Fix `NestedNameSpecifierLoc::getLocalSourceRange()`

[eduucaldas]

Summary

From the documentation comment above NestedNameSpecifierLoc::getLocalSourceRange()

/// Retrieve the source range covering just the last part of
/// this nested-name-specifier, not including the prefix.
///
/// For example, if this instance refers to a nested-name-specifier
/// \c \::std::vector<int>::, the returned source range would cover
/// from "vector" to the last '::'.

We would expect that for a nested-name-specifier with a dependent template specialization type: T::template ST<int>:: NestedNameSpecifierLoc::getLocalSourceRange() would return template ST<int>:: but instead it returns T::template ST<int>::.

The issue might as well be coming from DependentTemplateSpecializationTypeLoc::getLocalSourceRange() which is
indirectly called from NestedNameSpecifierLoc::getLocalSourceRange(). I couldn't determine what is the intended behaviour for that though.

[eduucaldas]

Don't assume the code is completely correct

[eduucaldas]

Update comment

[eduucaldas]

Unit tests will be provided once we decide that this is the way to go ;)

[eduucaldas]

.

clang/lib/AST/NestedNameSpecifier.cpp

return SourceRange(LoadSourceLocation(Data, Offset),
                        LoadSourceLocation(Data, Offset + sizeof(unsigned)));
 
   case NestedNameSpecifier::TypeSpecWithTemplate:
   case NestedNameSpecifier::TypeSpec: {
     // The "void*" that points at the TypeLoc data.
     // Note: the 'template' keyword is part of the TypeLoc.
     void *TypeData = LoadPointer(Data, Offset);
     TypeLoc TL(Qualifier->getAsType(), TypeData);
+    // For `T::template ST<int>::x`
+    // `NestedNameSpecifierLoc::getLocalSourceRange()` should return `template
+    // ST<int>::`.
+    // For any TypeLoc we can just use `TypeLoc::getBeginLoc()` to
+    // get the beginning of this LocalSourceRange but for dependent template
+    // specializations `getBeginLoc` returns the location of `T` yielding
+    // `T::template ST<int>::`.
+    if (auto DependentTL = TL.getAs<DependentTemplateSpecializationTypeLoc>()) {
+      if (DependentTL.getTemplateKeywordLoc().isValid())
+        return SourceRange(DependentTL.getTemplateKeywordLoc(),
+                           LoadSourceLocation(Data, Offset + sizeof(void *)));
+      return SourceRange(DependentTL.getTemplateNameLoc(),
+                         LoadSourceLocation(Data, Offset + sizeof(void *)));
+    }
     return SourceRange(TL.getBeginLoc(),
                        LoadSourceLocation(Data, Offset + sizeof(void*)));
   }
   }
 
   llvm_unreachable("Invalid NNS Kind!");
 }