From b14d9655ca4e1c32cf3a055bd9d4be2f5bf3471f Mon Sep 17 00:00:00 2001
From: Pavel Yaskevich <pyaskevich@apple.com>
Date: Wed, 3 Jun 2020 10:26:59 -0700
Subject: [PATCH] [CSGen] Allow `is` patterns to infer type from enclosing
 context

Instead of assuming type of its sub-pattern `is` should be able
to infer type from context and then propagate that to the sub-pattern
via conversion. This enables support for patterns like `.foo(_ as Foo)`
where `is` would have a type different from `_` which gets inferred
from associated value of `foo` and converted to `Foo` that becomes
a type of `_` pattern.

Resolves: rdar://problem/63510989
---
 lib/Sema/CSGen.cpp              | 12 +++++++++++-
 test/Constraints/patterns.swift | 17 +++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib/Sema/CSGen.cpp b/lib/Sema/CSGen.cpp
index 1486e63f7f839..3f432cfa3f9c7 100644
--- a/lib/Sema/CSGen.cpp
+++ b/lib/Sema/CSGen.cpp
@@ -2544,7 +2544,17 @@ namespace {
             ConstraintKind::CheckedCast, subPatternType, castType,
             locator.withPathElement(LocatorPathElt::PatternMatch(pattern)));
 
-        return setType(subPatternType);
+        // Allow `is` pattern to infer type from context which is then going
+        // to be propaged down to its sub-pattern via conversion. This enables
+        // correct handling of patterns like `_ as Foo` where `_` would
+        // get a type of `Foo` but `is` pattern enclosing it could still be
+        // inferred from enclosing context.
+        auto isType = CS.createTypeVariable(CS.getConstraintLocator(pattern),
+                                            TVO_CanBindToNoEscape);
+        CS.addConstraint(
+            ConstraintKind::Conversion, subPatternType, isType,
+            locator.withPathElement(LocatorPathElt::PatternMatch(pattern)));
+        return setType(isType);
       }
 
       case PatternKind::Bool:
diff --git a/test/Constraints/patterns.swift b/test/Constraints/patterns.swift
index f184629fa37f4..a3a2de3f5ee24 100644
--- a/test/Constraints/patterns.swift
+++ b/test/Constraints/patterns.swift
@@ -478,3 +478,20 @@ func rdar_60048356() {
     }
   }
 }
+
+// rdar://problem/63510989 - valid pattern doesn't type-check
+func rdar63510989() {
+  enum Value : P {
+    func p() {}
+  }
+
+  enum E {
+    case foo(P?)
+  }
+
+  func test(e: E) {
+    if case .foo(_ as Value) = e {} // Ok
+    if case .foo(let v as Value) = e {} // Ok
+    // expected-warning@-1 {{immutable value 'v' was never used; consider replacing with '_' or removing it}}
+  }
+}