[test] Make sure we prefer local .swiftmodules to SDK interfaces

Author: Harlan Haskins

test/ParseableInterface/ModuleCache/prefer-local-module-to-sdk.swift

@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t/BuildDir)
+// RUN: %empty-directory(%t/SecondBuildDir/Lib.swiftmodule)
+// RUN: %empty-directory(%t/ModuleCache)
+
+// RUN: echo 'public func showsUpInBothPlaces() {}' > %t/Lib.swift
+
+// 1. Create a .swiftinterface file containing just one API, and put it inside a second build dir (without a .swiftmodule)
+// RUN: %target-swift-frontend -typecheck %t/Lib.swift -emit-parseable-module-interface-path %t/SecondBuildDir/Lib.swiftmodule/%target-cpu.swiftinterface
+
+// 2. Add a new API to the module, and compile just the serialized version in the build dir.
+// RUN: echo 'public func onlyInTheCompiledModule() {}' >> %t/Lib.swift
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -o %t/BuildDir/Lib.swiftmodule -emit-parseable-module-interface-path %t/BuildDir/Lib.swiftinterface
+
+// 3. Make sure when we compile this test file, we can access both APIs since we'll
+//    load the compiled .swiftmodule instead of the .swiftinterface in the SDK.
+// RUN: %target-swift-frontend -typecheck %s -I %t/BuildDir -I %t/SecondBuildDir -module-cache-path %t/ModuleCache
+
+// 4. Make sure we didn't compile any .swiftinterfaces into the module cache.
+// RUN: ls %t/ModuleCache | not grep 'swiftmodule'
+
+import Lib
+
+showsUpInBothPlaces()
+onlyInTheCompiledModule()