[Clang] Avoid null deref in lambda attribute compat warning (#161096)

Fixes #161070

---

This PR addresses the issue in `ext_decl_attrs_on_lambda` by using
`%0`=_attribute name_ and `%1`=_selector_, which prevents a null
`IdentifierInfo*`.


https://github.com/llvm/llvm-project/blob/48a6f2f85c8269d8326c185016801a4eb8d5dfd6/clang/lib/Parse/ParseExprCXX.cpp#L1299-L1302


https://github.com/llvm/llvm-project/blob/48a6f2f85c8269d8326c185016801a4eb8d5dfd6/clang/include/clang/Basic/DiagnosticParseKinds.td#L1143-L1145


https://github.com/llvm/llvm-project/blob/48a6f2f85c8269d8326c185016801a4eb8d5dfd6/clang/include/clang/Basic/DiagnosticParseKinds.td#L1149-L1152

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -434,6 +434,7 @@ Bug Fixes to C++ Support
 - Suppress ``-Wdouble-promotion`` when explicitly asked for with C++ list initialization (#GH33409).
 - Fix the result of `__builtin_is_implicit_lifetime` for types with a user-provided constructor. (#GH160610)
 - Correctly deduce return types in ``decltype`` expressions. (#GH160497) (#GH56652) (#GH116319) (#GH161196)
+- Fixed a crash in the pre-C++23 warning for attributes before a lambda declarator (#GH161070).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1141,7 +1141,7 @@ def warn_cxx23_compat_binding_pack : Warning<
 def err_capture_default_first : Error<
   "capture default must be first">;
 def ext_decl_attrs_on_lambda : ExtWarn<
-  "%select{an attribute specifier sequence|%0}1 in this position "
+  "%select{an attribute specifier sequence|%1}0 in this position "
   "is a C++23 extension">, InGroup<CXX23AttrsOnLambda>;
 def ext_lambda_missing_parens : ExtWarn<
   "lambda without a parameter clause is a C++23 extension">,

clang/lib/Parse/ParseExprCXX.cpp

@@ -1299,7 +1299,7 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
     Diag(Tok, getLangOpts().CPlusPlus23
                   ? diag::warn_cxx20_compat_decl_attrs_on_lambda
                   : diag::ext_decl_attrs_on_lambda)
-        << Tok.getIdentifierInfo() << Tok.isRegularKeywordAttribute();
+        << Tok.isRegularKeywordAttribute() << Tok.getIdentifierInfo();
     MaybeParseCXX11Attributes(D);
   }
 

clang/test/Parser/cxx2b-lambdas-ext-warns.cpp

@@ -1,9 +1,7 @@
-// RUN: %clang_cc1 -std=c++20 %s -verify=cxx20
-// RUN: %clang_cc1 -std=c++23 %s -verify=cxx23
-// RUN: %clang_cc1 -std=c++23 -Wpre-c++23-compat %s -verify=precxx23
-// RUN: %clang_cc1 -std=c++23 -pedantic %s -verify=cxx23
-
-//cxx23-no-diagnostics
+// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -target-feature +sme -std=c++20 %s -verify=cxx20
+// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -target-feature +sme -std=c++23 %s -verify=cxx23
+// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -target-feature +sme -std=c++23 -Wpre-c++23-compat %s -verify=precxx23
+// RUN: %clang_cc1 -triple aarch64-unknown-linux-gnu -target-feature +sme -std=c++23 -pedantic %s -verify=cxx23
 
 auto L1 = [] constexpr {};
 // cxx20-warning@-1 {{lambda without a parameter clause is a C++23 extension}}
@@ -14,3 +12,25 @@ auto L3 = [] static {};
 // cxx20-warning@-1 {{lambda without a parameter clause is a C++23 extension}}
 // cxx20-warning@-2 {{static lambdas are a C++23 extension}}
 // precxx23-warning@-3 {{static lambdas are incompatible with C++ standards before C++23}}
+
+namespace GH161070 {
+void t1() { int a = [] __arm_streaming; }
+// precxx23-error@-1 {{'__arm_streaming' cannot be applied to a declaration}}
+// precxx23-error@-2 {{expected body of lambda expression}}
+// cxx23-error@-3 {{'__arm_streaming' cannot be applied to a declaration}}
+// cxx23-error@-4 {{expected body of lambda expression}}
+// cxx20-error@-5 {{'__arm_streaming' cannot be applied to a declaration}}
+// cxx20-error@-6 {{expected body of lambda expression}}
+// cxx20-warning@-7 {{'__arm_streaming' in this position is a C++23 extension}}
+// precxx23-warning@-8 {{'__arm_streaming' in this position is incompatible with C++ standards before C++23}}
+
+void t2() { int a = [] [[assume(true)]]; }
+// precxx23-error@-1 {{'assume' attribute cannot be applied to a declaration}}
+// precxx23-error@-2 {{expected body of lambda expression}}
+// cxx23-error@-3 {{'assume' attribute cannot be applied to a declaration}}
+// cxx23-error@-4 {{expected body of lambda expression}}
+// cxx20-error@-5 {{'assume' attribute cannot be applied to a declaration}}
+// cxx20-error@-6 {{expected body of lambda expression}}
+// cxx20-warning@-7 {{an attribute specifier sequence in this position is a C++23 extension}}
+// precxx23-warning@-8 {{an attribute specifier sequence in this position is incompatible with C++ standards before C++23}}
+}