[clang][ASTImporter] Import source location of explicit object parameter instead of copying it (#124305)

We used to copy the `SourceLocation` instead of importing it, which
isn't correct since the `SourceManager`'s of the source and target
ASTContext might differ.

Also adds test that confirms that we import the explicit object
parameter location for `ParmVarDecl`s. This is how Clang determines
whether a parameter `isExplicitObjectParamater`. The LLDB expression
evaluator relies on this for calling "explicit object member functions".

Author: Michael137

clang/lib/AST/ASTImporter.cpp

@@ -4701,9 +4701,13 @@ ExpectedDecl ASTNodeImporter::VisitImplicitParamDecl(ImplicitParamDecl *D) {
 
 Error ASTNodeImporter::ImportDefaultArgOfParmVarDecl(
     const ParmVarDecl *FromParam, ParmVarDecl *ToParam) {
+
+  if (auto LocOrErr = import(FromParam->getExplicitObjectParamThisLoc()))
+    ToParam->setExplicitObjectParameterLoc(*LocOrErr);
+  else
+    return LocOrErr.takeError();
+
   ToParam->setHasInheritedDefaultArg(FromParam->hasInheritedDefaultArg());
-  ToParam->setExplicitObjectParameterLoc(
-      FromParam->getExplicitObjectParamThisLoc());
   ToParam->setKNRPromoted(FromParam->isKNRPromoted());
 
   if (FromParam->hasUninstantiatedDefaultArg()) {

clang/unittests/AST/ASTImporterTest.cpp

@@ -3441,13 +3441,33 @@ TEST_P(ASTImporterOptionSpecificTestBase, ImportParmVarDecl) {
   ASSERT_TRUE(FromVar);
   ASSERT_TRUE(FromVar->hasUninstantiatedDefaultArg());
   ASSERT_TRUE(FromVar->getUninstantiatedDefaultArg());
+  ASSERT_FALSE(FromVar->isExplicitObjectParameter());
 
   const auto *ToVar = Import(FromVar, Lang_CXX11);
   EXPECT_TRUE(ToVar);
   EXPECT_TRUE(ToVar->hasUninstantiatedDefaultArg());
   EXPECT_TRUE(ToVar->getUninstantiatedDefaultArg());
   EXPECT_NE(FromVar->getUninstantiatedDefaultArg(),
             ToVar->getUninstantiatedDefaultArg());
+  EXPECT_FALSE(ToVar->isExplicitObjectParameter());
+}
+
+TEST_P(ASTImporterOptionSpecificTestBase, ImportParmVarDecl_Explicit) {
+  const auto *Code = R"(
+    struct Wrapper {
+      void func(this Wrapper) {}
+    };
+    )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX23);
+  auto *FromVar = FirstDeclMatcher<ParmVarDecl>().match(FromTU, parmVarDecl());
+  ASSERT_TRUE(FromVar);
+  ASSERT_TRUE(FromVar->isExplicitObjectParameter());
+
+  const auto *ToVar = Import(FromVar, Lang_CXX23);
+  EXPECT_TRUE(ToVar);
+  EXPECT_TRUE(ToVar->isExplicitObjectParameter());
+  EXPECT_NE(ToVar->getExplicitObjectParamThisLoc(),
+            FromVar->getExplicitObjectParamThisLoc());
 }
 
 TEST_P(ASTImporterOptionSpecificTestBase, ImportOfNonEquivalentField) {