(The Reference page phrasing is also somewhat vague because it has a paragraph specifically about "the second overload"—the one containing an x parameter—but the text about undefined results is not contained in this paragraph.)
FWIW, the SPIR-V definition of Atan2 matches the GLSL specification's text (undefined when x and y are both 0), not the Reference pages text (x=0).
The Reference pages for
atanindicate that "the result is undefined if x=0".https://registry.khronos.org/OpenGL-Refpages/gl4/html/atan.xhtml
However, the GLSL specification does not match, and for atan it states "results are undefined if x and y are both 0."
https://registry.khronos.org/OpenGL/specs/gl/GLSLangSpec.4.60.pdf
(The Reference page phrasing is also somewhat vague because it has a paragraph specifically about "the second overload"—the one containing an x parameter—but the text about undefined results is not contained in this paragraph.)
FWIW, the SPIR-V definition of Atan2 matches the GLSL specification's text (undefined when x and y are both 0), not the Reference pages text (x=0).