[CS] Fix DeclContext for multi-statement captures

Make sure we set the correct DeclContext for CSGen
of multi-statement captures, since otherwise the
DeclContext is set to the closure itself, which is
wrong.

Author: hamishknight

lib/Sema/CSSyntacticElement.cpp

@@ -818,7 +818,20 @@ class SyntacticElementConstraintGenerator
 
     if (patternBinding->isInitializerChecked(index))
       return;
-
+ 
+    // Capture list bindings in multi-statement closures get solved as part of
+    // the closure's conjunction, which has the DeclContext set to the closure.
+    // This is wrong for captures though, which are semantically bound outside
+    // of the closure body. So we need to re-adjust their DeclContext here for
+    // constraint generation. The constraint system's DeclContext will be wrong
+    // for solving, but CSGen should ensure that constraints carry the correct
+    // DeclContext.
+    std::optional<llvm::SaveAndRestore<DeclContext *>> DCScope;
+    if (auto *var = patternBinding->getSingleVar()) {
+      if (var->getParentCaptureList())
+        DCScope.emplace(cs.DC, var->getDeclContext());
+    }
+    
     auto contextualPattern =
         ContextualPattern::forPatternBindingDecl(patternBinding, index);
     Type patternType = TypeChecker::typeCheckPattern(contextualPattern);

test/Constraints/issue-79444.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift
+
+// https://github.com/swiftlang/swift/issues/79444
+class C {
+  func foo() {
+    _ = { [x = "\(self)"] in } // expected-warning {{capture 'x' was never used}}
+    _ = { [x = "\(self)"] in x }
+    _ = { [x = "\(self)"] in 
+      let y = x
+      return y 
+    }
+    _ = { [x = "\(self)"] in
+      let fn = { [y = "\(x)"] in
+        let z = y
+        return z 
+      }
+      return fn() 
+    }
+  }
+}