https://github.com/AaronBallman updated
https://github.com/llvm/llvm-project/pull/137368
>From e8234c9f374783f3b3fde586464037f52eda2f77 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aa...@aaronballman.com>
Date: Fri, 25 Apr 2025 13:16:39 -0400
Subject: [PATCH 1/3] [C] Diagnose declarations hidden in C++
This introduces a new diagnostic, -Wc++-hidden-decl, which is grouped
under -Wc++-compat, that diagnoses declarations which are valid in C
but invalid in C++ due to the type being at the wrong scope. e.g.,
struct S {
struct T {
int x;
} t;
};
struct T t; // Valid C, invalid C++
This is implementing the other half of #21898
---
clang/docs/ReleaseNotes.rst | 12 ++++
clang/include/clang/AST/DeclBase.h | 6 +-
clang/include/clang/Basic/DiagnosticGroups.td | 5 +-
.../clang/Basic/DiagnosticSemaKinds.td | 4 ++
clang/include/clang/Sema/Sema.h | 1 +
clang/lib/AST/DeclBase.cpp | 11 +++
clang/lib/Sema/SemaDecl.cpp | 33 +++++++++
clang/test/Sema/decl-hidden-in-c++.c | 68 +++++++++++++++++++
8 files changed, 138 insertions(+), 2 deletions(-)
create mode 100644 clang/test/Sema/decl-hidden-in-c++.c
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6ecb97825ab8d..262c863c602d0 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -150,6 +150,18 @@ C Language Changes
- Added ``-Wimplicit-void-ptr-cast``, grouped under ``-Wc++-compat``, which
diagnoses implicit conversion from ``void *`` to another pointer type as
being incompatible with C++. (#GH17792)
+- Added ``-Wc++-hidden-decl``, grouped under ``-Wc++-compat``, which diagnoses
+ use of tag types which are visible in C but not visible in C++ due to scoping
+ rules. e.g.,
+
+ .. code-block:: c
+
+ struct S {
+ struct T {
+ int x;
+ } t;
+ };
+ struct T t; // Invalid C++, valid C, now diagnosed
C2y Feature Support
^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/AST/DeclBase.h
b/clang/include/clang/AST/DeclBase.h
index 2fb9d5888bce4..375e9e2592502 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -2239,10 +2239,14 @@ class DeclContext {
return DC && this->getPrimaryContext() == DC->getPrimaryContext();
}
- /// Determine whether this declaration context encloses the
+ /// Determine whether this declaration context semantically encloses the
/// declaration context DC.
bool Encloses(const DeclContext *DC) const;
+ /// Determine whether this declaration context lexically encloses the
+ /// declaration context DC.
+ bool LexicallyEncloses(const DeclContext *DC) const;
+
/// Find the nearest non-closure ancestor of this context,
/// i.e. the innermost semantic parent of this context which is not
/// a closure. A context may be its own non-closure ancestor.
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td
b/clang/include/clang/Basic/DiagnosticGroups.td
index 31e2cfa7ab485..47b24d5d6c112 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -154,10 +154,13 @@ def BuiltinRequiresHeader :
DiagGroup<"builtin-requires-header">;
def C99Compat : DiagGroup<"c99-compat">;
def C23Compat : DiagGroup<"c23-compat">;
def : DiagGroup<"c2x-compat", [C23Compat]>;
+def HiddenCppDecl : DiagGroup<"c++-hidden-decl">;
def DefaultConstInitUnsafe : DiagGroup<"default-const-init-unsafe">;
def DefaultConstInit : DiagGroup<"default-const-init",
[DefaultConstInitUnsafe]>;
def ImplicitVoidPtrCast : DiagGroup<"implicit-void-ptr-cast">;
-def CXXCompat: DiagGroup<"c++-compat", [ImplicitVoidPtrCast,
DefaultConstInit]>;
+def CXXCompat: DiagGroup<"c++-compat",
+ [ImplicitVoidPtrCast, DefaultConstInit,
+ HiddenCppDecl]>;
