Reject unsorted t in munge_data for matrix/N-d u

src/interpolation_utils.jl

@@ -328,6 +328,13 @@ function munge_data(U::AbstractMatrix, t::AbstractVector)
     TU = nonmissingtype(eltype(U))
     Tt = nonmissingtype(eltype(t))
     if TU === eltype(U) && Tt === eltype(t)
+        if !issorted(t; by = ForwardDiff.value)
+            throw(
+                ArgumentError(
+                    "The second argument (`t`), which is used for the interpolation domain, is not sorted."
+                )
+            )
+        end
         return U, t
     end
 
@@ -347,6 +354,13 @@ function munge_data(U::AbstractArray{T, N}, t) where {T, N}
     TU = nonmissingtype(eltype(U))
     Tt = nonmissingtype(eltype(t))
     if TU === eltype(U) && Tt === eltype(t)
+        if !issorted(t; by = ForwardDiff.value)
+            throw(
+                ArgumentError(
+                    "The second argument (`t`), which is used for the interpolation domain, is not sorted."
+                )
+            )
+        end
         return U, t
     end
 

test/interpolation_tests.jl

@@ -2214,6 +2214,16 @@ f_cubic_spline = c -> square(CubicSpline, c)
     end
 end
 
+@testset "munge_data rejects unsorted t for matrix/N-d u" begin
+    # The vector `munge_data` method already rejects unsorted `t`; the matrix and
+    # N-d methods silently accepted it and returned wrong values
+    # (https://github.com/SciML/DataInterpolations.jl, candidate 5).
+    us = [1.0, 2.0, 3.0, 4.0]
+    ts = [1.0, 3.0, 2.0, 4.0]
+    @test_throws ArgumentError LinearInterpolation([us'; us'], ts)
+    @test_throws ArgumentError LinearInterpolation(reshape(vcat(us, us), 2, 1, 4), ts)
+end
+
 @testset "user error" begin
     @test_throws ArgumentError LinearInterpolation(rand(10), rand(10))
     @test_throws ArgumentError LinearInterpolation(1:10, rand(10))