AST: Work around shortcoming in abstract conformance representation when computing override substitutions

Fixes rdar://problem/98404650.

Author: slavapestov

lib/AST/SubstitutionMap.cpp

@@ -534,10 +534,25 @@ OverrideSubsInfo::OverrideSubsInfo(const NominalTypeDecl *baseNominal,
   if (auto baseNominalSig = baseNominal->getGenericSignature()) {
     BaseDepth = baseNominalSig.getGenericParams().back()->getDepth() + 1;
 
+    auto *genericEnv = derivedNominal->getGenericEnvironment();
     auto derivedNominalTy = derivedNominal->getDeclaredInterfaceType();
+
+    // FIXME: Map in and out of context to get more accurate
+    // conformance information. If the base generic signature
+    // is <T: P> and the derived generic signature is <T: C>
+    // where C is a class that conforms to P, then we want the
+    // substitution map to store the concrete conformance C: P
+    // and not the abstract conformance T: P.
+    if (genericEnv) {
+      derivedNominalTy = genericEnv->mapTypeIntoContext(
+          derivedNominalTy);
+    }
+
     BaseSubMap = derivedNominalTy->getContextSubstitutionMap(
-        baseNominal->getParentModule(), baseNominal);
-    assert(!BaseSubMap.hasArchetypes());
+        baseNominal->getParentModule(), baseNominal,
+        genericEnv);
+
+    BaseSubMap = BaseSubMap.mapReplacementTypesOutOfContext();
   }
 
   if (auto derivedNominalSig = derivedNominal->getGenericSignature())

test/SILGen/default_arguments_inherited_generic.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+class Base<T: P> {
+  init(x: Int = 0) {}
+}
+
+class Derived<T: C> : Base<T> {}
+
+protocol P {}
+class C: P {}
+
+_ = Derived<C>()
+
+// CHECK-LABEL: sil [ossa] @main : $@convention(c) (Int32, UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>) -> Int32 {
+// CHECK: [[FN:%.*]] = function_ref @$s35default_arguments_inherited_generic4BaseC1xACyxGSi_tcfcfA_ : $@convention(thin) <τ_0_0 where τ_0_0 : P> () -> Int
+// CHECK: apply [[FN]]<C>() : $@convention(thin) <τ_0_0 where τ_0_0 : P> () -> Int