def ExternCCompat : DiagGroup<"extern-c-compat">;
def KeywordCompat : DiagGroup<"keyword-compat">;
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 7090cbe7acbe6..4be5654e6a63f 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -496,6 +496,10 @@ def warn_unused_lambda_capture: Warning<"lambda capture %0
is not "
"%select{used|required to be captured for this use}1">,
InGroup<UnusedLambdaCapture>, DefaultIgnore;
+def warn_decl_hidden_in_cpp : Warning<
+ "%select{struct|union|enum}0 defined within a struct or union is not visible
"
+ "in C++">, InGroup<HiddenCppDecl>, DefaultIgnore;
+
def warn_reserved_extern_symbol: Warning<
"identifier %0 is reserved because %select{"
"<ERROR>|" // ReservedIdentifierStatus::NotReserved
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 0c77c5b5ca30a..7bad162afa66f 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -3536,6 +3536,7 @@ class Sema final : public SemaBase {
}
void warnOnReservedIdentifier(const NamedDecl *D);
+ void warnOnCTypeHiddenInCPlusPlus(const NamedDecl *D);
Decl *ActOnDeclarator(Scope *S, Declarator &D);
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 1afda9aac9e18..d80bdeec092d0 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1428,6 +1428,17 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
return false;
}
+bool DeclContext::LexicallyEncloses(const DeclContext* DC) const {
+ if (getPrimaryContext() != this)
+ return getPrimaryContext()->LexicallyEncloses(DC);
+
+ for (; DC; DC = DC->getLexicalParent())
+ if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+ DC->getPrimaryContext() == this)
+ return true;
+ return false;
+}
+
DeclContext *DeclContext::getNonTransparentContext() {
DeclContext *DC = this;
while (DC->isTransparentContext()) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index fe61b92e087d7..a71a6906febf6 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6500,6 +6500,8 @@ NamedDecl *Sema::HandleDeclarator(Scope *S, Declarator &D,
if (!New)
return nullptr;
+ warnOnCTypeHiddenInCPlusPlus(New);
+
// If this has an identifier and is not a function template specialization,
// add it to the scope stack.
if (New->getDeclName() && AddToScope)
@@ -15213,6 +15215,32 @@ void
Sema::CheckFunctionOrTemplateParamDeclarator(Scope *S, Declarator &D) {
}
}
+void Sema::warnOnCTypeHiddenInCPlusPlus(const NamedDecl *D) {
+ // This only matters in C.
+ if (getLangOpts().CPlusPlus)
+ return;
+
+ // This only matters if the declaration has a type.
+ const auto *VD = dyn_cast<ValueDecl>(D);
+ if (!VD)
+ return;
+
+ // Get the type, this only matters for tag types.
+ QualType QT = VD->getType();
+ const auto *TD = QT->getAsTagDecl();
+ if (!TD)
+ return;
+
+ // Check if the tag declaration is lexically declared somewhere different
+ // from the lexical declaration of the given object, then it will be hidden
+ // in C++ and we should warn on it.
+ if (!TD->getLexicalParent()->LexicallyEncloses(D->getLexicalDeclContext())) {
+ unsigned Kind = TD->isEnum() ? 2 : TD->isUnion() ? 1 : 0;
+ Diag(D->getLocation(), diag::warn_decl_hidden_in_cpp) << Kind;
+ Diag(TD->getLocation(), diag::note_declared_at);
+ }
+}
+
static void CheckExplicitObjectParameter(Sema &S, ParmVarDecl *P,
SourceLocation ExplicitThisLoc) {
if (!ExplicitThisLoc.isValid())
@@ -15334,6 +15362,8 @@ Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator
&D,
New->setScopeInfo(S->getFunctionPrototypeDepth() - 1,
S->getNextFunctionPrototypeIndex());
+ warnOnCTypeHiddenInCPlusPlus(New);
+
// Add the parameter declaration into this scope.
