[OpenMP] Diagnose bad 'omp declare variant' that references itself.

When an a variant is specified that is the same as the base function
the compiler will end up crashing in CodeGen. Give an error instead.

Differential Revision: https://reviews.llvm.org/D119979

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10830,6 +10830,8 @@ def err_omp_interop_type_not_found : Error<
 def err_omp_declare_variant_incompat_types : Error<
   "variant in '#pragma omp declare variant' with type %0 is incompatible with"
   " type %1%select{| with appended arguments}2">;
+def err_omp_declare_variant_same_base_function : Error<
+  "variant in '#pragma omp declare variant' is the same as the base function">;
 def warn_omp_declare_variant_marked_as_declare_variant : Warning<
   "variant function in '#pragma omp declare variant' is itself marked as '#pragma omp declare variant'"
   >, InGroup<SourceUsesOpenMP>;

clang/lib/Sema/SemaOpenMP.cpp

@@ -7171,6 +7171,13 @@ Sema::checkOpenMPDeclareVariantFunction(Sema::DeclGroupPtrTy DG,
     return None;
   }
 
+  if (FD->getCanonicalDecl() == NewFD->getCanonicalDecl()) {
+    Diag(VariantRef->getExprLoc(),
+         diag::err_omp_declare_variant_same_base_function)
+        << VariantRef->getSourceRange();
+    return None;
+  }
+
   // Check if function types are compatible in C.
   if (!LangOpts.CPlusPlus) {
     QualType NewType =

clang/test/OpenMP/declare_variant_messages.c

@@ -113,6 +113,15 @@ int bar(void) {
   return after_use();
 }
 
+// expected-error@+1 {{variant in '#pragma omp declare variant' is the same as the base function}}
+#pragma omp declare variant (self) \
+  match(construct={dispatch}, device={arch(arm)})
+void self(int n);
+
+void self_test(int n, int d_no) {
+  #pragma omp dispatch device(d_no) nowait
+  self(n);
+}
 
 #pragma omp declare variant(after_use_variant) match(xxx={}) // expected-warning {{'xxx' is not a valid context set in a `declare variant`; set ignored}} expected-warning {{'#pragma omp declare variant' cannot be applied for function after first usage; the original function might be used}} expected-note {{context set options are: 'construct' 'device' 'implementation' 'user'}} expected-note {{the ignored set spans until here}}
 int after_use(void);