Fix a Family of Crashers in Availability Checking

NIO exposed an issue in the new availability walkers where an implicit
check was not being performed. They were able to get this to crash by
using a defer statement - the body of which contains implicit
declarations that got run through the walker. This exposed a wider hole
in availability checking of defer statements. Namely, that it wasn't
happening.

Let's dilute the meaning of `isImplicit` in the availability walkers to
account for this desugaring.

rdar://74484150

Author: CodaFi

lib/Sema/TypeCheckAvailability.cpp

@@ -147,7 +147,11 @@ static void computeExportContextBits(ASTContext &Ctx, Decl *D,
   if (D->isSPI())
     *spi = true;
 
-  if (D->isImplicit())
+  // Defer bodies are desugared to an implicit closure expression. We need to
+  // dilute the meaning of "implicit" to make sure we're still checking
+  // availability inside of defer statements.
+  const auto isDeferBody = isa<FuncDecl>(D) && cast<FuncDecl>(D)->isDeferBody();
+  if (D->isImplicit() && !isDeferBody)
     *implicit = true;
 
   if (D->getAttrs().getDeprecated(Ctx))

test/Sema/availability.swift

@@ -195,3 +195,32 @@ struct VarToFunc {
   }
 }
 
+struct DeferBody {
+  func foo() {
+    enum No: Error {
+      case no
+    }
+
+    defer {
+      do {
+        throw No.no
+      } catch No.no {
+      } catch {
+      }
+    }
+    _ = ()
+  }
+
+  func bar() {
+    @available(*, unavailable)
+    enum No: Error { // expected-note 2 {{'No' has been explicitly marked unavailable here}}
+      case no
+    }
+    do {
+      throw No.no
+      // expected-error@-1 {{'No' is unavailable}}
+    } catch No.no {} catch _ {}
+    // expected-error@-1 {{'No' is unavailable}}
+  }
+}
+

test/attr/attr_inlinable.swift

@@ -357,3 +357,12 @@ public struct PrivateInlinableCrash {
     // expected-error@-2 {{initializer 'init()' is private and cannot be referenced from an '@inlinable' function}}
   }
 }
+
+// Just make sure we don't crash.
+private func deferBodyTestCall() {} // expected-note {{global function 'deferBodyTestCall()' is not '@usableFromInline' or public}}
+@inlinable public func deferBodyTest() {
+  defer {
+    deferBodyTestCall() // expected-error {{global function 'deferBodyTestCall()' is private and cannot be referenced from an '@inlinable' function}}
+  }
+  _ = ()
+}