[clang-tools-extra] [clangd] Show definition of underlying struct when hovering over a typedef (PR #89570)

[Younan Zhang via cfe-commits]

================
@@ -136,6 +136,41 @@ std::string getNamespaceScope(const Decl *D) {
   return "";
 }
 
+void printDeclAndWrappers(const TypedefNameDecl *TND,
+                          llvm::raw_string_ostream &OS, PrintingPolicy PP) {
+  TND->print(OS, PP);
+  const Decl *LastPrintedDecl = TND;
+
+  auto PrintDeclForType = [&](QualType T) {
+    Decl *D = nullptr;
+    if (const auto *TT = dyn_cast<TagType>(T.getTypePtr())) {
+      D = TT->getDecl();
+    } else if (const auto *TT = dyn_cast<TypedefType>(T.getTypePtr())) {
+      D = TT->getDecl();
+    }
+    if (D == LastPrintedDecl) {
+      return false;
+    }
+    if (D) {
+      OS << ";\n";
+      D->print(OS, PP);
+      LastPrintedDecl = D;
+    }
+    // In case of D == nullptr, return true. We might have a layer of type
+    // sugar like ElaboratedType that doesn't itself have a distinct Decl,
+    // but a subsequent layer of type sugar might.
+    return true;
+  };
+
+  QualType Type = TND->getUnderlyingType();
+  while (PrintDeclForType(Type)) {
+    QualType Desugared = Type->getLocallyUnqualifiedSingleStepDesugaredType();
----------------
zyn0217 wrote:

My thought: what do you think of resolving pointer types as well? I could envision cases where pointers are involved in an alias type e.g.
```cpp
typedef struct _PROCESS_INFORMATION {
  HANDLE hProcess;
  HANDLE hThread;
  DWORD  dwProcessId;
  DWORD  dwThreadId;
} PROCESS_INFORMATION, *PPROCESS_INFORMATION, *LPPROCESS_INFORMATION;
```
(excerpted from [`PROCESS_INFORMATION`](https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/ns-processthreadsapi-process_information))
People may want to peek at the definition of the struct while hovering over `LPPROCESS_INFORMATION` too.

https://github.com/llvm/llvm-project/pull/89570