[CSSimplify] Increase impact of a generic argument mismatch if mismatch is contextual

If generic arguments mismatch ends up being recorded on the result
of the chain or `try` expression it means that there is a contextual
conversion mismatch.

For optional conversions the solver currently generates a disjunction
with two choices - bind and optional-to-optional conversion which is
anchored on the contextual expression. If we can get a fix recorded
there that would result in a better diagnostic. It's only possible
for optional-to-optional choice because it doesn't bind the
variable immediately, so we need to downgrade direct fixes to prevent
`bind` choice from considered better.

Author: xedin

include/swift/Sema/CSFix.h

@@ -560,6 +560,11 @@ class ConstraintFix {
     return false;
   }
 
+  template <typename E>
+  bool directlyAt() const {
+    return getLocator()->directlyAt<E>();
+  }
+
   void print(llvm::raw_ostream &Out) const;
 
   SWIFT_DEBUG_DUMP;

lib/Sema/CSSimplify.cpp

@@ -15696,6 +15696,22 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     unsigned impact = 1;
     if (type1->isMarkerExistential() || type2->isMarkerExistential())
       ++impact;
+
+    // If generic arguments mismatch ends up being recorded on the result
+    // of the chain or a try expression it means that there is a contextual 
+    // conversion mismatch.
+    //
+    // For optional conversions the solver currently generates a disjunction
+    // with two choices - bind and optional-to-optional conversion which is
+    // anchored on the contextual expression. If we can get a fix recorded
+    // there that would result in a better diagnostic. It's only possible
+    // for optional-to-optional choice because it doesn't bind the
+    // variable immediately, so we need to downgrade direct fixes to prevent
+    // `bind` choice from considered better.
+    if (fix->directlyAt<OptionalEvaluationExpr>() ||
+        fix->directlyAt<AnyTryExpr>())
+      impact += 2;
+
     return recordFix(fix, impact) ? SolutionKind::Error : SolutionKind::Solved;
   }