[Diagnostics] Special case assignment between nominal types with optional promotion

Skip fixing situation where source and destination of assignment are both
nominal types with different optionality until restriction is attempted.

Otherwise fix could be too greedy and diagnose valid code if all of the
types are known in advance.

Resolves: SR-13951
Resolves: rdar://problem/72166791

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -3722,14 +3722,15 @@ bool ConstraintSystem::repairFailures(
         if (lhs->is<FunctionType>() && rhs->is<FunctionType>())
           return false;
 
-        // If either object type is a bound generic or existential it means
-        // that follow-up to value-to-optional is going to be:
+        // If either object type is a generic, nominal or existential type
+        // it means that follow-up to value-to-optional is going to be:
         //
         // 1. "deep equality" check, which is handled by generic argument(s)
-        //    mismatch fix, or
+        //    or contextual mismatch fix, or
         // 2. "existential" check, which is handled by a missing conformance
         //    fix.
         if ((lhs->is<BoundGenericType>() && rhs->is<BoundGenericType>()) ||
+            (lhs->is<NominalType>() && rhs->is<NominalType>()) ||
             rhs->isAnyExistentialType())
           return false;
       }

test/Constraints/sr13951.swift

@@ -0,0 +1,37 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol TheProtocol {}
+struct TheType1: TheProtocol {}
+
+enum TheEnum: RawRepresentable {
+  typealias RawValue = TheProtocol
+
+  case case1
+  case case2
+
+  init?(rawValue: TheProtocol) {
+    self = .case1
+  }
+
+  var rawValue: TheProtocol {
+    return TheType1()
+  }
+}
+
+func aTransformer(input: Int) -> TheEnum {
+  if input % 2 == 0 {
+    return .case1
+  } else {
+    return .case2
+  }
+}
+
+func theProblem(input: Int?) {
+  var enumValue: TheEnum?
+
+  if let input = input {
+    enumValue = aTransformer(input: input) // Ok
+  }
+
+  _ = enumValue // To silence the warning
+}