[Name lookup] Implement name shadowing rule for Data.withUnsafeBytes.

Address an annoying source compatibility issue with the introduction
of Data.withUnsafeBytes, which is ambiguous with Swift NIO's
implementatio of the same function. Do so with a narrow hackish name
shadowing rule (the Swift NIO version shadows the Foundation version)
that we will eventually generalize to something sensible.

Fixes rdar://problem/46850346.

Author: DougGregor

lib/AST/NameLookup.cpp

@@ -271,6 +271,42 @@ static void recordShadowedDeclsAfterSignatureMatch(
         }
       }
 
+      // The Foundation overlay introduced Data.withUnsafeBytes, which is
+      // treated as being ambiguous with SwiftNIO's Data.withUnsafeBytes
+      // extension. Apply a special-case name shadowing rule to use the
+      // latter rather than the former, which be the consequence of a more
+      // significant change to name shadowing in the future.
+      if (auto owningStruct1
+            = firstDecl->getDeclContext()->getSelfStructDecl()) {
+        if (auto owningStruct2
+              = secondDecl->getDeclContext()->getSelfStructDecl()) {
+          if (owningStruct1 == owningStruct2 &&
+              owningStruct1->getName().is("Data") &&
+              isa<FuncDecl>(firstDecl) && isa<FuncDecl>(secondDecl) &&
+              firstDecl->getFullName() == secondDecl->getFullName() &&
+              firstDecl->getBaseName().userFacingName() == "withUnsafeBytes") {
+            // If the second module is the Foundation module and the first
+            // is the NIOFoundationCompat module, the second is shadowed by the
+            // first.
+            if (firstDecl->getModuleContext()->getName()
+                  .is("NIOFoundationCompat") &&
+                secondDecl->getModuleContext()->getName().is("Foundation")) {
+              shadowed.insert(secondDecl);
+              continue;
+            }
+
+            // If it's the other way around, the first declaration is shadowed
+            // by the second.
+            if (secondDecl->getModuleContext()->getName()
+                  .is("NIOFoundationCompat") &&
+                firstDecl->getModuleContext()->getName().is("Foundation")) {
+              shadowed.insert(firstDecl);
+              break;
+            }
+          }
+        }
+      }
+
       // Prefer declarations in an overlay to similar declarations in
       // the Clang module it customizes.
       if (firstDecl->hasClangNode() != secondDecl->hasClangNode()) {

test/NameBinding/Inputs/NIOFoundationCompat.swift

@@ -0,0 +1,9 @@
+import Foundation
+
+extension Data {
+  @_inlineable
+  public func withUnsafeBytes<R>(_ body: (UnsafeRawBufferPointer) throws -> R) rethrows -> R {
+    let r: R? = nil
+    return r!
+  }
+}

test/NameBinding/nio_shadowing.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t %S/Inputs/NIOFoundationCompat.swift
+// RUN: %target-swift-frontend -typecheck %s -I %t -verify
+
+// REQUIRES: objc_interop
+import Foundation
+import NIOFoundationCompat
+
+func test(data: Data) {
+  data.withUnsafeBytes { x in print(x) }
+}