[cxx-interop] Update ABIArgInfo::Indirect case in expandExternalSignatureTypes to handle thin metatype parameters.

Add second list of SILParameterInfos that excludes thin metatypes so
that the "Indirect" argument path below will select the correct
parameter info.

Author: zoecarver

lib/IRGen/GenCall.cpp

@@ -1315,6 +1315,17 @@ void SignatureExpansion::expandExternalSignatureTypes() {
   // Swift parameters list?
   size_t clangToSwiftParamOffset = paramTys.size();
 
+  // This is exactly the same as "params" but without thin metatypes. This must
+  // exist so that when the path for "Indirect" arguments looks for the
+  // SILParameterInfo for a given index, we get the parameter info for the
+  // coorisponding clang type's index and not a "random" parameter's info.
+  //
+  // This is only an issue in very rare cases when we have a constructor that
+  // looks like this: (T, @thin U.Type) -> @out U. In this case "params" will
+  // contain two elements and "paramTys" will contain two elements so the
+  // "Indirect" argument path gets confused and selects the second parameter
+  // (the thin metatype) instead of the first one.
+  SmallVector<SILParameterInfo, 4> adjustedSILParams;
   // Convert each parameter to a Clang type.
   for (auto param : params) {
     auto clangTy = IGM.getClangType(param, FnType);
@@ -1324,6 +1335,7 @@ void SignatureExpansion::expandExternalSignatureTypes() {
       continue;
     }
     paramTys.push_back(clangTy);
+    adjustedSILParams.push_back(param);
   }
 
   // Generate function info for this signature.
@@ -1413,7 +1425,7 @@ void SignatureExpansion::expandExternalSignatureTypes() {
     case clang::CodeGen::ABIArgInfo::Indirect: {
       assert(i >= clangToSwiftParamOffset &&
              "Unexpected index for indirect byval argument");
-      auto &param = params[i - clangToSwiftParamOffset];
+      auto &param = adjustedSILParams[i - clangToSwiftParamOffset];
       auto paramTy = getSILFuncConventions().getSILType(
           param, IGM.getMaximalTypeExpansionContext());
       auto &paramTI = cast<FixedTypeInfo>(IGM.getTypeInfo(paramTy));