		q, g = 1, func() int { return q }
+		if never {
+			g = func() int { return 2 }
+		}
+		if g() != 1 {
+			panic("g() != 1")
+		}
+	}
 }