Add an API to map substitution map into a type expansion context.

Author: aschwaighofer

include/swift/AST/SubstitutionMap.h

@@ -20,6 +20,7 @@
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/ProtocolConformanceRef.h"
 #include "swift/AST/Type.h"
+#include "swift/AST/TypeExpansionContext.h"
 #include "swift/Basic/Debug.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMapInfo.h"
@@ -176,7 +177,13 @@ class SubstitutionMap {
   SubstitutionMap subst(TypeSubstitutionFn subs,
                         LookupConformanceFn conformances,
                         SubstOptions options=None) const;
-  
+
+  /// Apply type expansion lowering to all types in the substitution map. Opaque
+  /// archetypes will be lowered to their underlying types if the type expansion
+  /// context allows.
+  SubstitutionMap mapIntoTypeExpansionContext(
+      TypeExpansionContext context) const;
+
   /// Create a substitution map for a protocol conformance.
   static SubstitutionMap
   getProtocolSubstitutions(ProtocolDecl *protocol,

lib/AST/SubstitutionMap.cpp

@@ -715,3 +715,11 @@ bool SubstitutionMap::isIdentity() const {
 
   return !hasNonIdentityReplacement;
 }
+
+SubstitutionMap SubstitutionMap::mapIntoTypeExpansionContext(
+    TypeExpansionContext context) const {
+  ReplaceOpaqueTypesWithUnderlyingTypes replacer(
+      context.getContext(), context.getResilienceExpansion(),
+      context.isWholeModuleContext());
+  return this->subst(replacer, replacer, SubstFlags::SubstituteOpaqueArchetypes);
+}