[ClangImporter] Import builtins returning size_t as returning Int. (#9753)

...which is consistent with other library functions return size_t. But
don't break compatibility with Swift 3.

("Builtins" are functions that Clang has, well, built-in knowledge
about, and whose declarations can therefore be produced directly in
the compiler. This bypassed some of our checking for special typedefs
like size_t. We discovered this when 'wcslen' got added as a new
built-in in the Swift 4 timeframe.)

rdar://problem/30545505

Author: jrose-apple

lib/ClangImporter/ImportType.cpp

@@ -1540,6 +1540,34 @@ Type ClangImporter::Implementation::importFunctionReturnType(
     DeclContext *dc,
     const clang::FunctionDecl *clangDecl,
     bool allowNSUIntegerAsInt) {
+
+  // Hardcode handling of certain result types for builtins.
+  auto builtinID = clangDecl->getBuiltinID();
+  switch (builtinID) {
+  case clang::Builtin::NotBuiltin:
+    break;
+  case clang::Builtin::BIstrxfrm:
+  case clang::Builtin::BIstrcspn:
+  case clang::Builtin::BIstrspn:
+  case clang::Builtin::BIstrlen:
+  case clang::Builtin::BIstrlcpy:
+  case clang::Builtin::BIstrlcat:
+    // This is a list of all built-ins with a result type of 'size_t' that
+    // existed in Swift 3. We didn't have special handling for builtins at
+    // that time, and so these had a result type of UInt.
+    if (SwiftContext.isSwiftVersion3())
+      break;
+    LLVM_FALLTHROUGH;
+  default:
+    switch (getClangASTContext().BuiltinInfo.getTypeString(builtinID)[0]) {
+    case 'z': // size_t
+    case 'Y': // ptrdiff_t
+      return SwiftContext.getIntDecl()->getDeclaredType();
+    default:
+      break;
+    }
+  }
+
   // CF function results can be managed if they are audited or
   // the ownership convention is explicitly declared.
   assert(clangDecl && "expected to have a decl to import");

test/ClangImporter/clang_builtin.swift renamed to test/ClangImporter/clang_builtin_pcm.swift

@@ -0,0 +1,44 @@
+// RUN: not %target-swift-frontend -swift-version 3 -typecheck %s 2>&1 | %FileCheck -check-prefix=CHECK-3 -check-prefix=CHECK-%target-runtime-3 %s
+// RUN: not %target-swift-frontend -swift-version 4 -typecheck %s 2>&1 | %FileCheck -check-prefix=CHECK-4 -check-prefix=CHECK-%target-runtime-4 %s
+
+#if os(OSX) || os(iOS) || os(watchOS) || os(tvOS)
+  import Darwin
+#else
+  import Glibc
+#endif
+
+func test() {
+  let _: Int = strxfrm
+  // CHECK-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+
+  let _: Int = strcspn
+  // CHECK-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+
+  let _: Int = strspn
+  // CHECK-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+
+  let _: Int = strlen
+  // CHECK-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+}
+
+// These functions aren't consistently available across platforms, so only
+// test for them on Apple platforms.
+func testApple() {
+  let _: Int = strlcpy
+  // CHECK-objc-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-objc-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+
+  let _: Int = strlcat
+  // CHECK-objc-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> UInt'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-objc-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+
+  // wcslen is different: it wasn't a builtin until Swift 4, and so its return
+  // type has always been 'Int'.
+  let _: Int = wcslen
+  // CHECK-objc-3: [[@LINE-1]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+  // CHECK-objc-4: [[@LINE-2]]:16: error: cannot convert value of type '({{.+}}) -> Int'{{( [(]aka .+[)])?}} to specified type 'Int'
+}