Scanners getting added even when tool is not present, LaTeX scanner error when subst exceptions are ignored

[DeeeeLAN]

Describe the bug

At the top of SCons/Tool/__init__.py, five scanners are configured with default file extensions, with no regard for their respective backend program's presence.

CScanner = SCons.Scanner.C.CScanner()
DScanner = SCons.Scanner.D.DScanner()
JavaScanner = SCons.Scanner.Java.JavaScanner()
LaTeXScanner = SCons.Scanner.LaTeX.LaTeXScanner()
PDFLaTeXScanner = SCons.Scanner.LaTeX.PDFLaTeXScanner()
ProgramScanner = SCons.Scanner.Prog.ProgramScanner()
SourceFileScanner = SCons.Scanner.ScannerBase({}, name='SourceFileScanner')
SWIGScanner = SCons.Scanner.SWIG.SWIGScanner()

CSuffixes = [".c", ".C", ".cxx", ".cpp", ".c++", ".cc",
             ".h", ".H", ".hxx", ".hpp", ".hh",
             ".F", ".fpp", ".FPP",
             ".m", ".mm",
             ".S", ".spp", ".SPP", ".sx"]

DSuffixes = ['.d']

IDLSuffixes = [".idl", ".IDL"]

LaTeXSuffixes = [".tex", ".ltx", ".latex"]

SWIGSuffixes = ['.i']

for suffix in CSuffixes:
    SourceFileScanner.add_scanner(suffix, CScanner)

for suffix in DSuffixes:
    SourceFileScanner.add_scanner(suffix, DScanner)

for suffix in SWIGSuffixes:
    SourceFileScanner.add_scanner(suffix, SWIGScanner)

# FIXME: what should be done here? Two scanners scan the same extensions,
# but look for different files, e.g., "picture.eps" vs. "picture.pdf".
# The builders for DVI and PDF explicitly reference their scanners
# I think that means this is not needed???
for suffix in LaTeXSuffixes:
    SourceFileScanner.add_scanner(suffix, LaTeXScanner)
    SourceFileScanner.add_scanner(suffix, PDFLaTeXScanner)

For at least the LaTeXScanner, this can lead to an error if AllowSubstExceptions() has been set to disallow undefined env variables, because LaTeXScanner is looking for a $LATEXSUFFIXES variable, which gets a default definition from the LaTeX tool.

Required information

• Link to SCons Users thread discussing your issue.
  • https://discord.com/channels/571796279483564041/571796280146133047/1228014186613702697
• Version of SCons
  • 4.7.0
• Version of Python
  • 3.11.0
• What Platform are you on? (Linux/Windows and which version)
  • Linux RHEL7
• How to reproduce your issue? Please include a small self contained reproducer. Likely a SConstruct should do for most issues.

env = Environment(tools=[])
AllowSubstExceptions()
env.Command('file2.tex', 'file1.tex', 'cp $SOURCE $TARGET')

• How you invoke scons
  • scons