Fix #if-skipping to *really* skip the skipped group.

Previously we were avoiding printing within a skipped group, but we
were still evluating directives such as #define and #undef and still
emitting diagnostics for things such as macro calls with the wrong
number of arguments.

Add a test for this and fix it with a high-priority rule in the lexer
that consumes the skipped content.

glcpp-lex.l

@@ -47,6 +47,45 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 
 %%
 
+{HASH}if/.*\n {
+	yyextra->lexing_if = 1;
+	yyextra->space_tokens = 0;
+	return HASH_IF;
+}
+
+{HASH}elif/.*\n {
+	yyextra->lexing_if = 1;
+	yyextra->space_tokens = 0;
+	return HASH_ELIF;
+}
+
+{HASH}else/.*\n {
+	yyextra->space_tokens = 0;
+	return HASH_ELSE;
+}
+
+{HASH}endif/.*\n {
+	yyextra->space_tokens = 0;
+	return HASH_ENDIF;
+}
+
+	/* When skipping (due to an #if 0 or similar) consume anything
+	 * up to a newline. We do this less priroty than any
+	 * #if-related directive (#if, #elif, #else, #endif), but with
+	 * more priority than any other directive or token to avoid
+	 * any side-effects from skipped content.
+	 *
+	 * We use the lexing_if flag to avoid skipping any part of an
+	 * if conditional expression. */
+[^\n]+/\n {
+	if (yyextra->lexing_if ||
+	    yyextra->skip_stack == NULL ||
+	    yyextra->skip_stack->type == SKIP_NO_SKIP)
+	{
+		REJECT;
+	}
+}
+
 {HASH}define{HSPACE}+/{IDENTIFIER}"(" {
 	yyextra->space_tokens = 0;
 	return HASH_DEFINE_FUNC;
@@ -62,26 +101,6 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 	return HASH_UNDEF;
 }
 
-{HASH}if {
-	yyextra->space_tokens = 0;
-	return HASH_IF;
-}
-
-{HASH}elif {
-	yyextra->space_tokens = 0;
-	return HASH_ELIF;
-}
-
-{HASH}else {
-	yyextra->space_tokens = 0;
-	return HASH_ELSE;
-}
-
-{HASH}endif {
-	yyextra->space_tokens = 0;
-	return HASH_ENDIF;
-}
-
 {HASH} {
 	yyextra->space_tokens = 0;
 	return HASH;
@@ -163,6 +182,7 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 }
 
 \n {
+	yyextra->lexing_if = 0;
 	return NEWLINE;
 }
 

glcpp-parse.y

@@ -159,19 +159,11 @@ input:
 
 line:
 	control_line {
-		if (parser->skip_stack == NULL ||
-		    parser->skip_stack->type == SKIP_NO_SKIP)
-		{
-			printf ("\n");
-		}
+		printf ("\n");
 	}
 |	text_line {
-		if (parser->skip_stack == NULL ||
-		    parser->skip_stack->type == SKIP_NO_SKIP)
-		{
-			_glcpp_parser_print_expanded_token_list (parser, $1);
-			printf ("\n");
-		}
+		_glcpp_parser_print_expanded_token_list (parser, $1);
+		printf ("\n");
 		talloc_free ($1);
 	}
 |	expanded_line
@@ -889,6 +881,7 @@ glcpp_parser_create (void)
 	parser->defines = hash_table_ctor (32, hash_table_string_hash,
 					   hash_table_string_compare);
 	parser->active = _string_list_create (parser);
+	parser->lexing_if = 0;
 	parser->space_tokens = 1;
 	parser->newline_as_space = 0;
 	parser->in_control_line = 0;

glcpp.h

@@ -127,6 +127,7 @@ struct glcpp_parser {
 	yyscan_t scanner;
 	struct hash_table *defines;
 	string_list_t *active;
+	int lexing_if;
 	int space_tokens;
 	int newline_as_space;
 	int in_control_line;

tests/062-if-0-skips-garbage.c

@@ -0,0 +1,5 @@
+#define foo(a,b)
+#if 0
+foo(bar)
+foo(
+#endif