[CS] Perform limited exhaustiveness checking if we couldn't apply the solution

Currently ExprPatterns are type-checked during
CSApply. As such, they can cause solution
application to fail with a pattern that isn't
well-formed. I'm planning on moving their
type-checking into the solver, but until then,
lets only do limited exhaustiveness checking for
the switch if there was an error.

rdar://105781521

Author: hamishknight

lib/Sema/CSSyntacticElement.cpp

@@ -1746,8 +1746,11 @@ class SyntacticElementSolutionApplication
       }
     }
 
+    // Note we perform a limited exhaustiveness check if we weren't able to
+    // apply the solution, as the subject and patterns may not be well-formed.
     TypeChecker::checkSwitchExhaustiveness(
-        switchStmt, context.getAsDeclContext(), limitExhaustivityChecks);
+        switchStmt, context.getAsDeclContext(),
+        /*limited*/ limitExhaustivityChecks || hadError);
 
     return switchStmt;
   }

test/Constraints/rdar105781521.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift
+
+// rdar://105781521
+enum MyEnum {
+  case first(String)
+}
+
+func takeClosure(_ x: () -> Void) {}
+
+func test(value: MyEnum) {
+  takeClosure {
+    switch value {
+    case .first(true):
+      // expected-error@-1 {{expression pattern of type 'Bool' cannot match values of type 'String'}}
+      // expected-note@-2 {{overloads for '~=' exist with these partially matching parameter lists: (Substring, String)}}
+      break
+    default:
+      break
+    }
+  }
+}