Extend the "fuzzy" forward scan matching to support multiple trailing closures

The "fuzzy" forward scan matching algorithm was only applied when there
was a single, unlabeled trailing closure, but was disabled in the
presence of multiple trailing closures. Extend the "fuzzy" match to
account for multiple trailing closures, by restricting the search for
"a later parameter that needs an argument" to stop when we find a
parameter that matches the first (labeled) trailing closure.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -199,6 +199,37 @@ static ConstraintSystem::TypeMatchOptions getDefaultDecompositionOptions(
   return flags | ConstraintSystem::TMF_GenerateConstraints;
 }
 
+/// Determine whether any parameter from the given index up until the end
+/// requires an argument to be provided.
+///
+/// \param params The parameters themselves.
+/// \param paramInfo Declaration-provided information about the parameters.
+/// \param firstParamIdx The first parameter to examine to determine whether any
+/// parameter in the range \c [paramIdx, params.size()) requires an argument.
+/// \param beforeLabel If non-empty, stop examining parameters when we reach
+/// a parameter with this label.
+static bool anyParameterRequiresArgument(
+    ArrayRef<AnyFunctionType::Param> params,
+    const ParameterListInfo &paramInfo,
+    unsigned firstParamIdx,
+    Optional<Identifier> beforeLabel) {
+  for (unsigned paramIdx : range(firstParamIdx, params.size())) {
+    // If have been asked to stop when we reach a parameter with a particular
+    // label, and we see a parameter with that label, we're done: no parameter
+    // requires an argument.
+    if (beforeLabel && *beforeLabel == params[paramIdx].getLabel())
+      break;
+
+    // If this parameter requires an argument, tell the caller.
+    if (!paramInfo.hasDefaultArgument(paramIdx)
+        && !params[paramIdx].isVariadic())
+      return true;
+  }
+
+  // No parameters required arguments.
+  return false;
+}
+
 // FIXME: This should return ConstraintSystem::TypeMatchResult instead
 //        to give more information to the solver about the failure.
 bool constraints::
@@ -393,17 +424,17 @@ matchCallArguments(SmallVectorImpl<AnyFunctionType::Param> &args,
         return;
       }
 
-      // If there is only one trailing closure argument, consider applying
-      // a "fuzzy" match rule that skips this parameter if it is defaulted but
-      // later parameters require an argument.
-      if (nextArgIdx + 1 == numArgs &&
-          paramInfo.hasDefaultArgument(paramIdx) &&
+      // If this parameter has a default argument, consider applying a "fuzzy"
+      // match rule that skips this parameter if doing so is the only way to
+      // satisfy
+      if (paramInfo.hasDefaultArgument(paramIdx) &&
           param.getPlainType()->getASTContext().LangOpts
               .EnableFuzzyForwardScanTrailingClosureMatching &&
-          llvm::any_of(range(paramIdx + 1, params.size()), [&](unsigned idx) {
-            return !paramInfo.hasDefaultArgument(idx)
-              && !params[idx].isVariadic();
-          })) {
+          anyParameterRequiresArgument(
+              params, paramInfo, paramIdx + 1,
+              nextArgIdx + 1 < numArgs
+                ? Optional<Identifier>(args[nextArgIdx + 1].getLabel())
+                : Optional<Identifier>(None))) {
         haveUnfulfilledParams = true;
         return;
       }

test/expr/postfix/call/forward_trailing_closure_fuzzy.swift

@@ -10,4 +10,21 @@ func testDoSomething() {
   doSomething(onError: nil) { x in
     print(x)
   }
+
+  doSomething { e in
+    print(e)
+  } onCompletion: { x in
+    print(x)
+  }
+}
+
+func trailingClosures(
+  arg1: () -> Void = {},
+  arg2: () -> Void,
+  arg3: () -> Void = {}
+) {}
+
+func testTrailingClosures() {
+  trailingClosures { print("Hello!") }
+  trailingClosures { print("Hello,") } arg3: { print("world!") }
 }