[Sema][GSB] Fix crash on cond conformances with invalid req

Fixes crashes related to retrieving a generic signature that is currently being built.
This happens when a conditional conformance has a invalid requirement where the subject
is the 'self' type and the constraint is a protocol to which 'self' already conforms.
The most trivial case is when this latter conformance is the conditional conformance itself.

Author: AnthonyLatsis

lib/AST/GenericSignatureBuilder.cpp

@@ -4721,12 +4721,21 @@ ConstraintResult GenericSignatureBuilder::addTypeRequirement(
         // type that makes sense to use here, but, in practice, all
         // getLookupConformanceFns used in here don't use that parameter anyway.
         auto dependentType = CanType();
-        auto conformance =
-            getLookupConformanceFn()(dependentType, subjectType, proto->getDecl());
 
-        // FIXME: diagnose if there's no conformance.
-        if (conformance) {
-          addConditionalRequirements(*this, *conformance, inferForModule);
+        auto D = getGenericParams().front()->getDecl();
+        auto DC = D ? D->getDeclContext() : nullptr;
+        auto NTD = DC ? DC->getAsNominalTypeOrNominalTypeExtensionContext()
+                      : nullptr;
+        auto subjectNTD = subjectType->getAnyNominal();
+
+        if (!NTD || !subjectNTD || NTD != subjectNTD) {
+          auto conformance =
+            getLookupConformanceFn()(dependentType, subjectType,
+                                     proto->getDecl());
+          // FIXME: diagnose if there's no conformance.
+          if (conformance) {
+            addConditionalRequirements(*this, *conformance, inferForModule);
+          }
         }
       }
     }

test/decl/ext/generic.swift

@@ -144,6 +144,23 @@ extension GenericClass where Self : P3 { }
 // expected-error@-1{{'Self' is only available in a protocol or as the result of a method in a class; did you mean 'GenericClass'?}} {{30-34=GenericClass}}
 // expected-error@-2{{type 'GenericClass<T>' in conformance requirement does not refer to a generic parameter or associated type}}
 
+protocol Prot1 {}
+protocol Prot2 {}
+protocol Prot3 {}
+protocol Prot4 {}
+protocol Prot5 {}
+
+extension GenericClass: Prot1 where T: Prot1 {}
+extension GenericClass: Prot2 where Self: Prot1 {}
+// expected-error@-1 {{'Self' is only available in a protocol or as the result of a method in a class; did you mean 'GenericClass'?}}
+// expected-error@-2 {{type 'GenericClass<T>' in conformance requirement does not refer to a generic parameter or associated type}}
+extension GenericClass: Prot3 where GenericClass<T>: Prot1 {}
+// expected-error @-1 {{type 'GenericClass<T>' in conformance requirement does not refer to a generic parameter or associated type}}
+extension GenericClass: Prot4 where GenericClass<Int>: Prot1 {}
+// expected-error @-1 {{type 'GenericClass<Int>' in conformance requirement does not refer to a generic parameter or associated type}}
+extension GenericClass: Prot5 where GenericClass: Prot5 {}
+// expected-error @-1 {{type 'GenericClass<T>' in conformance requirement does not refer to a generic parameter or associated type}}
+
 protocol P4 {
   associatedtype T
   init(_: T)