[clang] [Clang][Sema] Warn when 'exclude_from_explicit_instantiation' attribute is used on local classes and members thereof (PR #88777)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Krystian Stasiowski (sdkrystian)

<details>
<summary>Changes</summary>

A local class and its members declared in a function template are instantiated alongside the definition of that template. It therefore does not make sense to apply the `exclude_from_explicit_instantiation` attribute to such declarations, and this patch adds a warning to diagnose these cases (in addition to ignoring the attribute). 

(The motivation for this patch is to fix a failing test in libc++ for #<!-- -->84050. In particular, [this line](https://github.com/llvm/llvm-project/blob/main/libcxx/include/__memory/uses_allocator_construction.h#L199) in libc++ contains the expression `this->__value_`  (reduced repro [here](https://godbolt.org/z/KqEerKWPd)) which will be looked up prior to instantiation (the lookup context is the current instantiation) once #<!-- -->84050 lands. `_LIBCPP_HIDE_FROM_ABI` includes `__attribute__((exclude_from_explicit_instantiation))`, which TLDR in the reduced repro results in `Local` being instantiated with `Local::operator A` as its `DeclContext`)


---
Full diff: https://github.com/llvm/llvm-project/pull/88777.diff


3 Files Affected:

- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+3) 
- (modified) clang/lib/Sema/SemaDeclAttr.cpp (+18) 
- (added) clang/test/SemaCXX/attr-exclude_from_explicit_instantiation.local-class.cpp (+63) 


``````````diff
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 5ec0218aedfe86..5385c634ca7c45 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3669,6 +3669,9 @@ def warn_attribute_dllexport_explicit_instantiation_decl : Warning<
 def warn_attribute_dllexport_explicit_instantiation_def : Warning<
   "'dllexport' attribute ignored on explicit instantiation definition">,
   InGroup<IgnoredAttributes>;
+def warn_attribute_exclude_from_explicit_instantiation_local_class : Warning<
+  "%0 attribute ignored on local class%select{| member}1">,
+  InGroup<IgnoredAttributes>;
 def warn_invalid_initializer_from_system_header : Warning<
   "invalid constructor from class in system header, should not be explicit">,
   InGroup<DiagGroup<"invalid-initializer-from-system-header">>;
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index b7b1fbc625a150..dec9fc4cea234d 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -983,6 +983,21 @@ static void handleErrorAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
     D->addAttr(EA);
 }
 
+static void handleExcludeFromExplicitInstantiationAttr(Sema &S, Decl *D,
+                                                       const ParsedAttr &AL) {
+  const auto *PD = isa<CXXRecordDecl>(D)
+                       ? cast<DeclContext>(D)
+                       : D->getDeclContext()->getRedeclContext();
+  if (const auto *RD = dyn_cast<CXXRecordDecl>(PD); RD && RD->isLocalClass()) {
+    S.Diag(AL.getLoc(),
+           diag::warn_attribute_exclude_from_explicit_instantiation_local_class)
+        << AL << /*IsMember=*/!isa<CXXRecordDecl>(D);
+    return;
+  }
+  D->addAttr(::new (S.Context)
+                 ExcludeFromExplicitInstantiationAttr(S.Context, AL));
+}
+
 namespace {
 /// Determines if a given Expr references any of the given function's
 /// ParmVarDecls, or the function's implicit `this` parameter (if applicable).
@@ -9315,6 +9330,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_Error:
     handleErrorAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_ExcludeFromExplicitInstantiation:
+    handleExcludeFromExplicitInstantiationAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_DiagnoseIf:
     handleDiagnoseIfAttr(S, D, AL);
     break;
diff --git a/clang/test/SemaCXX/attr-exclude_from_explicit_instantiation.local-class.cpp b/clang/test/SemaCXX/attr-exclude_from_explicit_instantiation.local-class.cpp
new file mode 100644
index 00000000000000..bac11d9cffbc23
--- /dev/null
+++ b/clang/test/SemaCXX/attr-exclude_from_explicit_instantiation.local-class.cpp
@@ -0,0 +1,63 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Test that the exclude_from_explicit_instantiation attribute is ignored
+// for local classes and members thereof.
+
+#define EXCLUDE_FROM_EXPLICIT_INSTANTIATION __attribute__((exclude_from_explicit_instantiation)) // expected-note 0+{{expanded from macro}}
+
+namespace N0 {
+
+template<typename T>
+void f() {
+  struct EXCLUDE_FROM_EXPLICIT_INSTANTIATION A { // expected-warning {{attribute ignored on local class}}
+                                                 // expected-note at -1 2{{in instantiation of}}
+    EXCLUDE_FROM_EXPLICIT_INSTANTIATION void g(T t) { // expected-warning {{attribute ignored on local class member}}
+      *t; // expected-error {{indirection requires pointer operand ('int' invalid)}}
+    }
+
+    struct EXCLUDE_FROM_EXPLICIT_INSTANTIATION B { // expected-warning {{attribute ignored on local class}}
+      void h(T t) {
+        *t; // expected-error {{indirection requires pointer operand ('int' invalid)}}
+      }
+    };
+  };
+}
+
+template void f<int>(); // expected-note 2{{in instantiation of}}
+
+}
+
+// This is a reduced example from libc++ which required that 'value'
+// be prefixed with 'this->' because the definition of 'Local::operator A'
+// was not instantiated when the definition of 'g' was.
+namespace N1 {
+
+  struct A { };
+
+  struct B {
+    operator A() {
+      return A();
+    }
+  };
+
+  auto f(auto&& x) {
+    return A(x);
+  }
+
+  template<typename T>
+  auto g(T x) {
+    struct Local {
+      T value;
+
+      EXCLUDE_FROM_EXPLICIT_INSTANTIATION // expected-warning {{attribute ignored on local class member}}
+      operator A() {
+        return A(value);
+      }
+    };
+
+    return f(Local(x));
+  }
+
+  auto x = g(B());
+
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/88777