cmd/compile: //go:build file version ignored when calling generic fn which has related type params (reproduces with 1.21 and 1.20, not at tip) [1.20 backport]

[gopherbot]

@griesemer requested issue #64759 to be considered for backport to the next 1.20 minor release.

@gopherbot Please consider this for backport to 1.21 and 1.20, this causes a compilation failure for code that chooses to select a future Go version in one of its source files.

[griesemer]

Closing this as not feasible (see comment). The code environment in 1.20 doesn't exist for this.