[coroutines] Do not check coroutine wrappers for skipped function bodies (#76729)

Without function bodies, we cannot tell whether a function is a
coroutine or not.
The analysis of coroutine wrappers is not useful when this information
is not available.

We therefore now skip this analysis for skipped function bodies.

Author: usx95

clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp

@@ -420,6 +420,62 @@ TEST(DiagnosticTest, MakeUnique) {
                        "no matching constructor for initialization of 'S'")));
 }
 
+TEST(DiagnosticTest, CoroutineInHeader) {
+  StringRef CoroutineH = R"cpp(
+namespace std {
+template <class Ret, typename... T>
+struct coroutine_traits { using promise_type = typename Ret::promise_type; };
+
+template <class Promise = void>
+struct coroutine_handle {
+  static coroutine_handle from_address(void *) noexcept;
+  static coroutine_handle from_promise(Promise &promise);
+  constexpr void* address() const noexcept;
+};
+template <>
+struct coroutine_handle<void> {
+  template <class PromiseType>
+  coroutine_handle(coroutine_handle<PromiseType>) noexcept;
+  static coroutine_handle from_address(void *);
+  constexpr void* address() const noexcept;
+};
+
+struct awaitable {
+  bool await_ready() noexcept { return false; }
+  void await_suspend(coroutine_handle<>) noexcept {}
+  void await_resume() noexcept {}
+};
+} // namespace std
+  )cpp";
+
+  StringRef Header = R"cpp(
+#include "coroutine.h"
+template <typename T> struct [[clang::coro_return_type]] Gen {
+  struct promise_type {
+    Gen<T> get_return_object() {
+      return {};
+    }
+    std::awaitable  initial_suspend();
+    std::awaitable  final_suspend() noexcept;
+    void unhandled_exception();
+    void return_value(T t);
+  };
+};
+
+Gen<int> foo_coro(int b) { co_return b; }
+  )cpp";
+  Annotations Main(R"cpp(
+// error-ok
+#include "header.hpp"
+Gen<int> $[[bar_coro]](int b) { return foo_coro(b); }
+  )cpp");
+  TestTU TU = TestTU::withCode(Main.code());
+  TU.AdditionalFiles["coroutine.h"] = std::string(CoroutineH);
+  TU.AdditionalFiles["header.hpp"] = std::string(Header);
+  TU.ExtraArgs.push_back("--std=c++20");
+  EXPECT_THAT(TU.build().getDiagnostics(), ElementsAre(hasRange(Main.range())));
+}
+
 TEST(DiagnosticTest, MakeShared) {
   // We usually miss diagnostics from header functions as we don't parse them.
   // std::make_shared is only parsed when --parse-forwarding-functions is set

clang/lib/Sema/SemaDecl.cpp

@@ -15845,8 +15845,6 @@ static void diagnoseImplicitlyRetainedSelf(Sema &S) {
 }
 
 void Sema::CheckCoroutineWrapper(FunctionDecl *FD) {
-  if (!FD)
-    return;
   RecordDecl *RD = FD->getReturnType()->getAsRecordDecl();
   if (!RD || !RD->getUnderlyingDecl()->hasAttr<CoroReturnTypeAttr>())
     return;
@@ -15869,7 +15867,8 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
   sema::AnalysisBasedWarnings::Policy WP = AnalysisWarnings.getDefaultPolicy();
   sema::AnalysisBasedWarnings::Policy *ActivePolicy = nullptr;
 
-  if (getLangOpts().Coroutines) {
+  // If we skip function body, we can't tell if a function is a coroutine.
+  if (getLangOpts().Coroutines && FD && !FD->hasSkippedBody()) {
     if (FSI->isCoroutine())
       CheckCompletedCoroutineBody(FD, Body);
     else