SIL: Add -disable-subst-sil-function-types flag to work around type lowering bugs

Author: slavapestov

include/swift/Basic/LangOptions.h

@@ -283,6 +283,9 @@ namespace swift {
     /// Whether to dump debug info for request evaluator cycles.
     bool DebugDumpCycles = false;
 
+    /// Disable SIL substituted function types.
+    bool DisableSubstSILFunctionTypes = false;
+
     /// Whether to diagnose an ephemeral to non-ephemeral conversion as an
     /// error.
     bool DiagnoseInvalidEphemeralnessAsError = false;

include/swift/Option/FrontendOptions.td

@@ -464,6 +464,10 @@ def enable_experimental_static_assert :
   Flag<["-"], "enable-experimental-static-assert">,
   HelpText<"Enable experimental #assert">;
 
+def disable_subst_sil_function_types :
+  Flag<["-"], "disable-subst-sil-function-types">,
+  HelpText<"Disable substituted function types for SIL type lowering of function values">;
+
 def enable_experimental_named_opaque_types :
   Flag<["-"], "enable-experimental-named-opaque-types">,
   HelpText<"Enable experimental support for named opaque result types">;

lib/Frontend/CompilerInvocation.cpp

@@ -838,6 +838,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.DisableSubstSILFunctionTypes =
+      Args.hasArg(OPT_disable_subst_sil_function_types);
+
   if (auto A = Args.getLastArg(OPT_requirement_machine_EQ)) {
     auto value = llvm::StringSwitch<Optional<RequirementMachineMode>>(A->getValue())
         .Case("off", RequirementMachineMode::Disabled)

lib/SIL/IR/SILFunctionType.cpp

@@ -1928,6 +1928,9 @@ static CanSILFunctionType getSILFunctionType(
   }
 
   bool shouldBuildSubstFunctionType = [&]{
+    if (TC.Context.LangOpts.DisableSubstSILFunctionTypes)
+      return false;
+
     // If there is no genericity in the abstraction pattern we're lowering
     // against, we don't need to introduce substitutions into the lowered
     // type.