Deprecate no-op transpose fallback instead of error'ing.

NEWS.md

@@ -237,6 +237,9 @@ Deprecated or removed
 
   * `sub` and `slice` have been deprecated in favor of `view` ([#16972])
 
+  * The no-op `transpose` fallback has been deprecated. Consider introducing suitable
+    `transpose` methods or calling `permutedims(x, [2,1])` ([#13171], [#17075], [#17374]).
+
 [PkgDev]: https://github.com/JuliaLang/PkgDev.jl
 <!--- generated by NEWS-update.jl: -->
 [#1090]: https://github.com/JuliaLang/julia/issues/1090
@@ -251,6 +254,7 @@ Deprecated or removed
 [#11196]: https://github.com/JuliaLang/julia/issues/11196
 [#11242]: https://github.com/JuliaLang/julia/issues/11242
 [#13062]: https://github.com/JuliaLang/julia/issues/13062
+[#13171]: https://github.com/JuliaLang/julia/issues/13171
 [#13232]: https://github.com/JuliaLang/julia/issues/13232
 [#13338]: https://github.com/JuliaLang/julia/issues/13338
 [#13387]: https://github.com/JuliaLang/julia/issues/13387
@@ -299,5 +303,7 @@ Deprecated or removed
 [#16645]: https://github.com/JuliaLang/julia/issues/16645
 [#16731]: https://github.com/JuliaLang/julia/issues/16731
 [#16972]: https://github.com/JuliaLang/julia/issues/16972
-[#17266]: https://github.com/JuliaLang/julia/issues/17266
 [#17037]: https://github.com/JuliaLang/julia/issues/17037
+[#17075]: https://github.com/JuliaLang/julia/issues/17075
+[#17266]: https://github.com/JuliaLang/julia/issues/17266
+[#17374]: https://github.com/JuliaLang/julia/issues/17374

base/deprecated.jl

@@ -784,6 +784,15 @@ function symperm{Tv,Ti}(A::SparseMatrixCSC{Tv,Ti}, pinv::Vector{Ti})
         "Pkg.add(\"SuiteSparse\") to install SuiteSparse on Julia v0.5."))
 end
 
+# Deprecate no-op transpose fallback. Please see #13171 and #17075.
+function transpose(x)
+    depwarn(string("the no-op `transpose` fallback is deprecated, and no more specific ",
+        "`transpose` method for $(typeof(x)) exists. Consider `permutedims(x, [2, 1])` ",
+        "or writing a specific `transpose(x::$(typeof(x)))` method if appropriate."),
+        :transpose)
+    return x
+end
+
 # During the 0.5 development cycle, do not add any deprecations below this line
 # To be deprecated in 0.6
 

base/operators.jl

@@ -293,7 +293,6 @@ fldmod1{T<:Real}(x::T, y::T) = (fld1(x,y), mod1(x,y))
 fldmod1{T<:Integer}(x::T, y::T) = (fld1(x,y), mod1(x,y))
 
 # transpose
-transpose(x) = throw(ArgumentError("transpose not implemented for $(typeof(x)). Consider permutedims."))
 ctranspose(x) = conj(transpose(x))
 conj(x) = x