avoid incomplete type in a vector (#5730)

Swap the order of struct declarations to avoid using an incomplete type in a vector.

In C++20, using std::vector with an incomplete type often becomes a build failure due to increased usage of constexpr for vector members.

Author: walkingeyerobot

src/analysis/cfg.h

@@ -30,23 +30,7 @@
 
 namespace wasm::analysis {
 
-struct BasicBlock;
-
-struct CFG {
-  // Iterate through basic blocks.
-  using iterator = std::vector<BasicBlock>::const_iterator;
-  iterator begin() const { return blocks.cbegin(); }
-  iterator end() const { return blocks.cend(); }
-  size_t size() const { return blocks.size(); }
-
-  static CFG fromFunction(Function* func);
-
-  void print(std::ostream& os, Module* wasm = nullptr) const;
-
-private:
-  std::vector<BasicBlock> blocks;
-  friend BasicBlock;
-};
+struct CFG;
 
 struct BasicBlock {
   using iterator = std::vector<Expression*>::const_iterator;
@@ -72,6 +56,22 @@ struct BasicBlock {
   friend CFG;
 };
 
+struct CFG {
+  // Iterate through basic blocks.
+  using iterator = std::vector<BasicBlock>::const_iterator;
+  iterator begin() const { return blocks.cbegin(); }
+  iterator end() const { return blocks.cend(); }
+  size_t size() const { return blocks.size(); }
+
+  static CFG fromFunction(Function* func);
+
+  void print(std::ostream& os, Module* wasm = nullptr) const;
+
+private:
+  std::vector<BasicBlock> blocks;
+  friend BasicBlock;
+};
+
 } // namespace wasm::analysis
 
 #include "cfg-impl.h"