[Frontend] Add a flag for experimental type inference from default expressions

Author: xedin

include/swift/Basic/LangOptions.h

@@ -718,6 +718,10 @@ namespace swift {
     /// closures.
     bool EnableMultiStatementClosureInference = false;
 
+    /// Enable experimental support for generic parameter inference in
+    /// parameter positions from associated default expressions.
+    bool EnableTypeInferenceFromDefaultArguments = false;
+
     /// See \ref FrontendOptions.PrintFullConvention
     bool PrintFullConvention = false;
   };

include/swift/Option/FrontendOptions.td

@@ -857,6 +857,10 @@ def experimental_multi_statement_closures :
   Flag<["-"], "experimental-multi-statement-closures">,
   HelpText<"Enable experimental support for type inference in multi-statement closures">;
 
+def experimental_type_inference_from_defaults :
+  Flag<["-"], "enable-experimental-type-inference-from-defaults">,
+  HelpText<"Enable experimental support for generic parameter inference from default values">;
+
 def prebuilt_module_cache_path :
   Separate<["-"], "prebuilt-module-cache-path">,
   HelpText<"Directory of prebuilt modules for loading module interfaces">;

lib/Frontend/CompilerInvocation.cpp

@@ -1051,6 +1051,9 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
   Opts.EnableMultiStatementClosureInference |=
       Args.hasArg(OPT_experimental_multi_statement_closures);
 
+  Opts.EnableTypeInferenceFromDefaultArguments |=
+      Args.hasArg(OPT_experimental_type_inference_from_defaults);
+
   Opts.PrintFullConvention |=
       Args.hasArg(OPT_experimental_print_full_convention);