AST: Track platform unavailability and introduction as separate constraints

Potential unavailability of a declaration is always diagnosed in a context that
does not have a sufficient platform introduction constraint, even if that
context is also unavailable on that platform. This behavior is overly strict,
since the potential unavailability will never matter, but it's a longstanding
quirk of availability checking. As a result, some source code has been written
to work around this quirk by marking declarations as simultaneously unavailable
and introduced for a given platform:

```
@available(macOS, unavailable, introduced: 15)
func unavailableAndIntroducedInMacOS15() {
  // ... allowed to call functions introduced in macOS 15.
}
```

When availability checking was refactored to be based on a constraint engine in
swiftlang#79260, the compiler started effectively
treating `@available(macOS, unavailable, introduced: 15)` as just
`@available(macOS, unavailable)` because the introduction constraint was
treated as lower priority and therefore superseded by the unavailability
constraint. This caused a regression for the code that was written to work
around the availability checker's strictness.

We could try to match the behavior from previous releases, but it's actually
tricky to match the behavior well enough in the new availability checking
architecture to fully fix source compatibility. Consequently, it seems like the
best fix is actually to address this long standing issue and stop diagnosing
potential unavailability in unavailable contexts. The main risk of this
approach is source compatibility for regions of unavailable code. It's
theoretically possible that restricting available declarations by introduction
version in unavailable contexts is important to prevent ambiguities during
overload resolution in some codebases. If we find that is a problem that is too
prevalent, we may have to take a different approach.

Resolves rdar://147945883.

Author: tshortli

lib/AST/AvailabilityConstraint.cpp

@@ -113,25 +113,59 @@ DeclAvailabilityConstraints::getPrimaryConstraint() const {
   return result;
 }
 
+static bool canIgnoreConstraintInUnavailableContexts(
+    const Decl *decl, const AvailabilityConstraint &constraint) {
+  auto domain = constraint.getDomain();
+
+  switch (constraint.getReason()) {
+  case AvailabilityConstraint::Reason::UnconditionallyUnavailable:
+    // Always reject uses of universally unavailable declarations, regardless
+    // of context, since there are no possible compilation configurations in
+    // which they are available. However, make an exception for types and
+    // conformances, which can sometimes be awkward to avoid references to.
+    if (!isa<TypeDecl>(decl) && !isa<ExtensionDecl>(decl)) {
+      if (domain.isUniversal() || domain.isSwiftLanguage())
+        return false;
+    }
+    return true;
+
+  case AvailabilityConstraint::Reason::PotentiallyUnavailable:
+    switch (domain.getKind()) {
+    case AvailabilityDomain::Kind::Universal:
+    case AvailabilityDomain::Kind::SwiftLanguage:
+    case AvailabilityDomain::Kind::PackageDescription:
+    case AvailabilityDomain::Kind::Embedded:
+    case AvailabilityDomain::Kind::Custom:
+      return false;
+    case AvailabilityDomain::Kind::Platform:
+      // Platform availability only applies to the target triple that the
+      // binary is being compiled for. Since the same declaration can be
+      // potentially unavailable from a given context when compiling for one
+      // platform, but available from that context when compiling for a
+      // different platform, it is overly strict to enforce potential platform
+      // unavailability constraints in contexts that are unavailable to that
+      // platform.
+      return true;
+    }
+    return constraint.getDomain().isPlatform();
+
+  case AvailabilityConstraint::Reason::Obsoleted:
+  case AvailabilityConstraint::Reason::UnavailableForDeployment:
+    return false;
+  }
+}
+
 static bool
-isInsideCompatibleUnavailableDeclaration(const Decl *decl,
-                                         const SemanticAvailableAttr &attr,
-                                         const AvailabilityContext &context) {
+shouldIgnoreConstraintInContext(const Decl *decl,
+                                const AvailabilityConstraint &constraint,
+                                const AvailabilityContext &context) {
   if (!context.isUnavailable())
     return false;
 
-  if (!attr.isUnconditionallyUnavailable())
+  if (!canIgnoreConstraintInUnavailableContexts(decl, constraint))
     return false;
 
-  // Refuse calling universally unavailable functions from unavailable code,
-  // but allow the use of types.
-  auto domain = attr.getDomain();
-  if (!isa<TypeDecl>(decl) && !isa<ExtensionDecl>(decl)) {
-    if (domain.isUniversal() || domain.isSwiftLanguage())
-      return false;
-  }
-
-  return context.containsUnavailableDomain(domain);
+  return context.containsUnavailableDomain(constraint.getDomain());
 }
 
