[ConstraintSystem] NFC: Pass an underlying declaration to Solution::computeSubstitutions

This is going to be used to determine whether the substitutions are
computed for `DistributedActorSystem::remoteCall` and adjust the
generic signature with witness conformance requirements.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -1653,11 +1653,15 @@ class Solution {
   /// Compute the set of substitutions for a generic signature opened at the
   /// given locator.
   ///
+  /// \param decl The underlying declaration for which the substitutions are
+  /// computed.
+  ///
   /// \param sig The generic signature.
   ///
   /// \param locator The locator that describes where the substitutions came
   /// from.
-  SubstitutionMap computeSubstitutions(GenericSignature sig,
+  SubstitutionMap computeSubstitutions(NullablePtr<ValueDecl> decl,
+                                       GenericSignature sig,
                                        ConstraintLocator *locator) const;
 
   /// Resolves the contextual substitutions for a reference to a declaration

lib/Sema/CSApply.cpp

@@ -82,7 +82,8 @@ static bool isOpenedAnyObject(Type type) {
 }
 
 SubstitutionMap
-Solution::computeSubstitutions(GenericSignature sig,
+Solution::computeSubstitutions(NullablePtr<ValueDecl> decl,
+                               GenericSignature sig,
                                ConstraintLocator *locator) const {
   if (sig.isNull())
     return SubstitutionMap();
@@ -147,7 +148,7 @@ Solution::resolveConcreteDeclRef(ValueDecl *decl,
 
   // Get the generic signature of the decl and compute the substitutions.
   auto sig = decl->getInnermostDeclContext()->getGenericSignatureOfContext();
-  auto subst = computeSubstitutions(sig, locator);
+  auto subst = computeSubstitutions(decl, sig, locator);
 
   maybeInstantiateCXXMethodDefinition(decl);
 
@@ -7121,7 +7122,8 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
     auto opaqueLocator = solution.getConstraintSystem().getOpenOpaqueLocator(
         locator, opaqueDecl);
     SubstitutionMap substitutions = solution.computeSubstitutions(
-        opaqueDecl->getOpaqueInterfaceGenericSignature(), opaqueLocator);
+        opaqueDecl, opaqueDecl->getOpaqueInterfaceGenericSignature(),
+        opaqueLocator);
 
     // If we don't have substitutions, this is an opaque archetype from
     // another declaration being manipulated, and not an erasure of a

lib/Sema/TypeCheckProtocol.cpp

@@ -1231,8 +1231,8 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
     auto witnessSig =
       witness->getInnermostDeclContext()->getGenericSignatureOfContext();
     result.WitnessSubstitutions =
-      solution->computeSubstitutions(witnessSig, witnessLocator);
-    
+        solution->computeSubstitutions(witness, witnessSig, witnessLocator);
+
     return result;
   };