OpenMSX switching video gives errors

Accumulator, perhaps your computer contains an OpenGL version where pi constant is already defined for shaders. OpenMSX compiles scale2x.vert vertex shader program during execution. The file should have a similar code to this (my version is 0.14, a little bit old, but it should be similar):

uniform mat4 u_mvpMatrix;
uniform vec3 texSize;

attribute vec4 a_position;
attribute vec3 a_texCoord;

varying vec2 texStep; // could be uniform
varying vec2 coord2pi;
varying vec2 texCoord;
varying vec2 videoCoord;

float pi = 4.0 * atan(1.0);
float pi2 = 2.0 * pi;

void main()
{
	gl_Position = u_mvpMatrix * a_position;
	texCoord = a_texCoord.xy;
	coord2pi = a_texCoord.xy * texSize.xy * pi2;
	texStep = 1.0 / texSize.xy;

#if SUPERIMPOSE
	videoCoord = a_texCoord.xz;
#endif
}

I propose several solutions, but you must copy scale2x.vert with other name before trying them. Restore the file if my solutions don't work:

First possible solution: If pi is already defined by yout openGL version, try to comment pi definition:
//float pi = 4.0 * atan(1.0);

Second possible solution: If my earlier proposal doesn't work (you will get an error on pi2 definition), try to define pi as constant
const float pi = 4.0 * atan(1.0);

Third possible solution: If both earlier proposals don't work, try to rename pi as c_pi in definition and uses of pi variable:

...
float c_pi = 4.0 * atan(1.0);
float pi2 = 2.0 * c_pi;
...

Fourth possible solution is pi should be defined as uniform above, but this would be a openMSX error because openMSX code should pass pi as uniform variable before invoking shader, but I don't think so. Uniform variables are constant during shader execution. Could you tell us your OpenMSX version?

Anyway, I hope you can solve the problem.

On this laptop OpenMSX 17.0, HD Graphics 500, Mesa 22.2.5, i915

All my machines are running Linux OS, Fedora, Ubuntu and Debian, dual boot.

Personally I'd quickly try giving just:

float pi = 3.1415926538;

a whirl; the guess being that your OpenGL is failing to recognise that atan(1.0) is a constant expression. It unambiguously is a constant expression per the language rules — "The return value of any built-in function [is a constant expression] ... if all of the arguments to the function are themselves constant expressions." — but that test would certainly help to diagnose.

Best solution is from TomH, however

I changed it to:

const float pi = 3.1415926538;

works perfectly. No Errors.
removing pi, (//) results in pi undefined (const float & float)

uniform mat4 u_mvpMatrix;
uniform vec3 texSize;

attribute vec4 a_position;
attribute vec3 a_texCoord;

varying vec2 texStep; // could be uniform
varying vec2 coord2pi;
varying vec2 texCoord;
varying vec2 videoCoord;

const float pi = 3.1415926538;
const float pi2 = 2.0 * pi;

void main()
{
	gl_Position = u_mvpMatrix * a_position;
	texCoord = a_texCoord.xy;
	coord2pi = a_texCoord.xy * texSize.xy * pi2;
	texStep = 1.0 / texSize.xy;

#if SUPERIMPOSE
	videoCoord = a_texCoord.xz;
#endif
}