Heat is a problem for guitars. Glue softens at around 150 degrees F.
In the open air on a sunny day there won't be a problem with overheating. However, if the guitar is in a black (heat absorbing) case in the sun, it might get hot enough to soften the glue.
I had a sunny day problem with a guitar stand. The neck support is covered with black rubber. The rubber got so hot it melted the finish on my Guild 12 string where the support touched the neck. This happened indoors with sunlight coming through a window 
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2009 CA Cargo Raw, 2006 Collings OM-1 SS light build, 2004 Taylor 714ce, 2000 Taylor 310K, 1991 Martin HD-28, 1971 Martin 0-18, 1967 Guild F-30
2006 Ovation Legend 6756LX 12 string, 2004 Taylor 354ce 12 string, 1976 Guild G312-NT 12 string (dreadnaught shape)
1966 Martin T-15 tiple, Mele koa ukulele
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