AST: Stop diagnosing potentially unavailable declarations in unavailable contexts.

Potential unavailability of a declaration has always been diagnosed in contexts
that do not have a sufficient platform introduction constraint, even when those
contexts are also unavailable on the target 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
#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treated as lower priority and therefore superseded by the unavailability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

CHANGELOG.md

@@ -5,6 +5,33 @@
 
 ## Swift 6.2
 
+* The Swift compiler no longer diagnoses references to declarations that are
+  potentially unavailable because the platform version might not be new enough
+  when those references occur inside of contexts that are also unavailable to
+  that platform. This addresses a long-standing nuisance for multi-platform
+  code. However, there is also a chance that existing source code may become
+  ambiguous as a result:
+
+  ```swift
+  struct A {}
+  struct B {}
+
+  func potentiallyAmbiguous(_: A) {}
+
+  @available(macOS 99, *)
+  func potentiallyAmbiguous(_: B) {}
+
+  @available(macOS, unavailable)
+  func unavailableOnMacOS() {
+    potentiallyAmbiguous(.init()) // error: ambiguous use of 'init()'
+  }
+  ```
+
+  Code that is now ambiguous as a result should likely be restructured since
+  disambiguation based on platform introduction alone has never been a reliable
+  strategy, given that the code would eventually become ambiguous anyways when
+  the deployment target is raised.
+
 * [SE-0419][]:
   Introduced the new `Runtime` module, which contains a public API that can
   generate backtraces, presently supported on macOS and Linux.  Capturing a

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/Constraints/availability_macos.swift

@@ -0,0 +1,37 @@
+// RUN: %target-typecheck-verify-swift
+
+// REQUIRES: OS=macosx
+
+struct A {} // expected-note * {{found this candidate}}
+struct B {} // expected-note * {{found this candidate}}
+
+func ambiguousInFarFuture(_: A) {}
+
+@available(macOS 99, *)
+func ambiguousInFarFuture(_: B) {}
+
+struct S {
+  func ambiguousInFarFuture(_: A) {}
+}
+
+@available(macOS 99, *)
+extension S {
+  func ambiguousInFarFuture(_: B) {}
+}
+
+func testDeploymentTarget(_ s: S) {
+  ambiguousInFarFuture(.init())
+  s.ambiguousInFarFuture(.init())
+}
+
+@available(macOS 99, *)
+func testFarFuture(_ s: S) {
+  ambiguousInFarFuture(.init()) // expected-error {{ambiguous use of 'init()'}}
+  s.ambiguousInFarFuture(.init()) // expected-error {{ambiguous use of 'init()'}}
+}
+
+@available(macOS, unavailable)
+func testUnavailable(_ s: S) {
+  ambiguousInFarFuture(.init()) // expected-error {{ambiguous use of 'init()'}}
+  s.ambiguousInFarFuture(.init()) // expected-error {{ambiguous use of 'init()'}}
+}

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)
@@ -418,10 +418,25 @@ extension SomeEnum {
 
 // CHECK-NEXT: {{^}}  (decl version=50 unavailable=macOS decl=unavailableOnMacOSAndIntroduced()
 
-@available(macOS, unavailable, introduced: 52)
+@available(macOS, unavailable)
+@available(macOS, introduced: 52)
 func unavailableOnMacOSAndIntroduced() {
 }
 
+// CHECK-NEXT: {{^}}  (decl version=50 unavailable=macOS decl=introducedOnMacOSAndUnavailable()
+
+@available(macOS, introduced: 53)
+@available(macOS, unavailable)
+func introducedOnMacOSAndUnavailable() {
+}
+
+
+// CHECK-NEXT: {{^}}  (decl version=50 unavailable=macOS decl=unavailableOnMacOSAndIntroducedSameAttr()
+
+@available(macOS, unavailable, introduced: 54)
+func unavailableOnMacOSAndIntroducedSameAttr() {
+}
+
 // CHECK-NEXT: {{^}}  (decl version=50 unavailable=* decl=NeverAvailable
 // CHECK-NEXT: {{^}}    (decl version=50 unavailable=* decl=unavailableOnMacOS()
 

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()
 }