Allow some attributes on declarations after definitions

clang/docs/ReleaseNotes.rst

@@ -427,6 +427,11 @@ Bug Fixes to Attribute Support
 - No longer crashing on ``__attribute__((align_value(N)))`` during template
   instantiation when the function parameter type is not a pointer or reference.
   (#GH26612)
+- Now allowing the ``[[deprecated]]``, ``[[maybe_unused]]``, and
+  ``[[nodiscard]]`` to be applied to a redeclaration after a definition in both
+  C and C++ mode for the standard spellings (other spellings, such as
+  ``__attribute__((unused))`` are still ignored after the definition, though
+  this behavior may be relaxed in the future). (#GH135481)
 
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -2996,6 +2996,21 @@ static void checkNewAttributesAfterDef(Sema &S, Decl *New, const Decl *Old) {
       // msvc will allow a subsequent definition to add an uuid to a class
       ++I;
       continue;
+    } else if (isa<DeprecatedAttr, WarnUnusedResultAttr, UnusedAttr>(
+                   NewAttribute) &&
+               NewAttribute->isStandardAttributeSyntax()) {
+      // C++14 [dcl.attr.deprecated]p3: A name or entity declared without the
+      // deprecated attribute can later be re-declared with the attribute and
+      // vice-versa.
+      // C++17 [dcl.attr.unused]p4: A name or entity declared without the
+      // maybe_unused attribute can later be redeclared with the attribute and
+      // vice versa.
+      // C++20 [dcl.attr.nodiscard]p2: A name or entity declared without the
+      // nodiscard attribute can later be redeclared with the attribute and
+      // vice-versa.
+      // C23 6.7.13.3p3, 6.7.13.4p3. and 6.7.13.5p5 give the same allowances.
+      ++I;
+      continue;
     } else if (const AlignedAttr *AA = dyn_cast<AlignedAttr>(NewAttribute)) {
       if (AA->isAlignas()) {
         // C++11 [dcl.align]p6:

clang/test/Sema/attr-decl-after-definition-std.c

@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -fsyntax-only -Wignored-attributes -verify -std=c23 %s
+// RUN: %clang_cc1 -fsyntax-only -Wignored-attributes -verify -x c++ %s
+// RUN: %clang_cc1 -fsyntax-only -ast-dump %s | FileCheck %s
+
+inline int frob(int x) { return x; }
+
+[[deprecated]] int frob(int); // expected-note 2 {{'frob' has been explicitly marked deprecated here}}
+
+void use1() {
+  // Using this should give a deprecation warning, but not a nodiscard warning.	
+  frob(0); // expected-warning {{'frob' is deprecated}}
+}
+
+[[nodiscard]] int frob(int);
+
+void use2() {
+  // This should give both warnings.
+  frob(0); // expected-warning {{'frob' is deprecated}} \
+              expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}
+
+[[maybe_unused]] int frob(int);
+
+// Currently, this is only allowed for the standard spelling of the attributes.
+void blob() {}                           // expected-note {{previous definition is here}}
+__attribute__((deprecated)) void blob(); // expected-warning {{attribute declaration must precede definition}}
+
+// CHECK: FunctionDecl {{.*}} frob
+
+// CHECK: FunctionDecl {{.*}} prev {{.*}} frob
+// CHECK:  DeprecatedAttr
+
+// CHECK: FunctionDecl {{.*}} prev {{.*}} frob
+// CHECK:  DeprecatedAttr
+// CHECK:  WarnUnusedResultAttr
+
+// CHECK: FunctionDecl {{.*}} prev {{.*}} frob
+// CHECK:  DeprecatedAttr
+// CHECK:  WarnUnusedResultAttr
+// CHECK:  UnusedAttr
+