GSB: Handle unresolved member types with a concrete base in substituteConcreteType()

Type::subst() does not support DependentMemberTypes that only have
an identifier and not an associated type. Instead, when we hit one
of these, replace the generic parameter at the root with its
replacement type only, then rely on maybeResolveEquivalenceClass()
getting called again, which will perform the name lookup to resolve
the member type.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -3829,6 +3829,22 @@ static Type substituteConcreteType(Type parentType,
   auto subMap = parentType->getContextSubstitutionMap(
       dc->getParentModule(), dc);
 
+  // If the type has unresolved DependentMemberTypes, we can't use
+  // Type::subst() if a generic parameter is replaced with a concrete
+  // type. Instead, perform a "shallow" substitution where we replace
+  // generic parameter types but leave DependentMemberTypes as-is.
+  // This means we will end up back in maybeResolveEquivalenceClass(),
+  // where we will perform the name lookup required to resolve any
+  // DependentMemberTypes with a concrete base.
+  if (type->findUnresolvedDependentMemberType()) {
+    return type.transform([&](Type t) {
+      if (t->is<GenericTypeParamType>()) {
+        return t.subst(subMap);
+      }
+      return t;
+    });
+  }
+
   return type.subst(subMap);
 }