[dev.typealias] cmd/compile: various minor cleanups

Also: Don't allow type pragmas with type alias declarations.

For #18130.

Change-Id: Ie54ea5fefcd677ad87ced03466bbfd783771e974
Reviewed-on: https://go-review.googlesource.com/35102
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/main.go

@@ -344,8 +344,9 @@ func Main() {
 	// Don't use range--typecheck can add closures to xtop.
 	timings.Start("fe", "typecheck", "top1")
 	for i := 0; i < len(xtop); i++ {
-		if xtop[i].Op != ODCL && xtop[i].Op != OAS && xtop[i].Op != OAS2 {
-			xtop[i] = typecheck(xtop[i], Etop)
+		n := xtop[i]
+		if op := n.Op; op != ODCL && op != OAS && op != OAS2 {
+			xtop[i] = typecheck(n, Etop)
 		}
 	}
 
@@ -355,8 +356,9 @@ func Main() {
 	// Don't use range--typecheck can add closures to xtop.
 	timings.Start("fe", "typecheck", "top2")
 	for i := 0; i < len(xtop); i++ {
-		if xtop[i].Op == ODCL || xtop[i].Op == OAS || xtop[i].Op == OAS2 {
-			xtop[i] = typecheck(xtop[i], Etop)
+		n := xtop[i]
+		if op := n.Op; op == ODCL || op == OAS || op == OAS2 {
+			xtop[i] = typecheck(n, Etop)
 		}
 	}
 	resumecheckwidth()
@@ -366,8 +368,9 @@ func Main() {
 	timings.Start("fe", "typecheck", "func")
 	var fcount int64
 	for i := 0; i < len(xtop); i++ {
-		if xtop[i].Op == ODCLFUNC || xtop[i].Op == OCLOSURE {
-			Curfn = xtop[i]
+		n := xtop[i]
+		if op := n.Op; op == ODCLFUNC || op == OCLOSURE {
+			Curfn = n
 			decldepth = 1
 			saveerrors()
 			typecheckslice(Curfn.Nbody.Slice(), Etop)
@@ -459,8 +462,9 @@ func Main() {
 	timings.Start("be", "compilefuncs")
 	fcount = 0
 	for i := 0; i < len(xtop); i++ {
-		if xtop[i].Op == ODCLFUNC {
-			funccompile(xtop[i])
+		n := xtop[i]
+		if n.Op == ODCLFUNC {
+			funccompile(n)
 			fcount++
 		}
 	}

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

@@ -153,11 +153,7 @@ func (p *noder) importDecl(imp *syntax.ImportDecl) {
 
 func (p *noder) varDecl(decl *syntax.VarDecl) []*Node {
 	names := p.declNames(decl.NameList)
-
-	var typ *Node
-	if decl.Type != nil {
-		typ = p.typeExpr(decl.Type)
-	}
+	typ := p.typeExprOrNil(decl.Type)
 
 	var exprs []*Node
 	if decl.Values != nil {
@@ -170,11 +166,7 @@ func (p *noder) varDecl(decl *syntax.VarDecl) []*Node {
 
 func (p *noder) constDecl(decl *syntax.ConstDecl) []*Node {
 	names := p.declNames(decl.NameList)
-
-	var typ *Node
-	if decl.Type != nil {
-		typ = p.typeExpr(decl.Type)
-	}
+	typ := p.typeExprOrNil(decl.Type)
 
 	var exprs []*Node
 	if decl.Values != nil {
@@ -190,12 +182,14 @@ func (p *noder) typeDecl(decl *syntax.TypeDecl) *Node {
 	}
 
 	name := typedcl0(p.name(decl.Name))
-	name.Name.Param.Pragma = Pragma(decl.Pragma)
-
-	var typ *Node
-	if decl.Type != nil {
-		typ = p.typeExpr(decl.Type)
+	pragma := Pragma(decl.Pragma)
+	if pragma != 0 && decl.Alias {
+		yyerror("cannot specify directive with type alias")
+		pragma = 0
 	}
+	name.Name.Param.Pragma = pragma
+
+	typ := p.typeExprOrNil(decl.Type)
 
 	return typedcl1(name, typ, true)
 }
@@ -470,6 +464,13 @@ func (p *noder) typeExpr(typ syntax.Expr) *Node {
 	return p.expr(typ)
 }
 
+func (p *noder) typeExprOrNil(typ syntax.Expr) *Node {
+	if typ != nil {
+		return p.expr(typ)
+	}
+	return nil
+}
+
 func (p *noder) chanDir(dir syntax.ChanDir) ChanDir {
 	switch dir {
 	case 0: