[Associated type inference] Find tentative type witnesses across protocol hierarchy.

As a minor step toward more global associated type inference,
allow tentative type witnesses to be found across a protocol
hierarchy, eliminating some recursion through associated type
inference. Fixes a recent regression in validation-test/stdlib/CollectionTypes.swift, as well as rdar://problem/36453271.

Author: DougGregor

lib/AST/ProtocolConformance.cpp

@@ -577,18 +577,16 @@ NormalProtocolConformance::getTypeWitnessAndDecl(AssociatedTypeDecl *assocType,
   if (known != TypeWitnesses.end())
     return known->second;
 
-  // If this conformance is in a state where it is inferring type witnesses,
-  // check tentative witnesses.
-  if (getState() == ProtocolConformanceState::CheckingTypeWitnesses) {
-    // If there is a tentative-type-witness function, use it.
-    if (options.getTentativeTypeWitness) {
-     if (Type witnessType =
-           Type(options.getTentativeTypeWitness(this, assocType)))
-       return { witnessType, nullptr };
-    }
+  // If there is a tentative-type-witness function, use it.
+  if (options.getTentativeTypeWitness) {
+   if (Type witnessType =
+         Type(options.getTentativeTypeWitness(this, assocType)))
+     return { witnessType, nullptr };
+  }
 
-    // Otherwise, we fail; this is the only case in which we can return a
-    // null type.
+  // If this conformance is in a state where it is inferring type witnesses but
+  // we didn't find anything, fail.
+  if (getState() == ProtocolConformanceState::CheckingTypeWitnesses) {
     return { Type(), nullptr };
   }
 

validation-test/compiler_crashers_2_fixed/0138-rdar36453271.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend %s -emit-ir -o 0
+
+extension Slice where Base == UnsafeBufferPointer<UInt16> {
+  var rebased: UnsafeBufferPointer<UInt16> {
+    return UnsafeBufferPointer(rebasing: self)
+  }
+}
+
+extension Slice where Base == UnsafeMutableBufferPointer<UInt16> {
+  var rebased: UnsafeMutableBufferPointer<UInt16> {
+    return UnsafeMutableBufferPointer(rebasing: self)
+  }
+}