[Sema] Type inference from default expression doesn’t compile in free function (swiftlang#79989)

This is an attempt to solving
swiftlang#72199.
As I've already lined out in the issue, this seems to be a two-fold
problem.

The first one was a rather easy fix, as I've seen similar approaches in
different parts of the codebase. It is pretty much just un-currying the
generic function.

The second problem was, that `DependentMemberType`s were counted towards
the generic-parameter-should-only-be-mentioned-once-limit even though
you cannot infer the generic type from the dependent member type.

Author: ph1ps

lib/Sema/TypeCheckConstraints.cpp

@@ -639,17 +639,26 @@ Type TypeChecker::typeCheckParameterDefault(Expr *&defaultValue,
   // Check whether generic parameters are only mentioned once in
   // the anchor's signature.
   {
-    auto anchorTy = anchor->getInterfaceType()->castTo<GenericFunctionType>();
+    auto anchorTy = anchor->getInterfaceType()->castTo<AnyFunctionType>();
 
-    // Reject if generic parameters are used in multiple different positions
-    // in the parameter list.
+    if (anchor->hasCurriedSelf())
+      anchorTy = anchorTy->getResult()->castTo<AnyFunctionType>();
+
+    SmallPtrSet<CanType, 4> inferrableParams;
+    collectReferencedGenericParams(paramInterfaceTy, inferrableParams);
 
     llvm::SmallVector<unsigned, 2> affectedParams;
     for (unsigned i : indices(anchorTy->getParams())) {
-      const auto &param = anchorTy->getParams()[i];
 
-      if (containsTypes(param.getPlainType()))
-        affectedParams.push_back(i);
+      SmallPtrSet<CanType, 4> referencedGenericParams;
+      collectReferencedGenericParams(anchorTy->getParams()[i].getPlainType(), referencedGenericParams);
+
+      for (auto can : referencedGenericParams) {
+        if (can->is<GenericTypeParamType>() && inferrableParams.contains(can)) {
+          affectedParams.push_back(i);
+          break;
+        }
+      }
     }
 
     if (affectedParams.size() > 1) {

lib/Sema/TypeCheckGeneric.cpp

@@ -313,31 +313,15 @@ void TypeChecker::checkProtocolSelfRequirements(ValueDecl *decl) {
   }
 }
 
-/// All generic parameters of a generic function must be referenced in the
-/// declaration's type, otherwise we have no way to infer them.
-void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
-  // Don't do this check for accessors: they're not used directly, so we
-  // never need to infer their generic arguments.  This is mostly a
-  // compile-time optimization, but it also avoids problems with accessors
-  // like 'read' and 'modify' that would arise due to yields not being
-  // part of the formal type.
-  if (isa<AccessorDecl>(dc))
-    return;
-
-  auto *genericParams = dc->getGenericParams();
-  auto genericSig = dc->getGenericSignatureOfContext();
-  if (!genericParams)
-    return;
-
-  auto *decl = cast<ValueDecl>(dc->getInnermostDeclarationDeclContext());
+void TypeChecker::collectReferencedGenericParams(Type ty, SmallPtrSet<CanType, 4> &referenced) {
 
   // A helper class to collect referenced generic type parameters
   // and dependent member types.
   class ReferencedGenericTypeWalker : public TypeWalker {
-    SmallPtrSet<CanType, 4> ReferencedGenericParams;
+    SmallPtrSet<CanType, 4> &ReferencedGenericParams;
 
   public:
-    ReferencedGenericTypeWalker() {}
+    ReferencedGenericTypeWalker(SmallPtrSet<CanType, 4> &referenced) : ReferencedGenericParams(referenced) {}
     Action walkToTypePre(Type ty) override {
       // Find generic parameters or dependent member types.
       // Once such a type is found, don't recurse into its children.
@@ -368,24 +352,39 @@ void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
 
       return Action::Continue;
     }
-
-    SmallPtrSetImpl<CanType> &getReferencedGenericParams() {
-      return ReferencedGenericParams;
-    }
   };
 
