Support dereferencing function pointer

src/qbe_ir_generator.cpp

@@ -770,6 +770,15 @@ void QbeIrGenerator::Visit(const UnaryExprNode& unary_expr) {
       num_recorder.Record(res_num);
     } break;
     case UnaryOperator::kDeref: {
+      // Is function pointer.
+      if (unary_expr.operand->type->IsPtr() &&
+          dynamic_cast<PtrType*>((unary_expr.operand->type).get())
+              ->base_type()
+              .IsFunc()) {
+        // No-op; the function itself also evaluates to the address.
+        break;
+      }
+
       // Lhs can use res_num to map to the address, which reg_num currently
       // holds.
       const int reg_num = num_recorder.NumOfPrevExpr();

src/type_checker.cpp

@@ -330,11 +330,11 @@ void TypeChecker::Visit(FuncCallExprNode& call_expr) {
   if (call_expr.func_expr->type->IsFunc()) {
     func_type = std::dynamic_pointer_cast<FuncType>(
         std::shared_ptr<Type>{call_expr.func_expr->type->Clone()});
-    // In case that the expression is a function type, it should be an
-    // identifier expression of a declared identifier (function).
     const auto* id_expr =
         dynamic_cast<IdExprNode*>((call_expr.func_expr).get());
-    if (!id_expr || !env_.LookUp(id_expr->id)) {
+    // If is an identifier, either a function or a function pointer, it should
+    // be declared.
+    if (id_expr && !env_.LookUp(id_expr->id)) {
       // TODO: use of undeclared identifier 'id'
       assert(false);
     }

test/codegen/func_pointer.c

@@ -10,7 +10,7 @@ int main() {
   // Taking address of a function has the same effect as using the function
   // name.
   p = &add;
-  int d = p(1, 2);
+  int d = (*p)(1, 2);
   __builtin_print(d);
   return 0;
 }

test/typecheck/func_pointer.c

@@ -7,6 +7,6 @@ int main() {
   p = add;
   int c = p(2, 3);
   p = &add;
-  int d = p(1, 2);
+  int d = (*p)(1, 2);
   return 0;
 }

test/typecheck/func_pointer.exp

@@ -28,10 +28,11 @@ ProgramNode <1:1>
             IdExprNode <9:8> add: int (int, int)
       DeclVarNode <10:7> d: int
         FuncCallExprNode <10:11> int
-          IdExprNode <10:11> p: int (*)(int, int)
-          ArgExprNode <10:13> int
-            IntConstExprNode <10:13> 1: int
+          UnaryExprNode <10:12> int (int, int) *
+            IdExprNode <10:13> p: int (*)(int, int)
           ArgExprNode <10:16> int
-            IntConstExprNode <10:16> 2: int
+            IntConstExprNode <10:16> 1: int
+          ArgExprNode <10:19> int
+            IntConstExprNode <10:19> 2: int
       ReturnStmtNode <11:3>
         IntConstExprNode <11:10> 0: int