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

Change-Id: Ib500ee92ae1a3d15f7c9f3f46d238b75184b4304
Reviewed-on: https://go-review.googlesource.com/21382
Reviewed-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

@@ -372,8 +372,8 @@ func saveorignode(n *Node) {
 	n.Orig = norig
 }
 
-func maptype(key *Type, val *Type) *Type {
-	if key != nil {
+// 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
@@ -405,9 +405,6 @@ func maptype(key *Type, val *Type) *Type {
 	}
 }
 
-	return typMap(key, val)
-}
-
 // methcmp sorts by symbol, then by package path for unexported symbols.
 type methcmp []*Field
 

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