GSB: When merging two equivalence classes, don't forget to re-process delayed requirements

lib/AST/GenericSignatureBuilder.cpp

@@ -4926,8 +4926,11 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenTypeParameters(
   if (equivClass2)
     equivClass->modified(*this);
 
-  // Same-type requirements.
+  // Same-type requirements, delayed requirements.
   if (equivClass2) {
+    Impl->DelayedRequirements.append(equivClass2->delayedRequirements.begin(),
+                                     equivClass2->delayedRequirements.end());
+
     equivClass->sameTypeConstraints.insert(
                                    equivClass->sameTypeConstraints.end(),
                                    equivClass2->sameTypeConstraints.begin(),

test/Generics/same_type_constraints.swift

@@ -393,3 +393,33 @@ func testP12a<T: P12>(_: T) where T.A == X12<Int>, T.A == X12<T.B>, T.B == Int {
 func testP12b<T: P12>(_: T) where T.B == Int, T.A == X12<T.B>, X12<T.B> == T.A { }
 // expected-warning@-1{{redundant same-type constraint 'T.A' == 'X12<Int>'}}
 // expected-note@-2{{same-type constraint 'T.A' == 'X12<Int>' written here}}
+
+// rdar://45307061 - dropping delayed same-type constraints when merging
+// equivalence classes
+
+protocol FakeIterator {
+  associatedtype Element
+}
+
+protocol FakeSequence {
+  associatedtype Iterator : FakeIterator
+  associatedtype Element where Iterator.Element == Element
+}
+
+protocol ObserverType {
+  associatedtype E
+}
+
+struct Bad<S: FakeSequence, O> where S.Element : ObserverType, S.Element.E == O {}
+
+func good<S: FakeSequence, O>(_: S, _: O) where S.Element : ObserverType, O == S.Element.E {
+  _ = Bad<S, O>()
+}
+
+func bad<S: FakeSequence, O>(_: S, _: O) where S.Element : ObserverType, O == S.Iterator.Element.E {
+  _ = Bad<S, O>()
+}
+
+func ugly<S: FakeSequence, O>(_: S, _: O) where S.Element : ObserverType, O == S.Iterator.Element.E, O == S.Element.E {
+  _ = Bad<S, O>()
+}