[dev.unified] cmd/compile/internal/noder: explicit nil handling

Currently, uses of "nil" are handling as references to cmd/compile's
own untyped "nil" object, and then we rely on implicitly converting
that to its appropriate type. But there are cases where this can
subtly go wrong (e.g., the switch test case added in the previous CL).

Instead, explicitly handling "nil" expressions so that we can
construct them directly with the appropriate type, as computed already
by types2.

Change-Id: I587f044f60f24e87525dde6d7dad6c58f14478de
Reviewed-on: https://go-review.googlesource.com/c/go/+/418100
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -53,6 +53,7 @@ const (
 	exprConvert
 	exprNew
 	exprMake
+	exprNil
 )
 
 type codeAssign int

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

@@ -1684,6 +1684,11 @@ func (r *reader) expr() (res ir.Node) {
 		orig := r.String()
 		return typecheck.Expr(OrigConst(pos, typ, val, op, orig))
 
+	case exprNil:
+		pos := r.pos()
+		typ := r.typ()
+		return Nil(pos, typ)
+
 	case exprCompLit:
 		return r.compLit()
 

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

@@ -1413,6 +1413,13 @@ func (w *writer) expr(expr syntax.Expr) {
 			w.String(syntax.String(expr))
 			return
 		}
+
+		if _, isNil := obj.(*types2.Nil); isNil {
+			w.Code(exprNil)
+			w.pos(expr)
+			w.typ(tv.Type)
+			return
+		}
 	}
 
 	if obj != nil {