Sema: Infer default witnesses before finishing conformance checking

Otherwise, we'll never get a chance to drain the TypeChecker's
UsedConformances list.

Fixes <https://bugs.swift.org/browse/SR-6565>, <rdar://problem/35949729>.

Author: slavapestov

include/swift/Subsystems.h

@@ -184,10 +184,6 @@ namespace swift {
                            unsigned WarnLongExpressionTypeChecking = 0,
                            unsigned ExpressionTimeoutThreshold = 0);
 
-  /// Once type checking is complete, this walks protocol requirements
-  /// to resolve default witnesses.
-  void finishTypeCheckingFile(SourceFile &SF);
-
   /// Now that we have type-checked an entire module, perform any type
   /// checking that requires the full module, e.g., Objective-C method
   /// override checking.

lib/Frontend/Frontend.cpp

@@ -651,7 +651,6 @@ void CompilerInstance::finishTypeChecking(
       performWholeModuleTypeChecking(SF);
     });
   }
-  forEachFileToTypeCheck([&](SourceFile &SF) { finishTypeCheckingFile(SF); });
 }
 
 void CompilerInstance::performParseOnly(bool EvaluateConditionals) {

lib/Sema/TypeCheckDecl.cpp

@@ -3969,6 +3969,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
         TC.checkDeclCircularity(nominal);
       }
+      if (auto protocol = dyn_cast<ProtocolDecl>(decl)) {
+        if (!protocol->hasFixedLayout())
+          TC.inferDefaultWitnesses(protocol);
+      }
     }
   }
 

lib/Sema/TypeChecker.cpp

@@ -672,16 +672,6 @@ void swift::performTypeChecking(SourceFile &SF, TopLevelContext &TLC,
   }
 }
 
-void swift::finishTypeCheckingFile(SourceFile &SF) {
-  auto &Ctx = SF.getASTContext();
-  TypeChecker TC(Ctx);
-
-  for (auto D : SF.Decls)
-    if (auto PD = dyn_cast<ProtocolDecl>(D))
-      if (!PD->hasFixedLayout())
-        TC.inferDefaultWitnesses(PD);
-}
-
 void swift::performWholeModuleTypeChecking(SourceFile &SF) {
   SharedTimer("performWholeModuleTypeChecking");
   auto &Ctx = SF.getASTContext();

test/multifile/Inputs/default-witness.swift

@@ -0,0 +1,9 @@
+public protocol Node: Source { }
+
+public protocol Source {
+    associatedtype Output
+}
+
+public final class GraphNode<U>: Node {
+    public typealias Output = U
+}

test/multifile/default-witness.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -typecheck %S/Inputs/default-witness.swift -primary-file %s -enable-resilience
+
+public protocol GraphType {
+    func insert<U>(_: GraphNode<U>, _: GraphNode<U>)
+}
+
+public extension GraphType {
+    func insert<N: Node>(_: N, _: GraphNode<N.Output>) {}
+}
+