Sema: Compute the requirement signature before visiting protocol members

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2650,6 +2650,12 @@ public:
 
     TypeChecker::checkDeclAttributes(PD);
 
+    // Explicity compute the requirement signature to detect errors.
+    // Do this before visiting members, to avoid a request cycle if
+    // a member referenecs another declaration whose generic signature
+    // has a conformance requirement to this protocol.
+    auto reqSig = PD->getRequirementSignature().getRequirements();
+
     // Check the members.
     for (auto Member : PD->getMembers())
       visit(Member);
@@ -2663,9 +2669,6 @@ public:
       if (!SF || SF->Kind != SourceFileKind::Interface)
         TypeChecker::inferDefaultWitnesses(PD);
 
-    // Explicity compute the requirement signature to detect errors.
-    auto reqSig = PD->getRequirementSignature().getRequirements();
-
     if (PD->getASTContext().TypeCheckerOpts.DebugGenericSignatures) {
       auto requirementsSig =
         GenericSignature::get({PD->getProtocolSelfType()}, reqSig);

test/Generics/protocol_typealias_cycle_1a.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  associatedtype X
+  associatedtype Y where Y : Q
+}
+
+protocol Q {
+  associatedtype T
+}
+
+struct S: Q {
+  typealias T = Int
+}
+
+extension P where X == () {
+  typealias Y = S
+}