[dev.unified] cmd/compile: extract nil handling from exprType

Type switches are the only context where exprType was used and `nilOK`
was true. It'll simplify subsequent dictionary work somewhat if
exprType doesn't need to worry about `nil`, so extract this logic and
move it into switchStmt instead.

Change-Id: I3d810f465173f5bb2e2dee7bbc7843fff6a62ee5
Reviewed-on: https://go-review.googlesource.com/c/go/+/419474
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1547,7 +1547,11 @@ func (r *reader) switchStmt(label *types.Sym) ir.Node {
 				cases = nil // TODO(mdempsky): Unclear if this matters.
 			}
 			for i := range cases {
-				cases[i] = r.exprType(true)
+				if r.Bool() { // case nil
+					cases[i] = typecheck.Expr(types.BuiltinPkg.Lookup("nil").Def.(*ir.NilExpr))
+				} else {
+					cases[i] = r.exprType()
+				}
 			}
 		} else {
 			cases = r.exprList()
@@ -1734,7 +1738,7 @@ func (r *reader) expr() (res ir.Node) {
 	case exprAssert:
 		x := r.expr()
 		pos := r.pos()
-		typ := r.exprType(false)
+		typ := r.exprType()
 		srcRType := r.rtype(pos)
 
 		// TODO(mdempsky): Always emit ODYNAMICDOTTYPE for uniformity?
@@ -1800,7 +1804,7 @@ func (r *reader) expr() (res ir.Node) {
 
 	case exprMake:
 		pos := r.pos()
-		typ := r.exprType(false)
+		typ := r.exprType()
 		extra := r.exprs()
 		n := typecheck.Expr(ir.NewCallExpr(pos, ir.OMAKE, nil, append([]ir.Node{typ}, extra...))).(*ir.MakeExpr)
 		n.RType = r.rtype(pos)
@@ -1808,7 +1812,7 @@ func (r *reader) expr() (res ir.Node) {
 
 	case exprNew:
 		pos := r.pos()
-		typ := r.exprType(false)
+		typ := r.exprType()
 		return typecheck.Expr(ir.NewUnaryExpr(pos, ir.ONEW, typ))
 
 	case exprConvert:
@@ -2043,13 +2047,9 @@ func (r *reader) convRTTI(pos src.XPos) (typeWord, srcRType ir.Node) {
 	return
 }
 
-func (r *reader) exprType(nilOK bool) ir.Node {
+func (r *reader) exprType() ir.Node {
 	r.Sync(pkgbits.SyncExprType)
 
-	if nilOK && r.Bool() {
-		return typecheck.Expr(types.BuiltinPkg.Lookup("nil").Def.(*ir.NilExpr))
-	}
-
 	pos := r.pos()
 	setBasePos(pos)
 

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

@@ -1464,7 +1464,10 @@ func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
 		if iface != nil {
 			w.Len(len(cases))
 			for _, cas := range cases {
-				w.exprType(iface, cas, true)
+				if w.Bool(isNil(w.p.info, cas)) {
+					continue
+				}
+				w.exprType(iface, cas)
 			}
 		} else {
 			// As if w.exprList(clause.Cases),
@@ -1642,7 +1645,7 @@ func (w *writer) expr(expr syntax.Expr) {
 		w.Code(exprAssert)
 		w.expr(expr.X)
 		w.pos(expr)
-		w.exprType(iface, expr.Type, false)
+		w.exprType(iface, expr.Type)
 		w.rtype(iface)
 
 	case *syntax.Operation:
@@ -1702,7 +1705,7 @@ func (w *writer) expr(expr syntax.Expr) {
 
 				w.Code(exprMake)
 				w.pos(expr)
-				w.exprType(nil, expr.ArgList[0], false)
+				w.exprType(nil, expr.ArgList[0])
 				w.exprs(expr.ArgList[1:])
 
 				typ := w.p.typeOf(expr)
@@ -1725,7 +1728,7 @@ func (w *writer) expr(expr syntax.Expr) {
 
 				w.Code(exprNew)
 				w.pos(expr)
-				w.exprType(nil, expr.ArgList[0], false)
+				w.exprType(nil, expr.ArgList[0])
 				return
 
 			case "append":
@@ -1960,21 +1963,15 @@ func (w *writer) convRTTI(src, dst types2.Type) {
 	w.typ(dst)
 }
 
-func (w *writer) exprType(iface types2.Type, typ syntax.Expr, nilOK bool) {
+func (w *writer) exprType(iface types2.Type, typ syntax.Expr) {
 	base.Assertf(iface == nil || isInterface(iface), "%v must be nil or an interface type", iface)
 
 	tv, ok := w.p.info.Types[typ]
 	assert(ok)
-
-	w.Sync(pkgbits.SyncExprType)
-
-	if nilOK && w.Bool(tv.IsNil()) {
-		return
-	}
-
 	assert(tv.IsType())
 	info := w.p.typIdx(tv.Type, w.dict)
 
+	w.Sync(pkgbits.SyncExprType)
 	w.pos(typ)
 
 	if w.Bool(info.derived && iface != nil && !iface.Underlying().(*types2.Interface).Empty()) {
@@ -2386,6 +2383,14 @@ func isMultiValueExpr(info *types2.Info, expr syntax.Expr) bool {
 	return false
 }
 
+// isNil reports whether expr is a (possibly parenthesized) reference
+// to the predeclared nil value.
+func isNil(info *types2.Info, expr syntax.Expr) bool {
+	tv, ok := info.Types[expr]
+	assert(ok)
+	return tv.IsNil()
+}
+
 // recvBase returns the base type for the given receiver parameter.
 func recvBase(recv *types2.Var) *types2.Named {
 	typ := recv.Type()