Allow omitted block parameter at beginning of block only

parse.y

@@ -255,6 +255,7 @@ struct parser_params {
     int max_numparam;
 
     unsigned int command_start:1;
+    unsigned int chk_omitted_arg:1;
     unsigned int eofp: 1;
     unsigned int ruby__end__seen: 1;
     unsigned int debug: 1;
@@ -8545,6 +8546,14 @@ parse_atmark(struct parser_params *p, const enum lex_state_e last_state)
     return result;
 }
 
+static int
+dot_ahead(struct parser_params *p)
+{
+    int spaces = 0;
+    while (ISSPACE(peekc_n(p,spaces))) spaces++;
+    return peek_n(p, '.', spaces) ? TRUE : FALSE;
+}
+
 static enum yytokentype
 parse_ident(struct parser_params *p, int c, int cmd_state)
 {
@@ -8599,9 +8608,11 @@ parse_ident(struct parser_params *p, int c, int cmd_state)
 	    if (kw->id[0] == keyword_do) {
 		if (lambda_beginning_p()) {
 		    p->lex.lpar_beg = -1; /* make lambda_beginning_p() == FALSE in the body of "-> do ... end" */
+                    p->chk_omitted_arg = dot_ahead(p);
 		    return keyword_do_LAMBDA;
 		}
 		if (COND_P()) return keyword_do_cond;
+                p->chk_omitted_arg = dot_ahead(p);
 		if (CMDARG_P() && !IS_lex_state_for(state, EXPR_CMDARG))
 		    return keyword_do_block;
 		return keyword_do;
@@ -9075,6 +9086,8 @@ parser_yylex(struct parser_params *p)
 
       case '.': {
         int is_beg = IS_BEG();
+        int chk_omitted_arg = p->chk_omitted_arg;
+        p->chk_omitted_arg = FALSE;
 	SET_LEX_STATE(EXPR_BEG);
 	switch (c = nextc(p)) {
 	  case '.':
@@ -9115,7 +9128,7 @@ parser_yylex(struct parser_params *p)
 	    p->lex.ptok = p->lex.pcur;
 	    goto retry;
 	}
-	if (is_beg) {
+	if (is_beg && chk_omitted_arg) {
 	    if (parser_numbered_param(p, 0)) {
 		pushback(p, '.');
 		return tNUMPARAM;
@@ -9287,15 +9300,20 @@ parser_yylex(struct parser_params *p)
 	    COND_PUSH(0);
 	    CMDARG_PUSH(0);
 	    p->lex.paren_nest++;
+            p->chk_omitted_arg = dot_ahead(p);
 	    return tLAMBEG;
 	}
 	p->lex.paren_nest++;
 	if (IS_lex_state(EXPR_LABELED))
 	    c = tLBRACE;      /* hash */
-	else if (IS_lex_state(EXPR_ARG_ANY | EXPR_END | EXPR_ENDFN))
+	else if (IS_lex_state(EXPR_ARG_ANY | EXPR_END | EXPR_ENDFN)) {
 	    c = '{';          /* block (primary) */
-	else if (IS_lex_state(EXPR_ENDARG))
+            p->chk_omitted_arg = dot_ahead(p);
+        }
+	else if (IS_lex_state(EXPR_ENDARG)) {
 	    c = tLBRACE_ARG;  /* block (expr) */
+            p->chk_omitted_arg = dot_ahead(p);
+        }
 	else
 	    c = tLBRACE;      /* hash */
 	COND_PUSH(0);