Add a failing test for SR-13785 (#34462)

* Add a test that verifies that Swift correctly recognizes symbol visibility when dealing with @_implementationOnly imports

* Fix header guards

Author: scentini

test/Interop/C/implementation-only-imports/Inputs/helper.h

@@ -0,0 +1,8 @@
+#ifndef TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_HELPER_H
+#define TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_HELPER_H
+
+inline int getFortyTwo() {
+    return 42;
+};
+
+#endif // TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_HELPER_H

test/Interop/C/implementation-only-imports/Inputs/module.modulemap

@@ -0,0 +1,9 @@
+module UserA {
+    header "user_a.h"
+    export *
+}
+
+module UserB {
+    header "user_b.h"
+    export *
+}

test/Interop/C/implementation-only-imports/Inputs/user_a.h

@@ -0,0 +1,6 @@
+#ifndef TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERA_H
+#define TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERA_H
+
+#include "helper.h"
+
+#endif // TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERA_H

test/Interop/C/implementation-only-imports/Inputs/user_b.h

@@ -0,0 +1,6 @@
+#ifndef TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERB_H
+#define TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERB_H
+
+#include "helper.h"
+
+#endif // TEST_INTEROP_C_IMPLEMENTATION_ONLY_IMPORTS_INPUTS_USERB_H

test/Interop/C/implementation-only-imports/prefer-a-visible-symbol-over-implementation-only-ones.swift

@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/FortyTwo.swiftmodule -I %S/Inputs %s
+
+// REQUIRES: SR-13785
+
+// TODO: Fix @_implementationOnly to consider all symbol sources
+
+// If a symbol comes from two modules, one of which is marked as
+// @_implementationOnly, Swift may choose the @_implementationOnly source
+// and then error out due to the symbol being hidden.
+
+// Swift should consider all sources for the symbol and recognize that the
+// symbol is not hidden behind @_implementationOnly in all modules.
+
+// E.g:
+// In this test case, UserA and UserB both textually include `helper.h`,
+// therefore both export `getFortyTwo()`.
+// This test verifies that even though Swift chooses UserA.getFortyTwo(), we
+// shouldn't get an error, because the symbol is also exported from UserB.
+
+@_implementationOnly import UserA
+import UserB
+
+@_inlineable
+public func callFortyTwo() -> CInt {
+    return getFortyTwo()
+}