Add frontend flag -enable-experimental-opaque-parameters

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -313,6 +313,10 @@ namespace swift {
     /// `func f() -> <T> T`.
     bool EnableExperimentalNamedOpaqueTypes = false;
 
+    /// Enable experimental support for opaque parameter types, e.g.
+    /// `func f(collection: some Collection)`.
+    bool EnableExperimentalOpaqueParameters = false;
+
     /// Enable support for explicit existential types via the \c any
     /// keyword.
     bool EnableExplicitExistentialTypes = true;

include/swift/Option/FrontendOptions.td

@@ -502,6 +502,10 @@ def enable_experimental_named_opaque_types :
   Flag<["-"], "enable-experimental-named-opaque-types">,
   HelpText<"Enable experimental support for named opaque result types">;
 
+def enable_experimental_opaque_parameters :
+  Flag<["-"], "enable-experimental-opaque-parameters">,
+  HelpText<"Enable experimental support for opaque parameters">;
+
 def enable_explicit_existential_types :
   Flag<["-"], "enable-explicit-existential-types">,
   HelpText<"Enable experimental support for explicit existential types">;

lib/Frontend/CompilerInvocation.cpp

@@ -445,6 +445,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalNamedOpaqueTypes |=
       Args.hasArg(OPT_enable_experimental_named_opaque_types);
 
+  Opts.EnableExperimentalOpaqueParameters |=
+      Args.hasArg(OPT_enable_experimental_opaque_parameters);
+
   Opts.EnableExplicitExistentialTypes |=
       Args.hasArg(OPT_enable_explicit_existential_types);