+  ty.walk(ReferencedGenericTypeWalker(referenced));
+}
+
+/// All generic parameters of a generic function must be referenced in the
+/// declaration's type, otherwise we have no way to infer them.
+void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
+  // Don't do this check for accessors: they're not used directly, so we
+  // never need to infer their generic arguments.  This is mostly a
+  // compile-time optimization, but it also avoids problems with accessors
+  // like 'read' and 'modify' that would arise due to yields not being
+  // part of the formal type.
+  if (isa<AccessorDecl>(dc))
+    return;
+
+  auto *genericParams = dc->getGenericParams();
+  auto genericSig = dc->getGenericSignatureOfContext();
+  if (!genericParams)
+    return;
+
+  auto *decl = cast<ValueDecl>(dc->getInnermostDeclarationDeclContext());
+
+  // Set of generic params referenced in parameter types,
+  // return type or requirements.
+  SmallPtrSet<CanType, 4> referencedGenericParams;
+
   // Collect all generic params referenced in parameter types and
   // return type.
-  ReferencedGenericTypeWalker paramsAndResultWalker;
   auto *funcTy = decl->getInterfaceType()->castTo<GenericFunctionType>();
   for (const auto &param : funcTy->getParams())
-    param.getPlainType().walk(paramsAndResultWalker);
-  funcTy->getResult().walk(paramsAndResultWalker);
-
-  // Set of generic params referenced in parameter types,
-  // return type or requirements.
-  auto &referencedGenericParams =
-      paramsAndResultWalker.getReferencedGenericParams();
+    collectReferencedGenericParams(param.getPlainType(), referencedGenericParams);
+  collectReferencedGenericParams(funcTy->getResult(), referencedGenericParams);
 
   // Check if at least one of the generic params in the requirement refers
   // to an already referenced generic parameter. If this is the case,
@@ -409,13 +408,11 @@ void TypeChecker::checkReferencedGenericParams(GenericContext *dc) {
     }
 
     // Collect generic parameter types referenced by types used in a requirement.
-    ReferencedGenericTypeWalker walker;
+    SmallPtrSet<CanType, 4> genericParamsUsedByRequirementTypes;
     if (first && first->hasTypeParameter())
-      first.walk(walker);
+      collectReferencedGenericParams(first, genericParamsUsedByRequirementTypes);
     if (second && second->hasTypeParameter())
-      second.walk(walker);
-    auto &genericParamsUsedByRequirementTypes =
-        walker.getReferencedGenericParams();
+      collectReferencedGenericParams(second, genericParamsUsedByRequirementTypes);
 
     // If at least one of the collected generic types or a root generic
     // parameter of dependent member types is known to be referenced by

lib/Sema/TypeChecker.h

@@ -496,6 +496,8 @@ std::optional<BraceStmt *> applyResultBuilderBodyTransform(FuncDecl *func,
 
 bool typeCheckTapBody(TapExpr *expr, DeclContext *DC);
 
+void collectReferencedGenericParams(Type ty, SmallPtrSet<CanType, 4> &referenced);
+
 Type typeCheckParameterDefault(Expr *&defaultValue, DeclContext *DC,
                                Type paramType, bool isAutoClosure,
                                bool atCallerSide);

test/Concurrency/sendable_keypaths.swift

@@ -22,7 +22,7 @@ class NonSendable : Hashable {
 final class CondSendable<T> : Hashable {
   init(_: T) {}
   init(_: Int) {}
-  init(_: T, other: T = 42) {}
+  init<U>(_: T, other: U = 42) {}
   init<Q>(_: [Q] = []) {}
 
   static func == (x: CondSendable, y: CondSendable) -> Bool { false }

test/Constraints/type_inference_from_default_exprs.swift

@@ -168,6 +168,11 @@ func main() {
   takesCircle(.init()) // Ok
   takesRectangle(.init())
   // expected-error@-1 {{cannot convert default value of type 'Rectangle' to expected argument type 'Circle' for parameter #0}}
+  
+  testS72199_2(x: 0)
+  testS72199_3(xs: 0, 0)
+  testS72199_4(x: 0)
+  testS72199_5(x: 0)
 }
 
 func test_magic_defaults() {
@@ -269,3 +274,19 @@ do {
 
 func testInferenceFromClosureVarInvalid<T>(x: T = { let x = "" as Int; return x }()) {}
 // expected-error@-1 {{cannot convert value of type 'String' to type 'Int' in coercion}}
+
+// https://github.com/swiftlang/swift/issues/72199
+enum S72199_1 {
+  func testS72199_1<T>(_: T = 42, _: [T]) {}
+  // expected-error@-1 {{cannot use default expression for inference of 'T' because it is inferrable from parameters #0, #1}}
+}
+
+func testS72199_2<T: P>(x: T.X, y: T = S()) { } // Ok
+func testS72199_3<each T: P>(xs: repeat (each T).X, ys: (repeat each T) = (S(), S())) {} // Ok
+
+typealias S72199_4<T> = Int
+func testS72199_4<T>(x: S72199_4<T>, y: T = "") {} // Ok
+
+func testS72199_5<T: P>(x: T.X, y: (T, T.X) = (S(), 0)) {} // Ok
+func testS72199_6<T: P>(x: T, y: (T, T.X) = (S(), 0)) {}
+// expected-error@-1 {{cannot use default expression for inference of '(T, T.X)' because it is inferrable from parameters #0, #1}}