Concurrency: Skip Patterns when checking isolation

Due to a quirk of the ASTVisitor, we would then visit
accessors twice, the first time as part of the
PatternBindingDecl inside the TopLevelCodeDecl, and
at that point the accessor body has not yet been
type checked.

Author: slavapestov

lib/Sema/TypeCheckConcurrency.cpp

@@ -2610,6 +2610,13 @@ namespace {
       return MacroWalking::Expansion;
     }
 
+    PreWalkResult<Pattern *> walkToPatternPre(Pattern *pattern) override {
+      // Walking into patterns leads to nothing good because then we
+      // end up visiting the AccessorDecls of a top-level
+      // PatternBindingDecl twice.
+      return Action::SkipNode(pattern);
+    }
+
     PreWalkAction walkToDeclPre(Decl *decl) override {
       // Don't walk into local types because nothing in them can
       // change the outcome of our analysis, and we don't want to

test/Concurrency/sendable_checking_script_mode.swift

@@ -0,0 +1,17 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+class NonSendable {} // expected-note 2{{class 'NonSendable' does not conform to the 'Sendable' protocol}}
+
+var testLocalCaptures: Int {
+  let ns = NonSendable()
+
+  @Sendable func localFunc() -> NonSendable {
+    return ns // expected-error {{capture of 'ns' with non-sendable type 'NonSendable' in a `@Sendable` local function}}
+  }
+
+  let _: @Sendable () -> NonSendable = {
+    return ns // expected-error {{capture of 'ns' with non-sendable type 'NonSendable' in a `@Sendable` closure}}
+  }
+
+  return 3
+}