[ConstraintSystem] Use getNumApplications + ValueDecl::getNumCurryLevels to detect partial applications

We used to detect partial applications based on member locators
and parent expressions, but using function reference kind +
check to see if self is applied is such simpler and hits both
uses of `isPartialApplication`.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -6558,6 +6558,11 @@ Type getPatternTypeOfSingleUnlabeledPackExpansionTuple(Type type);
 /// Check whether this is a reference to one of the special result builder
 /// methods prefixed with `build*` i.e. `buildBlock`, `buildExpression` etc.
 bool isResultBuilderMethodReference(ASTContext &, UnresolvedDotExpr *);
+
+/// Determine the number of applications applied to the given overload.
+unsigned getNumApplications(ValueDecl *decl, bool hasAppliedSelf,
+                            FunctionRefKind functionRefKind);
+
 } // end namespace constraints
 
 template<typename ...Args>

lib/Sema/CSSimplify.cpp

@@ -10293,15 +10293,16 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       if (!Context.getProtocol(KnownProtocolKind::Sendable))
         return false;
 
-      // Static members are always sendable because they only capture
-      // metatypes which are Sendable.
-      if (baseObjTy->is<AnyMetatypeType>())
-        return false;
+      return llvm::any_of(lookup, [&](const auto &result) {
+        auto decl = result.getValueDecl();
+        if (!isa_and_nonnull<FuncDecl>(decl))
+          return false;
 
-      return isPartialApplication(memberLocator) &&
-             llvm::any_of(lookup, [&](const auto &result) {
-               return isa_and_nonnull<FuncDecl>(result.getValueDecl());
-             });
+        auto hasAppliedSelf = decl->hasCurriedSelf() &&
+                              doesMemberRefApplyCurriedSelf(baseObjTy, decl);
+        return getNumApplications(decl, hasAppliedSelf, functionRefKind) <
+               decl->getNumCurryLevels();
+      });
     };
 
     if (shouldCheckSendabilityOfBase()) {
@@ -10945,7 +10946,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
 
         return simplifyValueWitnessConstraint(
             ConstraintKind::ValueWitness, baseTy, makeIterator, memberTy, useDC,
-            FunctionRefKind::Compound, flags, locator);
+            FunctionRefKind::SingleApply, flags, locator);
       }
 
       // Handle `next` reference.
@@ -10963,7 +10964,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
 
         return simplifyValueWitnessConstraint(
             ConstraintKind::ValueWitness, baseTy, next, memberTy, useDC,
-            FunctionRefKind::Compound, flags, locator);
+            FunctionRefKind::SingleApply, flags, locator);
       }
     }
   }

lib/Sema/ConstraintSystem.cpp

@@ -1655,8 +1655,8 @@ static unsigned getNumRemovedArgumentLabels(ValueDecl *decl,
 }
 
 /// Determine the number of applications
-static unsigned getNumApplications(
-    ValueDecl *decl, bool hasAppliedSelf, FunctionRefKind functionRefKind) {
+unsigned constraints::getNumApplications(ValueDecl *decl, bool hasAppliedSelf,
+                                         FunctionRefKind functionRefKind) {
   switch (functionRefKind) {
   case FunctionRefKind::Unapplied:
   case FunctionRefKind::Compound:
@@ -1774,7 +1774,7 @@ FunctionType *ConstraintSystem::adjustFunctionTypeForConcurrency(
           adjustedTy =
               adjustedTy->withExtInfo(adjustedTy->getExtInfo().withSendable());
         }
-      } else if (isPartialApplication(getConstraintLocator(locator))) {
+      } else if (numApplies < decl->getNumCurryLevels()) {
         // Operators on protocols could be found via unqualified lookup and
         // won't have a base type.
         if (decl->isOperator() ||

test/Concurrency/sendable_methods.swift

@@ -294,3 +294,15 @@ do {
 
   test(<) // Ok
 }
+
+// Partially applied instance method
+do {
+  struct S {
+    func foo() {}
+  }
+
+  func bar(_ x: @Sendable () -> Void) {}
+
+  let fn = S.foo(S())
+  bar(fn) // Ok
+}