[clang][FMV] Allow declaration of function versions in namespaces.

Fixes a bug:

../llvm-project/clang/test/Sema/fmv-namespace.cpp:8:18: warning:
attribute declaration must precede definition [-Wignored-attributes]

    8 | int __attribute((target_version("sve"))) foo() { return 1; }
      |                  ^
../llvm-project/clang/test/Sema/fmv-namespace.cpp:4:46: note:
previous definition is here
    4 | int __attribute((target_version("default"))) foo() { return 0; }
      |                                              ^
../llvm-project/clang/test/Sema/fmv-namespace.cpp:8:42: error:
redefinition of 'foo'
    8 | int __attribute((target_version("sve"))) foo() { return 1; }
      |                                          ^
../llvm-project/clang/test/Sema/fmv-namespace.cpp:4:46: note:
previous definition is here
    4 | int __attribute((target_version("default"))) foo() { return 0; }
      |                                              ^
1 warning and 1 error generated.

Author: labrinea

clang/lib/Sema/SemaDecl.cpp

@@ -11868,8 +11868,10 @@ static bool CheckMultiVersionFunction(Sema &S, FunctionDecl *NewFD,
     return false;
 
   if (!OldDecl || !OldDecl->getAsFunction() ||
-      OldDecl->getDeclContext()->getRedeclContext() !=
-          NewFD->getDeclContext()->getRedeclContext()) {
+      (OldDecl->getDeclContext()->getRedeclContext() !=
+           NewFD->getDeclContext()->getRedeclContext() &&
+       OldDecl->getDeclContext()->getEnclosingNamespaceContext() !=
+           NewFD->getDeclContext()->getEnclosingNamespaceContext())) {
     // If there's no previous declaration, AND this isn't attempting to cause
     // multiversioning, this isn't an error condition.
     if (MVKind == MultiVersionKind::None)

clang/test/CodeGenCXX/fmv-namespace.cpp

@@ -0,0 +1,56 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-globals all --include-generated-funcs --version 5
+// RUN: %clang_cc1 -triple aarch64-linux-gnu -emit-llvm %s -o - | FileCheck %s
+
+namespace Name {
+int __attribute((target_version("default"))) foo() { return 0; }
+}
+
+namespace Name {
+int __attribute((target_version("sve"))) foo() { return 1; }
+}
+
+int bar() { return Name::foo(); }
+//.
+// CHECK: @__aarch64_cpu_features = external dso_local global { i64 }
+// CHECK: @_ZN4Name3fooEv.ifunc = weak_odr alias i32 (), ptr @_ZN4Name3fooEv
+// CHECK: @_ZN4Name3fooEv = weak_odr ifunc i32 (), ptr @_ZN4Name3fooEv.resolver
+//.
+// CHECK-LABEL: define dso_local noundef i32 @_ZN4Name3fooEv.default(
+// CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    ret i32 0
+//
+//
+// CHECK-LABEL: define dso_local noundef i32 @_ZN4Name3fooEv._Msve(
+// CHECK-SAME: ) #[[ATTR1:[0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    ret i32 1
+//
+//
+// CHECK-LABEL: define dso_local noundef i32 @_Z3barv(
+// CHECK-SAME: ) #[[ATTR0]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[CALL:%.*]] = call noundef i32 @_ZN4Name3fooEv()
+// CHECK-NEXT:    ret i32 [[CALL]]
+//
+//
+// CHECK-LABEL: define weak_odr ptr @_ZN4Name3fooEv.resolver() comdat {
+// CHECK-NEXT:  [[RESOLVER_ENTRY:.*:]]
+// CHECK-NEXT:    call void @__init_cpu_features_resolver()
+// CHECK-NEXT:    [[TMP0:%.*]] = load i64, ptr @__aarch64_cpu_features, align 8
+// CHECK-NEXT:    [[TMP1:%.*]] = and i64 [[TMP0]], 1073741824
+// CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i64 [[TMP1]], 1073741824
+// CHECK-NEXT:    [[TMP3:%.*]] = and i1 true, [[TMP2]]
+// CHECK-NEXT:    br i1 [[TMP3]], label %[[RESOLVER_RETURN:.*]], label %[[RESOLVER_ELSE:.*]]
+// CHECK:       [[RESOLVER_RETURN]]:
+// CHECK-NEXT:    ret ptr @_ZN4Name3fooEv._Msve
+// CHECK:       [[RESOLVER_ELSE]]:
+// CHECK-NEXT:    ret ptr @_ZN4Name3fooEv.default
+//
+//.
+// CHECK: attributes #[[ATTR0]] = { mustprogress noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+// CHECK: attributes #[[ATTR1]] = { mustprogress noinline nounwind optnone "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-features"="+fp-armv8,+fullfp16,+neon,+sve" }
+//.
+// CHECK: [[META0:![0-9]+]] = !{i32 1, !"wchar_size", i32 4}
+// CHECK: [[META1:![0-9]+]] = !{!"{{.*}}clang version {{.*}}"}
+//.

clang/test/Sema/fmv-namespace.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple aarch64-linux-gnu  -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+namespace Name {
+int __attribute((target_version("default"))) foo() { return 0; }
+}
+
+namespace Name {
+int __attribute((target_version("sve"))) foo() { return 1; }
+}
+
+int bar() { return Name::foo(); }