cannot specify functions as input files if there are rule dependencies

I cannot use the inputs of a rule as a dependency if any of those inputs are functions.

Reproducer:
Consider the following Snakefile:

rule one:
    input:
        a="a.txt",
        b=lambda wildcards: "b.txt",


rule two:
    input:
        rules.one.input.a,

When run like snakemake -n, rule two fails with error

TypeError in line 9 of /<path/to>/Snakefile:
expected string or bytes-like object
  File "/<path/to>/Snakefile", line 9, in <module>

Note that this happens even though two does not depend on the functional input b. The presence of one function as an input is enough to cause the failure.

Start with the reproduced Snakefile and run snakemake -n to confirm the TypeError at rules.one.input.a. Trace how rule input dependencies are resolved when another input is a function. Done means rule two can depend on rules.one.input.a without failing because rule one also has the functional input b.