Validation error "DXIL intrinsic overload must be valid"

Hey, I encountered a problem while compiling some shaders through dxcompile (normally we're using d3dcompile), with a cryptic validation error message.

Managed to narrow it down, and can reproduce with the attached shader: test.txt

Gives for output:

dxc -E ps_main -T ps_6_0 test.hlsl

error: validation errors
at 0x280ab609bc0 inside block #0 of function ps_main DXIL intrinsic overload must be valid

Validation failed.

My guess is that it can't decide whether to implicitly convert the ?: result from float3 down to float, or bump the 1.0 second dot argument up to float3. I think some improved error message for this would be useful (didn't get any line number or anything) :)

Start by compiling the attached test.txt shader with dxc -E ps_main -T ps_6_0 and reproduce the DXIL intrinsic overload validation error in ps_main. Trace the validation diagnostic to determine why it lacks a source location, then make the error identify the ambiguous expression and verify the improved output with this reproducer.