diff -r c5110aa667d8 sys/src/cmd/cc/lex.c
--- a/sys/src/cmd/cc/lex.c	Wed Jul 29 13:56:03 2020 +0930
+++ b/sys/src/cmd/cc/lex.c	Wed Jul 29 11:07:39 2020 -0700
@@ -444,7 +444,7 @@
 yylex(void)
 {
 	vlong vv;
-	long c, c1, t;
+	long c, c1, t, w;
 	char *cp;
 	Rune rune;
 	Sym *s;
@@ -844,7 +844,8 @@
 		yyerror("overflow in constant");
 
 	vv = yylval.vval;
-	if(c1 & Numvlong) {
+	w = (c1 & Numdec) ? 31 : 32;
+	if(c1 & Numvlong || (uvlong)vv >= 1ULL<<w){
 		if((c1 & Numuns) || convvtox(vv, TVLONG) < 0) {
 			c = LUVLCONST;
 			t = TUVLONG;