GLSL 1.10 spec section 5.3.2 says:
"It is an error to construct matrices from other matrices. This is reserved for future use."
But piglit/tests/glslparsertest/shaders/matrix.V110.frag (default #version 110) passes in mesa:
mat4 m1 = mat4(m);
p.s. This is allowed in GLSL 1.20.
Fixed with the glsl2 merge.
verified with mesa master d442a01ac14382d83cdaac87d2832315ceb3e963.