[Clang] Diagnose UB and emit error when identifier has both internal and external linkage (#192116)

C11 6.2.2p7 says if the same identifier appears with both internal and
external linkage in a translation unit, the behavior is undefined.

This patch adds a check in `Sema::MergeVarDecl` that uses
`LookupResult::isShadowed()` to detect when a block-scope `extern` found
a file-scope `static` through a shadow, and emits an error.

Fixes #54215

clang/docs/ReleaseNotes.rst

@@ -155,6 +155,12 @@ C Language Changes
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^
 
+- Clang now diagnoses the use of the same identifier with both internal and
+  external linkage within a translation unit, as made ill-formed by
+  `N3410 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3410.pdf>`_.
+  This is also diagnosed in older C language modes as the behavior was
+  undefined prior to C2y. (#GH54215)
+
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 - Clang now allows C23 ``constexpr`` struct member access through the dot operator in constant expressions. (#GH178349)

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6534,6 +6534,9 @@ def err_inline_declaration_block_scope : Error<
   "inline declaration of %0 not allowed in block scope">;
 def err_static_non_static : Error<
   "static declaration of %0 follows non-static declaration">;
+def err_internal_extern_mismatch : Error<
+  "variable %0 declared with both internal and external linkage "
+  "in the same translation unit%select{; behavior is undefined|}1">;
 def err_different_language_linkage : Error<
   "declaration of %0 has a different language linkage">;
 def ext_retained_language_linkage : Extension<

clang/lib/Sema/SemaDecl.cpp

@@ -4802,6 +4802,25 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
       return New->setInvalidDecl();
     }
   }
+
+  // C2y 6.7.1p7:
+  //   Within a translation unit, the same identifier shall not appear with
+  //   both internal and external linkage.
+  //
+  // In C11 through C23, this was undefined behavior (C11 6.2.2p7).
+  //
+  // This can occur when an extern declaration in block scope finds a file-scope
+  // static declaration, but a no-linkage local variable shadowed the static,
+  // preventing linkage inheritance per C2y 6.2.2p4.
+  if (!getLangOpts().CPlusPlus && New->isLocalVarDecl() &&
+      New->hasExternalStorage() && Old->isFileVarDecl() && Old->hasLinkage() &&
+      Previous.isShadowed() && Old->getFormalLinkage() == Linkage::Internal) {
+    Diag(New->getLocation(), diag::err_internal_extern_mismatch)
+        << New->getDeclName() << getLangOpts().C2y;
+    Diag(OldLocation, PrevDiag);
+    return New->setInvalidDecl();
+  }
+
   // C99 6.2.2p4:
   //   For an identifier declared with the storage-class specifier
   //   extern in a scope in which a prior declaration of that

clang/test/C/C2y/n3410.c

@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -Wno-unused %s
 
-/* WG14 N3410: No
+/* WG14 N3410: Clang 23
  * Slay Some Earthly Demons XI
  *
  * It is now ill-formed for the same identifier within a TU to have both
@@ -24,19 +24,17 @@ void func2() {
   extern int b; // Ok
 }
 
-static int c, d;
+static int c, d; // expected-note 2 {{previous definition is here}}
 void func3() {
   int c; // no linkage, different object from the one declared above.
   for (int d;;) {
     // This 'c' is the same as the one declared at file scope, but because of
     // the local scope 'c', the file scope 'c' is not visible.
-    // FIXME: This should be diagnosed under N3410.
-    extern int c;
+    extern int c; // expected-error {{declared with both internal and external linkage}}
     // This 'd' is the same as the one declared at file scope as well, but
     // because of the 'd' declared within the for loop, the file scope 'd' is
     // also not visible, same as with 'c'.
-    // FIXME: This should be diagnosed under N3410.
-    extern int d;
+    extern int d; // expected-error {{declared with both internal and external linkage}}
   }
   for (static int e;;) {
     extern int e; // Ok for the same reason as 'b' above.

clang/test/Sema/linkage-internal-extern.c

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+
+// C11 6.2.2p7: same identifier with both internal and external linkage is UB.
+
+// Conflicting linkage (UB)
+
+static int x; // expected-note {{previous}}
+void test_basic_shadow(void) {
+    int x;
+    { extern int x; } // expected-error {{declared with both internal and external linkage}}
+}
+
+static int y; // expected-note {{previous}}
+void test_deep_nesting(void) {
+    int y;
+    { int y; { { extern int y; } } } // expected-error {{declared with both internal and external linkage}}
+}
+
+static int p; // expected-note {{previous}}
+void test_param_shadow(int p) {
+    { extern int p; } // expected-error {{declared with both internal and external linkage}}
+}
+
+// Valid cases
+
+static int a;
+void test_no_shadow(void) {
+    extern int a;
+}
+
+void test_no_file_scope(void) {
+    for (static int b = 0;;) {
+        extern int b;
+        break;
+    }
+}

clang/test/Sema/linkage-internal-extern.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -std=c++17 %s
+
+// expected-no-diagnostics
+
+// CWG 426 / [basic.link]p6: the same identifier with both internal and
+// external linkage should be ill-formed in C++, but Clang does not yet
+// diagnose this due to ABI break concerns. This test documents current
+// behavior.
+
+static int x;
+void test_shadow(void) {
+    int x;
+    {
+        // FIXME: Per CWG 426, this should be ill-formed because the
+        // file-scope 'x' has internal linkage but this 'extern' gets
+        // external linkage due to the local shadow.
+        extern int x;
+    }
+}

clang/www/c_status.html

@@ -283,7 +283,7 @@ conformance.</p>
     <tr>
       <td>Slay Some Earthly Demons XI</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3410.pdf">N3410</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 23</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons XII</td>