RequirementMachine: Concrete contraction gives up on recursive same-type and superclass requirements

A requirement of the form T == G<T> is never valid, and T == G<T.A>
will leave behind a term T.[P:A] once the conformance T : P is
dropped in contraction. So just ignore recursive requirements
here, allowing them to be properly diagnosed later.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -164,6 +164,13 @@ static Type stripBoundDependentMemberTypes(Type t) {
   return t;
 }
 
+/// Returns true if \p lhs appears inside \p rhs.
+static bool typeOccursIn(Type lhs, Type rhs) {
+  return rhs.findIf([lhs](Type t) -> bool {
+    return t->isEqual(lhs);
+  });
+}
+
 namespace {
 
 /// Utility class to store some shared state.
@@ -550,6 +557,15 @@ bool ConcreteContraction::performConcreteContraction(
         break;
 
       subjectType = stripBoundDependentMemberTypes(subjectType);
+      if (typeOccursIn(subjectType,
+                       stripBoundDependentMemberTypes(constraintType))) {
+        if (Debug) {
+          llvm::dbgs() << "@ Subject type of same-type requirement "
+                       << subjectType << " == " << constraintType << " "
+                       << "occurs in the constraint type, skipping\n";
+        }
+        break;
+      }
       ConcreteTypes[subjectType->getCanonicalType()].insert(constraintType);
       break;
     }
@@ -559,6 +575,15 @@ bool ConcreteContraction::performConcreteContraction(
              "You forgot to call desugarRequirement()");
 
       subjectType = stripBoundDependentMemberTypes(subjectType);
+      if (typeOccursIn(subjectType,
+                       stripBoundDependentMemberTypes(constraintType))) {
+        if (Debug) {
+          llvm::dbgs() << "@ Subject type of superclass requirement "
+                       << subjectType << " : " << constraintType << " "
+                       << "occurs in the constraint type, skipping\n";
+        }
+        break;
+      }
       Superclasses[subjectType->getCanonicalType()].insert(constraintType);
       break;
     }