 /// Returns the `AvailabilityConstraint` that describes how \p attr restricts
@@ -218,8 +252,7 @@ static void getAvailabilityConstraintsForDecl(
   // declaration is unconditionally unavailable in a domain for which
   // the context is already unavailable.
   llvm::erase_if(constraints, [&](const AvailabilityConstraint &constraint) {
-    return isInsideCompatibleUnavailableDeclaration(decl, constraint.getAttr(),
-                                                    context);
+    return shouldIgnoreConstraintInContext(decl, constraint, context);
   });
 }
 

test/Sema/availability_scopes.swift

@@ -253,19 +253,19 @@ extension SomeClass {
 
 // CHECK-NEXT: {{^}}  (decl_implicit version=50 decl=extension.SomeClass
 // CHECK-NEXT: {{^}}    (decl version=50 unavailable=macOS decl=extension.SomeClass
-// CHECK-NEXT: {{^}}      (decl version=51 unavailable=macOS decl=functionWithStmtConditionsInUnavailableExt()
+// CHECK-NEXT: {{^}}      (decl version=50 unavailable=macOS decl=functionWithStmtConditionsInUnavailableExt()
 // CHECK-NEXT: {{^}}        (condition_following_availability version=52 unavailable=macOS
 // CHECK-NEXT: {{^}}          (condition_following_availability version=53 unavailable=macOS
 // CHECK-NEXT: {{^}}        (if_then version=53 unavailable=macOS
 // CHECK-NEXT: {{^}}          (condition_following_availability version=54 unavailable=macOS
 // CHECK-NEXT: {{^}}          (if_then version=54 unavailable=macOS
 // CHECK-NEXT: {{^}}            (condition_following_availability version=55 unavailable=macOS
-// CHECK-NEXT: {{^}}            (decl version=55 unavailable=macOS decl=funcInGuardElse()
+// CHECK-NEXT: {{^}}            (decl version=54 unavailable=macOS decl=funcInGuardElse()
 // CHECK-NEXT: {{^}}            (guard_fallthrough version=55 unavailable=macOS
 // CHECK-NEXT: {{^}}              (condition_following_availability version=56 unavailable=macOS
 // CHECK-NEXT: {{^}}              (guard_fallthrough version=56 unavailable=macOS
-// CHECK-NEXT: {{^}}          (decl version=57 unavailable=macOS decl=funcInInnerIfElse()
-// CHECK-NEXT: {{^}}        (decl version=53 unavailable=macOS decl=funcInOuterIfElse()
+// CHECK-NEXT: {{^}}          (decl version=53 unavailable=macOS decl=funcInInnerIfElse()
+// CHECK-NEXT: {{^}}        (decl version=50 unavailable=macOS decl=funcInOuterIfElse()
 @available(OSX, unavailable)
 extension SomeClass {
   @available(OSX 51, *)
@@ -401,7 +401,7 @@ extension SomeEnum {
 // CHECK-NEXT: {{^}}  (decl_implicit version=50 decl=extension.SomeEnum
 // CHECK-NEXT: {{^}}    (decl version=50 unavailable=macOS decl=extension.SomeEnum
 // CHECK-NEXT: {{^}}      (decl_implicit version=50 unavailable=macOS decl=availableMacOS_52
-// CHECK-NEXT: {{^}}        (decl version=52 unavailable=macOS decl=availableMacOS_52
+// CHECK-NEXT: {{^}}        (decl version=50 unavailable=macOS decl=availableMacOS_52
 // CHECK-NEXT: {{^}}      (decl version=50 unavailable=* decl=neverAvailable()
 
 @available(macOS, unavailable)

test/Sema/property_wrapper_availability.swift

@@ -99,7 +99,7 @@ struct UnavailableStruct {
   @UnavailableWrapper var unavailableInferred = S()
 
   @WrappedValueUnavailableOnMacOS var unavailableWrappedValue: S
-  @WrappedValueAvailable51 var wrappedValueAavailable51: S // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 var wrappedValueAavailable51: S
 }
 
 @available(macOS, unavailable)
@@ -117,7 +117,7 @@ struct UnavailableOnMacOSStruct {
   @UnavailableWrapper var unavailableInferred = S()
 
   @WrappedValueUnavailableOnMacOS var unavailableWrappedValue: S
-  @WrappedValueAvailable51 var wrappedValueAavailable51: S // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 var wrappedValueAavailable51: S
 }
 
 func alwaysAvailableFunc( // expected-note 4 {{add @available attribute to enclosing global function}}
@@ -160,14 +160,14 @@ func unavailableFunc(
   @DeprecatedWrapper _ deprecated: S,
   @UnavailableWrapper _ unavailable: S,
   @WrappedValueUnavailableOnMacOS _ unavailableWrappedValue: S,
-  @WrappedValueAvailable51 _ wrappedValueAavailable51: S // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 _ wrappedValueAavailable51: S
 ) {
   @AlwaysAvailableWrapper var alwaysAvailableLocal = S()
   @Available51Wrapper var available51Local = S()
   @DeprecatedWrapper var deprecatedLocal = S()
   @UnavailableWrapper var unavailableLocal = S()
   @WrappedValueUnavailableOnMacOS var unavailableWrappedValueLocal = S()
-  @WrappedValueAvailable51 var wrappedValueAavailable51 = S() // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 var wrappedValueAavailable51 = S()
 }
 
