Account for parse stack merges when shifting

Previously, when the parse stack was split into 3 or more heads, it was
possible for head 3 to be accidentally skipped if head 2 merged with head 1.

spec/fixtures/corpus/c/declarations.txt

@@ -76,3 +76,43 @@ int main() {
     (expression_statement (call_expression
       (identifier)
       (math_expression (identifier) (identifier)))))))
+
+==========================================
+function-like macros that produce types
+==========================================
+
+GIT_INLINE(int) x = 5;
+
+---
+
+(translation_unit (declaration
+  (macro_type (identifier) (identifier))
+  (identifier)
+  (initializer (number))))
+
+============================================
+3-way ambiguities (regression)
+============================================
+
+int main() {
+  /*
+   * Could be either:
+   *   - function call
+   *   - declaration w/ parenthesized declarator
+   *   - declaration w/ macro type, no declarator
+   */
+  ABC(d);
+
+  efg hij;
+}
+
+---
+
+(translation_unit
+  (function_definition
+    (identifier)
+    (function_declarator (identifier))
+    (compound_statement
+      (comment)
+      (expression_statement (call_expression (identifier) (identifier)))
+      (declaration (identifier) (identifier)))))

spec/fixtures/corpus/cpp/statements.txt

@@ -36,7 +36,7 @@ template function / relational expression ambiguities
 
 int main() {
   someVariable < someValue > 0.0;
-  someTemplate<SomeType>(y);
+  someTemplate<SomeType>(0, 0);
 }
 
 ---
@@ -52,7 +52,7 @@ int main() {
     (expression_statement
       (call_expression
         (template_call (identifier) (type_id (type_specifier (identifier))))
-        (identifier)))))))
+        (number) (number)))))))
 
 ==================================================
 template class / relational expression ambiguities

src/runtime/parser.c

@@ -20,6 +20,12 @@
 
 #define SYM_NAME(sym) self->language->symbol_names[sym]
 
+typedef enum {
+  ConsumeResultShifted,
+  ConsumeResultRemoved,
+  ConsumeResultFinished
+} ConsumeResult;
+
 /*
  *  Private
  */
@@ -139,13 +145,16 @@ static void ts_parser__get_next_lookahead(TSParser *self) {
  *  Parse Actions
  */
 
-static void ts_parser__shift(TSParser *self, int head, TSStateId parse_state) {
-  ts_stack_push(self->stack, head, parse_state, self->lookahead);
+static ConsumeResult ts_parser__shift(TSParser *self, int head, TSStateId parse_state) {
+  if (ts_stack_push(self->stack, head, parse_state, self->lookahead))
+    return ConsumeResultRemoved;
+  else
+    return ConsumeResultShifted;
 }
 
-static void ts_parser__shift_extra(TSParser *self, int head, TSStateId state) {
+static bool ts_parser__shift_extra(TSParser *self, int head, TSStateId state) {
   ts_tree_set_extra(self->lookahead);
-  ts_parser__shift(self, head, state);
+  return ts_parser__shift(self, head, state);
 }
 
 static TSTree *ts_parser__reduce(TSParser *self, int head, TSSymbol symbol,
@@ -288,12 +297,6 @@ static TSTree *ts_parser__finish(TSParser *self) {
   return root;
 }
 
-typedef enum {
-  ConsumeResultShifted,
-  ConsumeResultRemoved,
-  ConsumeResultFinished
-} ConsumeResult;
-
 /*
  * Continue performing parse actions for the given head until the current
  * lookahead symbol is consumed.
@@ -345,13 +348,11 @@ static ConsumeResult ts_parser__consume_lookahead(TSParser *self, int head) {
 
         case TSParseActionTypeShift:
           DEBUG("shift state:%u", action.data.to_state);
-          ts_parser__shift(self, current_head, action.data.to_state);
-          return ConsumeResultShifted;
+          return ts_parser__shift(self, current_head, action.data.to_state);
 
         case TSParseActionTypeShiftExtra:
           DEBUG("shift_extra");
-          ts_parser__shift_extra(self, current_head, state);
-          return ConsumeResultShifted;
+          return ts_parser__shift_extra(self, current_head, state);
 
         case TSParseActionTypeReduce:
           DEBUG("reduce sym:%s, child_count:%u", SYM_NAME(action.data.symbol),