S->AddDecl(New);
if (II)
@@ -18860,6 +18890,9 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name,
QualType T,
if (InvalidDecl)
NewFD->setInvalidDecl();
+ if (!InvalidDecl)
+ warnOnCTypeHiddenInCPlusPlus(NewFD);
+
if (PrevDecl && !isa<TagDecl>(PrevDecl) &&
!PrevDecl->isPlaceholderVar(getLangOpts())) {
Diag(Loc, diag::err_duplicate_member) << II;
diff --git a/clang/test/Sema/decl-hidden-in-c++.c
b/clang/test/Sema/decl-hidden-in-c++.c
new file mode 100644
index 0000000000000..c967d4b474024
--- /dev/null
+++ b/clang/test/Sema/decl-hidden-in-c++.c
@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wc++-hidden-decl %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wc++-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify=good %s
+// RUN: %clang_cc1 -fsyntax-only -verify=cxx -x c++ -std=c++2c %s
+// good-no-diagnostics
+
+struct A {
+ struct B { // #b-decl
+ int x;
+ } bs;
+ enum E { // #e-decl
+ One
+ } es;
+ int y;
+};
+
+struct C {
+ struct D {
+ struct F { // #f-decl
+ int x;
+ } f;
+ } d;
+};
+
+struct B b; // expected-warning {{struct defined within a struct or union is
not visible in C++}} \
+ expected-note@#b-decl {{declared here}} \
+ cxx-error {{variable has incomplete type 'struct B'}} \
+ cxx-note 3 {{forward declaration of 'B'}}
+enum E e; // expected-warning {{enum defined within a struct or union is not
visible in C++}} \
+ expected-note@#e-decl {{declared here}} \
+ cxx-error {{ISO C++ forbids forward references to 'enum'
types}} \
+ cxx-error {{variable has incomplete type 'enum E'}} \
+ cxx-note 3 {{forward declaration of 'E'}}
+struct F f; // expected-warning {{struct defined within a struct or union is
not visible in C++}} \
+ expected-note@#f-decl {{declared here}} \
+ cxx-error {{variable has incomplete type 'struct F'}} \
+ cxx-note {{forward declaration of 'F'}}
+
+void func(struct B b); // expected-warning {{struct defined within a
struct or union is not visible in C++}} \
+ expected-note@#b-decl {{declared here}}
+void other_func(enum E e) { // expected-warning {{enum defined within a struct
or union is not visible in C++}} \
+ expected-note@#e-decl {{declared here}} \
+ cxx-error {{variable has incomplete type 'enum
E'}}
+ struct B b; // expected-warning {{struct defined within a
struct or union is not visible in C++}} \
+ expected-note@#b-decl {{declared here}} \
+ cxx-error {{variable has incomplete type
'struct B'}}
+}
+
+struct X {
+ struct B b; // expected-warning {{struct defined within a struct or union is
not visible in C++}} \
+ expected-note@#b-decl {{declared here}} \
+ cxx-error {{field has incomplete type 'struct B'}}
+ enum E e; // expected-warning {{enum defined within a struct or union is
not visible in C++}} \
+ expected-note@#e-decl {{declared here}} \
+ cxx-error {{field has incomplete type 'enum E'}}
+};
+
+struct Y {
+ struct Z1 {
+ int x;
+ } zs;
+
+ struct Z2 {
+ // This is fine, it is still valid C++.
+ struct Z1 inner_zs;
+ } more_zs;
+};
+
>From 7df3587cd0e1d82a2f4c62ae0d116b5d17efa5d1 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aa...@aaronballman.com>
Date: Mon, 28 Apr 2025 07:15:37 -0400
Subject: [PATCH 2/3] Fix formatting; NFC
---
clang/lib/AST/DeclBase.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index d80bdeec092d0..c1a4fa2bc7af1 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1428,7 +1428,7 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
return false;
}
-bool DeclContext::LexicallyEncloses(const DeclContext* DC) const {
+bool DeclContext::LexicallyEncloses(const DeclContext *DC) const {
if (getPrimaryContext() != this)
return getPrimaryContext()->LexicallyEncloses(DC);
>From d0ccfe774ca705a60249b3f0018a555ee2c17d12 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aa...@aaronballman.com>
Date: Mon, 28 Apr 2025 13:49:26 -0400
Subject: [PATCH 3/3] Simplify; NFC
---
clang/lib/AST/DeclBase.cpp | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index c1a4fa2bc7af1..fead99c5f28a9 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1422,7 +1422,7 @@ bool DeclContext::Encloses(const DeclContext *DC) const {
return getPrimaryContext()->Encloses(DC);
for (; DC; DC = DC->getParent())
- if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+ if (!isa<LinkageSpecDecl, ExportDecl>(DC) &&
DC->getPrimaryContext() == this)
return true;
return false;
@@ -1433,7 +1433,7 @@ bool DeclContext::LexicallyEncloses(const DeclContext
*DC) const {
return getPrimaryContext()->LexicallyEncloses(DC);
for (; DC; DC = DC->getLexicalParent())
- if (!isa<LinkageSpecDecl>(DC) && !isa<ExportDecl>(DC) &&
+ if (!isa<LinkageSpecDecl, ExportDecl>(DC) &&
DC->getPrimaryContext() == this)
return true;
return false;
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