Disallow btf_type_tag in C++ mode (#107238)

This was always intended to be disallowed in C++ (see the definition in
Attr.td), but failed to add the correct checking code in SemaType.cpp to
ensure it was rejected.

Fixes #106864

clang/docs/ReleaseNotes.rst

@@ -225,6 +225,11 @@ Attribute Changes in Clang
   more cases where the returned reference outlives the object.
   (#GH100567)
 
+- Clang now correctly diagnoses the use of ``btf_type_tag`` in C++ and ignores
+  it; this attribute is a C-only attribute, and caused crashes with template
+  instantiation by accidentally allowing it in C++ in some circumstances.
+  (#GH106864)
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/lib/Sema/SemaType.cpp

@@ -6490,6 +6490,15 @@ static void HandleBTFTypeTagAttribute(QualType &Type, const ParsedAttr &Attr,
                                       TypeProcessingState &State) {
   Sema &S = State.getSema();
 
+  // This attribute is only supported in C.
+  // FIXME: we should implement checkCommonAttributeFeatures() in SemaAttr.cpp
+  // such that it handles type attributes, and then call that from
+  // processTypeAttrs() instead of one-off checks like this.
+  if (!Attr.diagnoseLangOpts(S)) {
+    Attr.setInvalid();
+    return;
+  }
+
   // Check the number of attribute arguments.
   if (Attr.getNumArgs() != 1) {
     S.Diag(Attr.getLoc(), diag::err_attribute_wrong_number_arguments)

clang/test/Sema/attr-btf_type_tag.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c -x c %s
+
+// c-no-diagnostics
+
+// Ensure that we diagnose the attribute as ignored in C++ but not in C.
+#ifdef __cplusplus
+static_assert(__builtin_is_implicit_lifetime(int __attribute__((btf_type_tag("user"))) *)); // expected-warning {{'btf_type_tag' attribute ignored}}
+#endif
+int __attribute__((btf_type_tag("user"))) *ptr; // expected-warning {{'btf_type_tag' attribute ignored}}
+

clang/test/SemaCXX/sugar-common-types.cpp

@@ -90,13 +90,6 @@ N t19 = 0 ? (__underlying_type(EnumsX::X)){} : (__underlying_type(EnumsY::Y)){};
 N t20 = 0 ? (__underlying_type(EnumsX::X)){} : (__underlying_type(EnumsY::X)){};
 // expected-error@-1 {{rvalue of type '__underlying_type(Enums::X)' (aka 'int')}}
 
-using SBTF1 = SS1 [[clang::btf_type_tag("1")]];
-using SBTF2 = ::SS1 [[clang::btf_type_tag("1")]];
-using SBTF3 = ::SS1 [[clang::btf_type_tag("2")]];
-
-N t21 = 0 ? (SBTF1){} : (SBTF3){}; // expected-error {{from 'SS1'}}
-N t22 = 0 ? (SBTF1){} : (SBTF2){}; // expected-error {{from 'SS1 __attribute__((btf_type_tag("1")))' (aka 'SS1')}}
-
 using QX = const SB1 *;
 using QY = const ::SB1 *;
 N t23 = 0 ? (QX){} : (QY){}; // expected-error {{rvalue of type 'const SB1 *' (aka 'const SS1 *')}}

clang/test/SemaCXX/type-traits.cpp

@@ -2052,7 +2052,6 @@ void is_implicit_lifetime(int n) {
   static_assert(__builtin_is_implicit_lifetime(float4));
   static_assert(__builtin_is_implicit_lifetime(align_value_int));
   static_assert(__builtin_is_implicit_lifetime(int[[clang::annotate_type("category2")]] *));
-  static_assert(__builtin_is_implicit_lifetime(int __attribute__((btf_type_tag("user"))) *));
   static_assert(__builtin_is_implicit_lifetime(EnforceReadOnlyPlacement));
   static_assert(__builtin_is_implicit_lifetime(int __attribute__((noderef)) *));
   static_assert(__builtin_is_implicit_lifetime(TypeVisibility));