[Constraint system] Cope with implicit patterns in closure checks.

Author: DougGregor

lib/Sema/ConstraintSystem.cpp

@@ -2241,10 +2241,16 @@ FunctionType::ExtInfo ConstraintSystem::closureEffects(ClosureExpr *expr) {
       // of is-patterns applied to an irrefutable pattern.
       pattern = pattern->getSemanticsProvidingPattern();
       while (auto isp = dyn_cast<IsPattern>(pattern)) {
-        const Type castType = TypeResolution::forContextual(
-                                  CS.DC, TypeResolverContext::InExpression,
-                                  /*unboundTyOpener*/ nullptr)
-                                  .resolveType(isp->getCastTypeRepr());
+        Type castType;
+        if (auto castTypeRepr = isp->getCastTypeRepr()) {
+          castType = TypeResolution::forContextual(
+                         CS.DC, TypeResolverContext::InExpression,
+                         /*unboundTyOpener*/ nullptr)
+                         .resolveType(castTypeRepr);
+        } else {
+          castType = isp->getCastType();
+        }
+
         if (castType->hasError()) {
           return false;
         }

test/decl/func/async.swift

@@ -2,7 +2,7 @@
 
 // Redeclaration checking
 func redecl1() async { } // expected-note{{previously declared here}}
-func redecl1() throws { } // expected-error{{invalid redeclaration of 'redecl1()'}}
+func redecl1() async throws { } // expected-error{{invalid redeclaration of 'redecl1()'}}
 
 // Override checking