Allow some attributes on declarations after definitions (#135791)

The deprecated, maybe_unused, and nodiscard standard attributes may all
be applied to a redeclaration after a definition has already appeared.
We were previously dropping the attribute in that case, now we retain
the attribute after the redeclaration.

Note: someday we may want to tablegen this as part of information from
Attr.td. We may also want to relax the restriction here so that the
syntax used does not matter. This is an intentionally conservative fix.

Fixes #135481

clang/docs/ReleaseNotes.rst

@@ -439,6 +439,11 @@ 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
+