[Constraint solver] Ignore type declarations in "common type" computation.

Type declarations are handled somewhat specially by the constraint
solver when applied, so temporarily exclude them from the "common
type" computation.

Author: DougGregor

lib/Sema/ConstraintSystem.cpp

@@ -1484,6 +1484,10 @@ static Type getEffectiveOverloadType(const OverloadChoice &overload) {
 
   auto decl = overload.getDecl();
 
+  // Ignore type declarations.
+  if (isa<TypeDecl>(decl))
+    return Type();
+
   // Retrieve the interface type.
   auto type = decl->getInterfaceType();
   if (!type) {