 @available(macOS, unavailable)
@@ -177,12 +177,12 @@ func unavailableOnMacOSFunc(
   @DeprecatedWrapper _ deprecated: S,
   @UnavailableWrapper _ unavailable: S,
   @WrappedValueUnavailableOnMacOS _ unavailableWrappedValue: S,
-  @WrappedValueAvailable51 _ wrappedValueAavailable51: S // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 _ wrappedValueAavailable51: S
 ) {
   @AlwaysAvailableWrapper var alwaysAvailableLocal = S()
   @Available51Wrapper var available51Local = S()
   @DeprecatedWrapper var deprecatedLocal = S()
   @UnavailableWrapper var unavailableLocal = S()
   @WrappedValueUnavailableOnMacOS var unavailableWrappedValueLocal = S()
-  @WrappedValueAvailable51 var wrappedValueAavailable51 = S() // expected-error {{'wrappedValue' is only available in macOS 51 or newer}}
+  @WrappedValueAvailable51 var wrappedValueAavailable51 = S()
 }

test/attr/attr_availability_transitive_osx.swift

@@ -87,8 +87,7 @@ func never_available_func(
 ) {
   always()
   never() // expected-error {{'never()' is unavailable}}
-  osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-  // expected-note@-1 {{add 'if #available' version check}}
+  osx_future()
   osx()
   osx_ios()
   osx_extension()
@@ -105,8 +104,7 @@ func osx_func(
 ) {
   always()
   never() // expected-error {{'never()' is unavailable}}
-  osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-  // expected-note@-1 {{add 'if #available' version check}}
+  osx_future()
   osx()
   osx_ios()
   osx_extension()
@@ -159,7 +157,7 @@ var never_var: (
 ) = (
   always(),
   never(), // expected-error {{'never()' is unavailable}}
-  osx_future(), // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
+  osx_future(),
   osx(),
   osx_ios(),
   osx_extension()
@@ -176,7 +174,7 @@ var osx_var: (
 ) = (
   always(),
   never(), // expected-error {{'never()' is unavailable}}
-  osx_future(), // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
+  osx_future(),
   osx(),
   osx_ios(),
   osx_extension()
@@ -218,7 +216,7 @@ struct AlwaysAvailableContainer { // expected-note 2 {{add @available attribute
 struct NeverAvailableContainer { // expected-note 2 {{'NeverAvailableContainer' has been explicitly marked unavailable here}}
   let always_var: AlwaysAvailable = always()
   let never_var: NeverAvailable = never() // expected-error {{'never()' is unavailable}}
-  let osx_future_var: OSXFutureAvailable = osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
+  let osx_future_var: OSXFutureAvailable = osx_future()
   let osx_var: OSXUnavailable = osx()
   let osx_ios_var: MultiPlatformUnavailable = osx_ios()
   let osx_extension_var: OSXAppExtensionsUnavailable = osx_extension()
@@ -228,7 +226,7 @@ struct NeverAvailableContainer { // expected-note 2 {{'NeverAvailableContainer'
 struct OSXUnavailableContainer { // expected-note 2 {{'OSXUnavailableContainer' has been explicitly marked unavailable here}}
   let always_var: AlwaysAvailable = always()
   let never_var: NeverAvailable = never() // expected-error {{'never()' is unavailable}}
-  let osx_future_var: OSXFutureAvailable = osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
+  let osx_future_var: OSXFutureAvailable = osx_future()
   let osx_var: OSXUnavailable = osx()
   let osx_ios_var: MultiPlatformUnavailable = osx_ios()
   let osx_extension_var: OSXAppExtensionsUnavailable = osx_extension()
@@ -301,8 +299,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -319,8 +316,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -337,15 +333,14 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
   }
 
   @available(OSXApplicationExtension, unavailable)
-  func never_available_extension_osx_app_extension_method( // expected-note {{add @available attribute to enclosing instance method}}
+  func never_available_extension_osx_app_extension_method(
     _: AlwaysAvailable,
     _: NeverAvailable,
     _: OSXFutureAvailable,
@@ -355,8 +350,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -380,8 +374,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -398,8 +391,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -416,15 +408,14 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
   }
 
