[CSSimplify] Detect and fix implicit Double <-> CGFloat conversion via optional chaining

Passing Double? to CGFloat? and vice versa is not supported but
there was one case which solver still allowed to get through -
optional chains, which is not fixed.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5485,6 +5485,19 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
             SmallVector<LocatorPathElt, 4> path;
             auto anchor = location.getLocatorParts(path);
 
+            // An attempt at Double/CGFloat conversion through
+            // optional chaining. This is not supported at the
+            // moment because solution application doesn't know
+            // how to map Double to/from CGFloat through optionals.
+            if (isExpr<OptionalEvaluationExpr>(anchor)) {
+              if (!shouldAttemptFixes())
+                return getTypeMatchFailure(locator);
+
+              conversionsOrFixes.push_back(ContextualMismatch::create(
+                  *this, nominal1, nominal2, getConstraintLocator(locator)));
+              break;
+            }
+
             // Drop all of the applied `value-to-optional` promotions.
             path.erase(llvm::remove_if(
                            path,