Objects in the symbol table are intentionally constant. However, when
converting "ExternalValue" AST nodes into symbols via symbol table
lookup, IRGenerator::convertIdentifier was casting away constness
because ExternalValueReference held a non-const pointer. Fixing this
involved significant ripple-effect additions of "const" throughout the
ExternalValue class and its subclasses.
These changes generally appear to be benign, but one interesting edge
case is `ExternalValue::write`, which intuitively does not seem to make
sense as a const method. However, invoking `write` should not alter the
ExternalValue object itself; rather, it is intended to alter the
*external value* that is being referenced. (In practice, nothing invokes
write() anyway except for one unit test, which continues to pass.)
This issue was discovered while converting casts to `as<T>()` calls.
Change-Id: I8ff6a477e475833d2a99c72f1c79c766b57767ee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311276
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
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