RFC: Neither advertise full keyword of qr nor test it on 0.7 (#558)

* Neither advertise `full` keyword of `qr` nor test it on 0.7

The `thin` keyword was replaced by `full`, but later abandoned
completely. To keep code working on Julia 0.6 that had adapted to
`full`, the interface is retained on Julia 0.6, but neither tested on
0.7 (where it doesn't work) nor advertised in the README.

* Add brief comments explaining the qr situation

README.md

@@ -299,7 +299,7 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `fetch` for `Task`s ([#25940]).
 
-* `Compat.qr` takes `pivot` as a `Val` _instance_ and keyword argument `full` ([#22475], [#24279]).
+* `Compat.qr` takes `pivot` as a `Val` _instance_ ([#22475]).
 
 * `Compat.rmul!` provides a subset of the functionality of `LinearAlgebra.rmul!` for
   use with Julia 0.6 ([#25701], [#25812]).
@@ -511,7 +511,6 @@ includes this fix. Find the minimum version from there.
 [#18510]: https://github.com/JuliaLang/julia/issues/18510
 [#18629]: https://github.com/JuliaLang/julia/issues/18629
 [#18727]: https://github.com/JuliaLang/julia/issues/18727
-[#18839]: https://github.com/JuliaLang/julia/issues/18839
 [#18977]: https://github.com/JuliaLang/julia/issues/18977
 [#19088]: https://github.com/JuliaLang/julia/issues/19088
 [#19246]: https://github.com/JuliaLang/julia/issues/19246
@@ -561,7 +560,6 @@ includes this fix. Find the minimum version from there.
 [#23931]: https://github.com/JuliaLang/julia/issues/23931
 [#24047]: https://github.com/JuliaLang/julia/issues/24047
 [#24182]: https://github.com/JuliaLang/julia/issues/24182
-[#24279]: https://github.com/JuliaLang/julia/issues/24279
 [#24282]: https://github.com/JuliaLang/julia/issues/24282
 [#24361]: https://github.com/JuliaLang/julia/issues/24361
 [#24372]: https://github.com/JuliaLang/julia/issues/24372

src/Compat.jl

@@ -1818,6 +1818,8 @@ if !isdefined(Base, :selectdim) # 0.7.0-DEV.3976
 end
 
 if VERSION < v"0.7.0-DEV.2337"
+    # qr doesn't take the full keyword anymore since 0.7.0-DEV.5211; we still support it
+    # here to avoid unneccesary breakage
     if VERSION < v"0.7.0-DEV.843"
         qr(A::Union{Number,AbstractMatrix}, pivot::Union{Val{false},Val{true}}=Val(false); full=false) =
             Base.qr(A, typeof(pivot), thin=!full)

test/runtests.jl

@@ -1674,24 +1674,36 @@ end
 
 # 0.7.0-DEV.843 / 0.7.0-DEV.2337
 let A = [1 2; 1 2; 1 2]
-    f = Compat.qr(A, Val(false), full=false)
-    @test f == Compat.qr(A, Val(false))
-    @test length(f) == 2
-    @test size(f[1]) == (3, 2)
-    @test f[1] * f[2] ≈ A
-    f = Compat.qr(A, Val(false), full=true)
-    @test length(f) == 2
-    @test size(f[1]) == (3, 3)
-    @test f[1] * [f[2]; [0 0]] ≈ A
-    f = Compat.qr(A, Val(true), full=false)
-    @test f == Compat.qr(A, Val(true))
-    @test length(f) == 3
-    @test size(f[1]) == (3, 2)
-    @test f[1] * f[2] ≈ A[:,f[3]]
-    f = Compat.qr(A, Val(true), full=true)
-    @test length(f) == 3
-    @test size(f[1]) == (3, 3)
-    @test f[1] * [f[2]; [0 0]] ≈ A[:,f[3]]
+    if VERSION < v"0.7.0-DEV.5211"
+        # the full keyword was only temporarily available in Base, so these  methods don't
+        # work on 0.7 anymore, but we test them for the time being to avoid accidentally
+        # breaking anyone's code
+        f = Compat.qr(A, Val(false), full=false)
+        @test f == Compat.qr(A, Val(false))
+        @test length(f) == 2
+        @test size(f[1]) == (3, 2)
+        @test f[1] * f[2] ≈ A
+        f = Compat.qr(A, Val(false), full=true)
+        @test length(f) == 2
+        @test size(f[1]) == (3, 3)
+        @test f[1] * [f[2]; [0 0]] ≈ A
+        f = Compat.qr(A, Val(true), full=false)
+        @test f == Compat.qr(A, Val(true))
+        @test length(f) == 3
+        @test size(f[1]) == (3, 2)
+        @test f[1] * f[2] ≈ A[:,f[3]]
+        f = Compat.qr(A, Val(true), full=true)
+        @test length(f) == 3
+        @test size(f[1]) == (3, 3)
+        @test f[1] * [f[2]; [0 0]] ≈ A[:,f[3]]
+    else
+        f = Compat.qr(A, Val(false))
+        @test size(f.Q) == (3, 3)
+        @test f.Q * [f.R; [0 0]] ≈ A
+        f = Compat.qr(A, Val(true))
+        @test size(f.Q) == (3, 3)
+        @test f.Q * [f.R; [0 0]] ≈ A[:,f.p]
+    end
 end
 
 let A = [1 2; 3 4]