[Sema] Transfer contextual type info when we inject UnresolvedMemberChainResultExpr

If we create an UnresolvedMemberChainResultExpr at the top level, we may end up losing contextual type information that was already attached to the end of the chain. To avoid this, we fetch any existing contextual type info and transfer it to the newly injected expression.

Author: Jumhyn

lib/Sema/CSGen.cpp

@@ -3847,8 +3847,14 @@ namespace {
         if (auto *UME = getUnresolvedMemberChainBase(expr)) {
           CG.setUnresolvedChainBase(expr, UME);
           if (!parent || !isa<UnresolvedMemberChainResultExpr>(parent)) {
-            auto &context = CG.getConstraintSystem().getASTContext();
+            auto &cs = CG.getConstraintSystem();
+            auto &context = cs.getASTContext();
+            auto typeInfo = cs.getContextualTypeInfo(expr);
             expr = new (context) UnresolvedMemberChainResultExpr(expr);
+            if (typeInfo) {
+              cs.setContextualType(expr, (*typeInfo).typeLoc,
+                                   (*typeInfo).purpose);
+            }
           }
         }
       }