[clang] [Clang][Diagnostics] Mention 'import std' in typeid diagnostic (PR #173236)

[via cfe-commits]

https://github.com/AltriaSuki updated https://github.com/llvm/llvm-project/pull/173236

>From e3b21647a8f9facebe13ecd4236cd3f7a72f2b3c Mon Sep 17 00:00:00 2001
From: feilun <feilun at feilundeMacBook-Pro.local>
Date: Mon, 22 Dec 2025 18:04:49 +0800
Subject: [PATCH 1/2] [Clang][Diagnostics] Mention 'import std' in typeid
 diagnostic

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 2 +-
 clang/test/SemaCXX/typeid-requires-typeinfo.cpp  | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/typeid-requires-typeinfo.cpp

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 49eee0c2fa617..a8bff4d789b38 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8429,7 +8429,7 @@ def err_bad_dynamic_cast_not_polymorphic : Error<"%0 is not polymorphic">;
 
 // Other C++ expressions
 def err_need_header_before_typeid : Error<
-  "you need to include <typeinfo> before using the 'typeid' operator">;
+  "you need to include <typeinfo> or import std before using the 'typeid' operator">;
 def err_need_header_before_placement_new : Error<
   "no matching %0 function for non-allocating placement new expression; "
   "include <new>">;
diff --git a/clang/test/SemaCXX/typeid-requires-typeinfo.cpp b/clang/test/SemaCXX/typeid-requires-typeinfo.cpp
new file mode 100644
index 0000000000000..ec8773b438c28
--- /dev/null
+++ b/clang/test/SemaCXX/typeid-requires-typeinfo.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -fsyntax-only -verify -std=c++17 %s
+
+class A{};
+
+auto f() {
+  return typeid(A);
+}
+
+// CHECK: error: you need to include <typeinfo> or import std before using the 'typeid' operator
\ No newline at end of file

>From e34e4c6de922f6289487d890a11a143086ccaa52 Mon Sep 17 00:00:00 2001
From: feilun <feilun at feilundeMacBook-Pro.local>
Date: Thu, 25 Dec 2025 10:35:06 +0800
Subject: [PATCH 2/2]   [Clang][Diagnostics] Suggest 'import std' in typeid
 diagnostic for C++20+

  In C++20 and later, when typeid is used without including <typeinfo>,
  the diagnostic now suggests either including <typeinfo> or importing std,
  since modules are available in C++20.

  For C++17 and earlier versions, the diagnostic continues to only suggest
  including <typeinfo>, as modules are not available in those standards.

  Close #172966
---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 4 +++-
 clang/lib/Sema/SemaExprCXX.cpp                   | 8 ++++++--
 clang/test/SemaCXX/typeid-requires-typeinfo.cpp  | 4 +++-
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index eb6d0d211524c..a4dbb07ec2de5 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8428,8 +8428,10 @@ def err_bad_dynamic_cast_not_ptr : Error<"cannot use dynamic_cast to convert fro
 def err_bad_dynamic_cast_not_polymorphic : Error<"%0 is not polymorphic">;
 
 // Other C++ expressions
-def err_need_header_before_typeid : Error<
+def err_need_header_or_std_before_typeid : Error<
   "you need to include <typeinfo> or import std before using the 'typeid' operator">;
+def err_need_header_before_typeid : Error<
+  "you need to include <typeinfo> before using the 'typeid' operator">;
 def err_need_header_before_placement_new : Error<
   "no matching %0 function for non-allocating placement new expression; "
   "include <new>">;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index 965ad55465db7..977f3216b8520 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -644,8 +644,12 @@ Sema::ActOnCXXTypeid(SourceLocation OpLoc, SourceLocation LParenLoc,
   }
 
   // Find the std::type_info type.
-  if (!getStdNamespace())
-    return ExprError(Diag(OpLoc, diag::err_need_header_before_typeid));
+  if (!getStdNamespace()){
+    auto DiagID = getLangOpts().CPlusPlus20
+                      ? diag::err_need_header_or_std_before_typeid
+                      : diag::err_need_header_before_typeid;
+    return ExprError(Diag(OpLoc, DiagID));
+  }
 
   if (!CXXTypeInfoDecl) {
     IdentifierInfo *TypeInfoII = &PP.getIdentifierTable().get("type_info");
diff --git a/clang/test/SemaCXX/typeid-requires-typeinfo.cpp b/clang/test/SemaCXX/typeid-requires-typeinfo.cpp
index ec8773b438c28..dc20c6b3dcb14 100644
--- a/clang/test/SemaCXX/typeid-requires-typeinfo.cpp
+++ b/clang/test/SemaCXX/typeid-requires-typeinfo.cpp
@@ -1,3 +1,4 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -fsyntax-only -verify -std=c++20 %s
 // RUN: %clang_cc1 -triple %itanium_abi_triple -fsyntax-only -verify -std=c++17 %s
 
 class A{};
@@ -6,4 +7,5 @@ auto f() {
   return typeid(A);
 }
 
-// CHECK: error: you need to include <typeinfo> or import std before using the 'typeid' operator
\ No newline at end of file
+// cxx20-error at -2 {{you need to include <typeinfo> or import std before using the 'typeid' operator}}
+// cxx17-error at -2 {{you need to include <typeinfo> before using the 'typeid' operator}}