Sema: Fix non-determinism with ConstraintSystem::ConstraintRestrictions

We iterate over the DenseMap and simplify both types appearing
in the key. If multiple keys simplify to the same type, later
keys overwrite earlier keys, so the outcome depends on hash
table order.

Fix this by introducing an arbitrary tie break: we prefer the
highest-numbered restriction.

Fixes rdar://146780049.

lib/Sema/CSSolver.cpp

@@ -124,6 +124,15 @@ Solution ConstraintSystem::finalize() {
 
       CanType first = simplifyType(types.first)->getCanonicalType();
       CanType second = simplifyType(types.second)->getCanonicalType();
+
+      // Pick the restriction with the highest value to avoid depending on
+      // iteration order.
+      auto found = solution.ConstraintRestrictions.find({first, second});
+      if (found != solution.ConstraintRestrictions.end() &&
+          (unsigned) restriction <= (unsigned) found->second) {
+        continue;
+      }
+
       solution.ConstraintRestrictions[{first, second}] = restriction;
     }
   }

test/SILGen/ambiguous_pointer_conversion.swift

@@ -0,0 +1,37 @@
+// RUN: %target-swift-frontend -emit-silgen %s | %FileCheck %s
+
+// rdar://146780049
+
+class Image {}
+
+class MyV {
+    var image: Image = Image()
+}
+
+func takesPtr(_: UnsafePointer<Image>?, _: UnsafePointer<Image>?, _: UnsafePointer<Image>?) {
+}
+
+func test(buffers: [MyV]) {
+    withUnsafePointer(to: buffers[0].image) { ptrA in
+        withUnsafePointer(to: buffers[1].image) { ptrB in
+            withUnsafePointer(to: buffers[2].image) { ptrC in
+                _ = takesPtr(ptrA, ptrB, ptrC)
+            }
+        }
+    }
+}
+
+// The other valid conversion here that should not be considered because it has a worse
+// score is pointer-to-pointer instead of value-to-optional. The generated SIL looks
+// quite different in that case, involving calls to intrinsics. Make sure we pick the
+// value-to-optional conversion, because it generates much simpler SIL:
+
+// CHECK-LABEL: sil private [ossa] @$s28ambiguous_pointer_conversion4test7buffersySayAA3MyVCG_tFySPyAA5ImageCGXEfU_yAIXEfU_yAIXEfU_ : $@convention(thin) @substituted <τ_0_0, τ_0_1, τ_0_2> (UnsafePointer<τ_0_0>, UnsafePointer<Image>, UnsafePointer<Image>) -> (@out τ_0_2, @error_indirect τ_0_1) for <Image, Never, ()> {
+// CHECK: bb0(%0 : $*(), %1 : $*Never, %2 : $UnsafePointer<Image>, %3 : @closureCapture $UnsafePointer<Image>, %4 : @closureCapture $UnsafePointer<Image>):
+// CHECK: [[X:%.*]] = enum $Optional<UnsafePointer<Image>>, #Optional.some!enumelt, %3
+// CHECK: [[Y:%.*]] = enum $Optional<UnsafePointer<Image>>, #Optional.some!enumelt, %4
+// CHECK: [[Z:%.*]] = enum $Optional<UnsafePointer<Image>>, #Optional.some!enumelt, %2
+// CHECK: [[FN:%.*]] = function_ref @$s28ambiguous_pointer_conversion8takesPtryySPyAA5ImageCGSg_A2FtF : $@convention(thin) (Optional<UnsafePointer<Image>>, Optional<UnsafePointer<Image>>, Optional<UnsafePointer<Image>>) -> ()
+// CHECK: apply %11([[X]], [[Y]], [[Z]]) : $@convention(thin) (Optional<UnsafePointer<Image>>, Optional<UnsafePointer<Image>>, Optional<UnsafePointer<Image>>) -> ()
+// CHECK: return
+