[ConstraintSystem] Allow Double<->CGFloat conversions only if they are AST representable

Allow Double <-> CGFloat conversion if the associated locator is
simplifiable down to a AST node (e.g. expression) this limits
applications of such conversions to e.g. argument-to-parameter,
contextual conversions and forbids conversions in generic arguments,
function result, collection elements etc.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5087,6 +5087,12 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
           nominal1->getDecl() != nominal2->getDecl() &&
           ((nominal1->isCGFloatType() || nominal2->isCGFloatType()) &&
            (nominal1->isDoubleType() || nominal2->isDoubleType()))) {
+        // Support implicit Double<->CGFloat conversions only for
+        // something which could be directly represented in the AST
+        // e.g. argument-to-parameter, contextual conversions etc.
+        if (!locator.trySimplifyToExpr())
+          return getTypeMatchFailure(locator);
+
         SmallVector<LocatorPathElt, 4> path;
         auto anchor = locator.getLocatorParts(path);