Referencing a static function defined in an opnemp clause, is

generating an erroneous warning.

See here: https://godbolt.org/z/ajKPc36M7

clang/lib/Sema/Sema.cpp

@@ -812,8 +812,21 @@ static void checkUndefinedButUsed(Sema &S) {
       // FIXME: We can promote this to an error. The function or variable can't
       // be defined anywhere else, so the program must necessarily violate the
       // one definition rule.
-      S.Diag(VD->getLocation(), diag::warn_undefined_internal)
-        << isa<VarDecl>(VD) << VD;
+      bool IsImplicitBase = false;
+      if (const auto *BaseD = dyn_cast<FunctionDecl>(VD)) {
+        auto *DVAttr = BaseD->getAttr<OMPDeclareVariantAttr>();
+        if (DVAttr && !DVAttr->getTraitInfo().isExtensionActive(
+                          llvm::omp::TraitProperty::
+                              implementation_extension_disable_implicit_base)) {
+          const auto *Func = cast<FunctionDecl>(
+              cast<DeclRefExpr>(DVAttr->getVariantFuncRef())->getDecl());
+          IsImplicitBase = BaseD->isImplicit() &&
+                           Func->getIdentifier()->isMangledOpenMPVariantName();
+        }
+      }
+      if (!S.getLangOpts().OpenMP || !IsImplicitBase)
+        S.Diag(VD->getLocation(), diag::warn_undefined_internal)
+            << isa<VarDecl>(VD) << VD;
     } else if (auto *FD = dyn_cast<FunctionDecl>(VD)) {
       (void)FD;
       assert(FD->getMostRecentDecl()->isInlined() &&

clang/test/OpenMP/declare_variant.cpp

@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -fopenmp -verify %s
+
+namespace {
+// TODO this must be fixed. This warning shouldn't be generated.
+// expected-warning@+1{{function '(anonymous namespace)::bar' has internal linkage but is not defined}}
+void bar();
+} // namespace
+
+#pragma omp begin declare variant match(user = {condition(1)})
+void bar() {
+}
+#pragma omp end declare variant
+
+// expected-warning@+1{{function 'baz' has internal linkage but is not defined}}
+static void baz();
+#pragma omp begin declare variant match(device = {kind(nohost)})
+static void baz() {}
+#pragma omp end declare variant
+
+#pragma omp begin declare variant match(device = {kind(host)})
+static void foo() {}
+#pragma omp end declare variant
+
+int main() {
+  foo();
+  // expected-note@+1{{used here}}
+  baz();
+  // expected-note@+1{{used here}}
+  bar();
+
+  return 0;
+}