OpenMP/C++: Fix 'declare variant' for struct-returning functions [PR118486]

To find the variant declaration, a call is constructed in
omp_declare_variant_finalize_one, which gives here:
  TARGET_EXPR <D.3010, variant_fn ()>

Extracting now the function declaration failed and gave the bogus
  error: could not find variant declaration

Solution: Use the 2nd argument of the TARGET_EXPR and continue.

	PR c++/118486

gcc/cp/ChangeLog:

	* decl.cc (omp_declare_variant_finalize_one): When resolving
	the variant to use, handle variant calls with TARGET_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/declare-variant-11.C: New test.

gcc/cp/decl.cc

@@ -8552,6 +8552,9 @@ omp_declare_variant_finalize_one (tree decl, tree attr)
   if (variant == error_mark_node && !processing_template_decl)
     return true;
 
+  if (TREE_CODE (variant) == TARGET_EXPR)
+    variant = TARGET_EXPR_INITIAL (variant);
+
   variant = cp_get_callee_fndecl_nofold (STRIP_REFERENCE_REF (variant));
   input_location = save_loc;
 

gcc/testsuite/g++.dg/gomp/declare-variant-11.C

@@ -0,0 +1,38 @@
+/* { dg-additional-options "-fdump-tree-gimple" }  */
+
+/* PR c++/118486  */
+
+struct NotAnInt {};
+
+// Wrong return type:
+NotAnInt var1();
+#pragma omp declare variant(var1) match(user={condition(true)})
+int base1();
+/* { dg-error "variant 'NotAnInt var1\\(\\)' and base 'int base1\\(\\)' have incompatible types" "" { target *-*-* } .-2 } */
+
+
+// Wrong return type:
+NotAnInt var2();
+float var2(float);
+#pragma omp declare variant(var2) match(user={condition(true)})
+int base2();
+/* { dg-error "variant 'NotAnInt var2\\(\\)' and base 'int base2\\(\\)' have incompatible types" "" { target *-*-* } .-2 } */
+
+
+// OK:
+NotAnInt var3();
+#pragma omp declare variant(var3) match(user={condition(true)})
+NotAnInt base3();
+
+void f()
+{
+  // int x;
+  NotAnInt y;
+
+  //x = base1 ();
+  //x = base2 ();
+  y = base3 ();
+}
+
+/* { dg-final { scan-tree-dump "var3 \\(\\);" "gimple" } }  */
+/* { dg-final { scan-tree-dump-not "base3" "gimple" } }  */