[Clang] Avoid null deref in lambda attribute compat warning

clang/docs/ReleaseNotes.rst

@@ -432,6 +432,8 @@ Bug Fixes to C++ Support
 - Fix an assertion failure when taking the address on a non-type template parameter argument of
   object type. (#GH151531)
 - Suppress ``-Wdouble-promotion`` when explicitly asked for with C++ list initialization (#GH33409).
+- Fixed a crash in the pre-C++23 warning for attributes before a lambda
+  declarator by unifying format args to ``%0`` as attribute name, ``%1`` as selector (#GH161070).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1147,7 +1147,7 @@ def ext_lambda_missing_parens : ExtWarn<
   "lambda without a parameter clause is a C++23 extension">,
   InGroup<CXX23>;
 def warn_cxx20_compat_decl_attrs_on_lambda : Warning<
-  "%select{an attribute specifier sequence|%1}0 in this position "
+  "%select{an attribute specifier sequence|%0}1 in this position "
   "is incompatible with C++ standards before C++23">,
   InGroup<CXXPre23Compat>, DefaultIgnore;
 

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}}
+}