cmd/compile: stop generating garbage when checking map key types

Change-Id: Ib500ee92ae1a3d15f7c9f3f46d238b75184b4304
Reviewed-on: https://go-review.googlesource.com/21382Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -3018,7 +3018,7 @@ func (p *parser) hidden_type_misc() *Type {
 		p.want(']')
 		s5 := p.hidden_type()
 
-		return maptype(s3, s5)
+		return typMap(s3, s5)
 
 	case LSTRUCT:
 		// LSTRUCT '{' ohidden_structdcl_list '}'

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

@@ -372,40 +372,37 @@ func saveorignode(n *Node) {
 	n.Orig = norig
 }
 
-func maptype(key *Type, val *Type) *Type {
-	if key != nil {
-		var bad *Type
-		atype := algtype1(key, &bad)
-		var mtype EType
-		if bad == nil {
-			mtype = key.Etype
-		} else {
-			mtype = bad.Etype
+// checkMapKeyType checks that Type key is valid for use as a map key.
+func checkMapKeyType(key *Type) {
+	var bad *Type
+	atype := algtype1(key, &bad)
+	var mtype EType
+	if bad == nil {
+		mtype = key.Etype
+	} else {
+		mtype = bad.Etype
+	}
+	switch mtype {
+	default:
+		if atype == ANOEQ {
+			Yyerror("invalid map key type %v", key)
 		}
-		switch mtype {
-		default:
-			if atype == ANOEQ {
-				Yyerror("invalid map key type %v", key)
-			}
 
-		case TANY:
-			// will be resolved later.
-			break
+	case TANY:
+		// will be resolved later.
+		break
 
-		case TFORW:
-			// map[key] used during definition of key.
-			// postpone check until key is fully defined.
-			// if there are multiple uses of map[key]
-			// before key is fully defined, the error
-			// will only be printed for the first one.
-			// good enough.
-			if key.Maplineno == 0 {
-				key.Maplineno = lineno
-			}
+	case TFORW:
+		// map[key] used during definition of key.
+		// postpone check until key is fully defined.
+		// if there are multiple uses of map[key]
+		// before key is fully defined, the error
+		// will only be printed for the first one.
+		// good enough.
+		if key.Maplineno == 0 {
+			key.Maplineno = lineno
 		}
 	}
-
-	return typMap(key, val)
 }
 
 // methcmp sorts by symbol, then by package path for unexported symbols.

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

@@ -275,6 +275,10 @@ func typChan(elem *Type, dir uint8) *Type {
 
 // typMap returns a new map Type with key type k and element (aka value) type v.
 func typMap(k, v *Type) *Type {
+	if k != nil {
+		checkMapKeyType(k)
+	}
+
 	t := typ(TMAP)
 	t.Down = k
 	t.Type = v

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

@@ -402,7 +402,7 @@ OpSwitch:
 			return n
 		}
 		n.Op = OTYPE
-		n.Type = maptype(l.Type, r.Type)
+		n.Type = typMap(l.Type, r.Type)
 		n.Left = nil
 		n.Right = nil
 
@@ -3595,7 +3595,7 @@ ret:
 
 		for _, n := range mapqueue {
 			lineno = n.Type.Maplineno
-			maptype(n.Type, Types[TBOOL])
+			checkMapKeyType(n.Type)
 		}
 
 		lineno = lno