From 09bb346b037ceb789f7d917371998fa1db3b2b51 Mon Sep 17 00:00:00 2001
From: Hamish Knight <hamish_github@mediocremail.com>
Date: Mon, 17 Jul 2023 17:32:46 +0100
Subject: [PATCH] [CS] Walk UnresolvedDeclRefExprs in UnresolvedVarCollector

This matches what we do in VarRefCollector, and is
needed because we currently delay the pre-checking
of patterns due to the fact that we don't resolve
them until CSGen. We ought to consider changing
this, but until then, adjust the logic here to
ensure we properly connect an ExprPattern that
references an outer var with any type variables
it may involve.

rdar://112264204
---
 lib/Sema/CSSyntacticElement.cpp      | 21 +++++++++++++++++++++
 test/Constraints/rdar112264204.swift | 15 +++++++++++++++
 2 files changed, 36 insertions(+)
 create mode 100644 test/Constraints/rdar112264204.swift

diff --git a/lib/Sema/CSSyntacticElement.cpp b/lib/Sema/CSSyntacticElement.cpp
index 26465899c3d..9ca79e476fd 100644
--- a/lib/Sema/CSSyntacticElement.cpp
+++ b/lib/Sema/CSSyntacticElement.cpp
@@ -275,6 +275,27 @@ public:
         }
       }
     }
+
+    // FIXME: We can see UnresolvedDeclRefExprs here because we don't walk into
+    // patterns when running preCheckExpression, since we don't resolve patterns
+    // until CSGen. We ought to consider moving pattern resolution into
+    // pre-checking, which would allow us to pre-check patterns normally.
+    if (auto *declRef = dyn_cast<UnresolvedDeclRefExpr>(expr)) {
+      auto name = declRef->getName();
+      auto loc = declRef->getLoc();
+      if (name.isSimpleName() && loc.isValid()) {
+        auto *varDecl =
+            dyn_cast_or_null<VarDecl>(ASTScope::lookupSingleLocalDecl(
+                CS.DC->getParentSourceFile(), name.getFullName(), loc));
+        if (varDecl) {
+          if (auto varType = CS.getTypeIfAvailable(varDecl)) {
+            SmallPtrSet<TypeVariableType *, 4> typeVars;
+            varType->getTypeVariables(typeVars);
+            Vars.insert(typeVars.begin(), typeVars.end());
+          }
+        }
+      }
+    }
     return Action::Continue(expr);
   }
 
diff --git a/test/Constraints/rdar112264204.swift b/test/Constraints/rdar112264204.swift
new file mode 100644
index 00000000000..68768df0774
--- /dev/null
+++ b/test/Constraints/rdar112264204.swift
@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+// rdar://112264204: Make sure we can type-check this.
+func foo(_ fn: (Int) -> Void) {}
+
+func bar(_ x: Int) {
+  foo { [x] y in
+    switch y {
+    case x:
+      ()
+    default:
+      ()
+    }
+  }
+}