RequirementMachine: Only add a concrete type adjustment step if the symbol has substitutions

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystemCompletion.cpp

@@ -450,7 +450,9 @@ RewriteSystem::computeCriticalPair(ArrayRef<Symbol>::const_iterator from,
     // perform the concrete type adjustment:
     //
     //     (σ - T)
-    if (xv.back().isSuperclassOrConcreteType() && t.size() > 0) {
+    if (xv.back().isSuperclassOrConcreteType() &&
+        !xv.back().getSubstitutions().empty() &&
+        t.size() > 0) {
       path.add(RewriteStep::forAdjustment(t.size(), /*inverse=*/true));
 
       xv.back() = xv.back().prependPrefixToConcreteSubstitutions(