[CSGen] Allow is patterns to infer type from enclosing context

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:

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}}
+  }
+}