[Type checker] Handle 'rethrows' checks for single-parameter functions.

The throw-checking code wasn't properly coping with functions that
take a single, labeled argument, due to the longstanding lie that
pretends that functions take a tuple argument vs. zero or more
separate arguments. Here, the lie manifests as spurious "call can
throw, but is not marked as such" errors.

Fixes rdar://problem/31794932.

(cherry picked from commit b07d995)

Author: DougGregor

lib/Sema/TypeCheckError.cpp

@@ -405,6 +405,7 @@ class ApplyClassifier {
     // But if the expression didn't type-check, suppress diagnostics.
     if (!E->getType() || E->getType()->hasError())
       return Classification::forInvalidCode();
+
     auto type = E->getFn()->getType();
     if (!type) return Classification::forInvalidCode();
     auto fnType = type->getAs<AnyFunctionType>();
@@ -673,11 +674,18 @@ class ApplyClassifier {
         return classifyShuffleRethrowsArgument(shuffle, paramTupleType);
       }
 
-      // Otherwise, we're passing an opaque tuple expression, and we
-      // should treat it as contributing to 'rethrows' if the original
-      // parameter type included a throwing function type.
-      return classifyArgumentByType(paramType,
+      int scalarElt = paramTupleType->getElementForScalarInit();
+      if (scalarElt < 0 ||
+          !paramTupleType->getElementType(scalarElt)->isEqual(arg->getType())) {
+        // Otherwise, we're passing an opaque tuple expression, and we
+        // should treat it as contributing to 'rethrows' if the original
+        // parameter type included a throwing function type.
+        return classifyArgumentByType(
+                                    paramType,
                                     PotentialReason::forRethrowsArgument(arg));
+      }
+
+      paramType = paramTupleType->getElementType(scalarElt);
     }
 
     // Otherwise, if the original parameter type was not a throwing

test/decl/func/rethrows.swift

@@ -476,3 +476,10 @@ func throwWhileGettingFoo() throws -> Foo.Type { return Foo.self }
 
 (throwWhileGettingFoo()).foo(Foo())() // expected-error {{can throw}}
 (try throwWhileGettingFoo()).foo(Foo())()
+
+// <rdar://problem/31794932> [Source compatibility] Call to sort(by):) can throw, but is not marked with 'try'
+func doRethrow(fn: (Int, Int) throws -> Int) rethrows { }
+
+func testDoRethrow() {
+  doRethrow(fn:) { (a, b) in return a }
+}