   @available(OSXApplicationExtension, unavailable)
-  func osx_extension_osx_app_extension_method( // expected-note {{add @available attribute to enclosing instance method}}
+  func osx_extension_osx_app_extension_method(
     _: AlwaysAvailable,
     _: NeverAvailable,
     _: OSXFutureAvailable,
@@ -434,8 +425,7 @@ extension ExtendMe {
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -477,8 +467,7 @@ extension ExtendMe { // expected-note * {{add @available attribute to enclosing
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -495,8 +484,7 @@ extension ExtendMe { // expected-note * {{add @available attribute to enclosing
   ) {
     always()
     never() // expected-error {{'never()' is unavailable}}
-    osx_future() // expected-error {{'osx_future()' is only available in macOS 99 or newer}}
-    // expected-note@-1 {{add 'if #available' version check}}
+    osx_future()
     osx()
     osx_ios()
     osx_extension()
@@ -534,7 +522,7 @@ func available_func_call_extension_methods(_ e: ExtendMe) { // expected-note {{a
 }
 
 @available(OSX, obsoleted: 10.9)
-struct OSXObsoleted {} // expected-note 2 {{'OSXObsoleted' was obsoleted in macOS 10.9}}
+struct OSXObsoleted {} // expected-note 4 {{'OSXObsoleted' was obsoleted in macOS 10.9}}
 
 @available(OSX, unavailable)
 @available(OSX, introduced: 99)
@@ -561,17 +549,46 @@ func osx_unavailable_func(
   OSXUnavailableAndIntroducedInFutureSameAttribute,
   OSXIntroducedInFutureAndUnavailable
 ) {
-  // FIXME: [availability] Stop diagnosing potential unavailability or obsoletion in an unavailable context.
-  _ = OSXFutureAvailable() // expected-error {{'OSXFutureAvailable' is only available in macOS 99 or newer}}
-  // expected-note@-1 {{add 'if #available' version check}}
+  // FIXME: [availability] Stop diagnosing obsoletion in an unavailable context.
+  _ = OSXFutureAvailable()
   _ = OSXObsoleted() // expected-error {{'OSXObsoleted' is unavailable in macOS}}
   _ = OSXUnavailableAndIntroducedInFuture()
   _ = OSXUnavailableAndIntroducedInFutureSameAttribute()
   _ = OSXIntroducedInFutureAndUnavailable()
 
   func takesType<T>(_ t: T.Type) {}
-  takesType(OSXFutureAvailable.self) // expected-error {{'OSXFutureAvailable' is only available in macOS 99 or newer}}
-  // expected-note@-1 {{add 'if #available' version check}}
+  takesType(OSXFutureAvailable.self)
+  takesType(OSXObsoleted.self) // expected-error {{'OSXObsoleted' is unavailable in macOS}}
+  takesType(OSXUnavailableAndIntroducedInFuture.self)
+  takesType(OSXUnavailableAndIntroducedInFutureSameAttribute.self)
+  takesType(OSXIntroducedInFutureAndUnavailable.self)
+
+  return (s1, s2, s3, s4, s5)
+}
+
+@available(OSX, unavailable, introduced: 99)
+func osx_unavailable_and_introduced_func(
+  _ s1: OSXFutureAvailable,
+  _ s2: OSXObsoleted,
+  _ s3: OSXUnavailableAndIntroducedInFuture,
+  _ s4: OSXUnavailableAndIntroducedInFutureSameAttribute,
+  _ s5: OSXIntroducedInFutureAndUnavailable,
+) -> (
+  OSXFutureAvailable,
+  OSXObsoleted,
+  OSXUnavailableAndIntroducedInFuture,
+  OSXUnavailableAndIntroducedInFutureSameAttribute,
+  OSXIntroducedInFutureAndUnavailable
+) {
+  // FIXME: [availability] Stop diagnosing obsoletion in an unavailable context.
+  _ = OSXFutureAvailable()
+  _ = OSXObsoleted() // expected-error {{'OSXObsoleted' is unavailable in macOS}}
+  _ = OSXUnavailableAndIntroducedInFuture()
+  _ = OSXUnavailableAndIntroducedInFutureSameAttribute()
+  _ = OSXIntroducedInFutureAndUnavailable()
+
+  func takesType<T>(_ t: T.Type) {}
+  takesType(OSXFutureAvailable.self)
   takesType(OSXObsoleted.self) // expected-error {{'OSXObsoleted' is unavailable in macOS}}
   takesType(OSXUnavailableAndIntroducedInFuture.self)
   takesType(OSXUnavailableAndIntroducedInFutureSameAttribute.self)