Make eval a reserved keyword

Like `set` and `read` before it, `eval` can be used to set variables,
and so it can't be shadowed by a function without loss of
functionality.

So this forbids it.

Incidentally, this means we will no longer try to autoload an
`eval.fish` file that's left over from an old version, which would
have helped with #8963.

src/parser_keywords.cpp

@@ -24,7 +24,7 @@ static const string_set_t block_keywords = {L"for",      L"while",  L"if",
 
 static const wcstring reserved_keywords[] = {
     L"end",  L"case",   L"else", L"return", L"continue", L"break", L"argparse",
-    L"read", L"string", L"set",  L"status", L"test",     L"[",     L"_"};
+    L"read", L"string", L"set",  L"status", L"test", L"[", L"_", L"eval"};
 
 // The lists above are purposely implemented separately from the logic below, so that future
 // maintainers may assume the contents of the list based off their names, and not off what the

tests/checks/eval.fish

@@ -60,3 +60,25 @@ echo empty block eval: $status # 0
 source /banana/\t/foo
 # CHECKERR: source: Error encountered while sourcing file '/banana/\t/foo':
 # CHECKERR: source: No such file or directory
+
+# See that eval can't be defined as a function
+function eval
+    builtin eval $argv
+end
+# CHECKERR: checks/eval.fish (line {{\d+}}): function: eval: cannot use reserved keyword as function name
+# CHECKERR: function eval
+# CHECKERR: ^
+
+
+function evil --no-scope-shadowing
+    eval $argv
+end
+
+# And this is why we do this: `eval` *can't* be cleanly shadowed with a function,
+# because that would always introduce a new block or at the very least clobber $argv.
+eval set -l argv this works
+echo $argv
+# CHECK: this works
+evil set -l argv this does not
+echo $argv
+# CHECK: this works