diff --git a/tests/lxdrgen.lxdr b/tests/lxdrgen.lxdr
index 6c53de5..41973f8 100644
--- a/tests/lxdrgen.lxdr
+++ b/tests/lxdrgen.lxdr
@@ -20,4 +20,9 @@ struct Struct {
 	case NOTHING:
 		void;
 	} u<>;
+
+	union Onion switch (Enum tag) {
+	case NOTHING:
+		void;
+	} o;
 };
diff --git a/tools/lxdrgen-c.awk b/tools/lxdrgen-c.awk
index 0d43785..e898aae 100644
--- a/tools/lxdrgen-c.awk
+++ b/tools/lxdrgen-c.awk
@@ -259,7 +259,7 @@ function codegen_union_struct( \
 		"\t\tbreak;\n")
 }
 
-function codegen_union(name, cg,    f, ctype, funcname) {
+function codegen_union(name, cg, exhaustive,    f, ctype, funcname) {
 	ctype = "union " PrefixLower cameltosnake(name)
 	print ""
 	print ctype " {"
diff --git a/tools/lxdrgen-go.awk b/tools/lxdrgen-go.awk
index f9163bb..f129bc1 100644
--- a/tools/lxdrgen-go.awk
+++ b/tools/lxdrgen-go.awk
@@ -467,7 +467,7 @@ function codegen_union_struct(name, casename, cg, scg,     structname, init) {
 		"\t\tu.Interface = &s\n")
 }
 
-function codegen_union(name, cg,    gotype, tagvar) {
+function codegen_union(name, cg, exhaustive,    gotype, tagvar) {
 	gotype = PrefixCamel name
 	print "type " gotype " struct {"
 	print "\tInterface any"
diff --git a/tools/lxdrgen-mjs.awk b/tools/lxdrgen-mjs.awk
index 01ee97d..4eaac63 100644
--- a/tools/lxdrgen-mjs.awk
+++ b/tools/lxdrgen-mjs.awk
@@ -208,7 +208,7 @@ function codegen_union_struct(name, casename, cg, scg,     structname) {
 		"\t}\n")
 }
 
-function codegen_union(name, cg,    tagvar) {
+function codegen_union(name, cg, exhaustive,    tagvar) {
 	tagvar = cg["tagname"]
 
 	print ""
diff --git a/tools/lxdrgen-swift.awk b/tools/lxdrgen-swift.awk
index 28554c9..33b1f73 100644
--- a/tools/lxdrgen-swift.awk
+++ b/tools/lxdrgen-swift.awk
@@ -246,7 +246,7 @@ function codegen_union_struct(name, casename, cg, scg,     swifttype) {
 		delete scg[i]
 }
 
-function codegen_union(name, cg,    swifttype, init) {
+function codegen_union(name, cg, exhaustive,    swifttype, init) {
 	# Classes don't have automatic member-wise initializers,
 	# thus using structs and protocols.
 	swifttype = PrefixCamel name
@@ -255,6 +255,10 @@ function codegen_union(name, cg,    swifttype, init) {
 	print "\tvar " cg["tagname"] ": " CodegenSwiftType[cg["tagtype"]] " { get }"
 	print "}"
 
+	if (!exhaustive)
+		append(cg, "cases", "\tdefault:\n" \
+			"\t\tthrow RelayReader.ReadError.unexpectedValue\n")
+
 	init = decapitalize(swifttype)
 	print ""
 	print "public func " init \
@@ -262,11 +266,7 @@ function codegen_union(name, cg,    swifttype, init) {
 	print "\tlet " cg["tagname"] ": " CodegenSwiftType[cg["tagtype"]] \
 		" = try from.read()"
 	print "\tswitch " cg["tagname"] " {"
-	# TODO: Only generate the default if there are remaining values,
-	# so that swiftc doesn't produce warnings.
-	print cg["cases"] "\tdefault:"
-	print "\t\tthrow RelayReader.ReadError.unexpectedValue"
-	print"\t}"
+	print cg["cases"] "\t}"
 	print "}"
 
 	CodegenSwiftType[name] = swifttype
diff --git a/tools/lxdrgen.awk b/tools/lxdrgen.awk
index f77f19f..e5259b7 100644
--- a/tools/lxdrgen.awk
+++ b/tools/lxdrgen.awk
@@ -217,7 +217,8 @@ function defstruct(    name, d, cg) {
 	return name
 }
 
-function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i, unseen) {
+function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i,
+		unseen, exhaustive) {
 	delete cg[0]
 	delete scg[0]
 	delete d[0]
@@ -258,9 +259,14 @@ function defunion(    name, tag, tagtype, tagvalue, cg, scg, d, a, i, unseen) {
 	if (tagvalue)
 		codegen_union_struct(name, tagvalue, cg, scg)
 
-	# What remains non-zero in unseen[2..] is simply not recognized/allowed.
+	# Unseen cases are simply not recognized/allowed.
+	exhaustive = 1
+	for (i in unseen)
+		if (i && unseen[i])
+			exhaustive = 0
+
 	Types[name] = "union"
-	codegen_union(name, cg)
+	codegen_union(name, cg, exhaustive)
 	return name
 }