introduce tyFromExpr; fixes #618

tests/compile/tmatrix3.nim

@@ -0,0 +1,41 @@
+discard """
+  output: '''0.0000000000000000e+00
+0.0000000000000000e+00
+0
+0
+0
+'''
+"""
+
+include compilehelpers
+
+type
+  Matrix*[M, N, T] = object 
+    aij*: array[M, array[N, T]]
+  
+  Matrix2*[T] = Matrix[range[0..1], range[0..1], T]
+  
+  Matrix3*[T] = Matrix[range[0..2], range[0..2], T]
+
+proc mn(x: Matrix): Matrix.T = x.aij[0][0]
+
+proc m2(x: Matrix2): Matrix2.T = x.aij[0][0]
+
+proc m3(x: Matrix3): auto = x.aij[0][0]
+
+var 
+  matn: Matrix[range[0..3], range[0..2], int]
+  mat2: Matrix2[int]
+  mat3: Matrix3[float]
+
+echo m3(mat3)
+echo mn(mat3)
+echo m2(mat2)
+echo mn(mat2)
+echo mn(matn)
+
+reject m3(mat2)
+reject m3(matn)
+reject m2(mat3)
+reject m2(matn)
+