diff --git a/stdlib/LinearAlgebra/src/tridiag.jl b/stdlib/LinearAlgebra/src/tridiag.jl
index 3f8eb5da9fc9d..84c79f57debc7 100644
--- a/stdlib/LinearAlgebra/src/tridiag.jl
+++ b/stdlib/LinearAlgebra/src/tridiag.jl
@@ -372,7 +372,7 @@ function tril!(M::SymTridiagonal{T}, k::Integer=0) where T
         return Tridiagonal(M.ev,M.dv,zero(M.ev))
     elseif k == 0
         return Tridiagonal(M.ev,M.dv,zero(M.ev))
-    elseif k >= 1
+    else # if k >= 1
         return Tridiagonal(M.ev,M.dv,copy(M.ev))
     end
 end
@@ -391,7 +391,7 @@ function triu!(M::SymTridiagonal{T}, k::Integer=0) where T
         return Tridiagonal(zero(M.ev),M.dv,M.ev)
     elseif k == 0
         return Tridiagonal(zero(M.ev),M.dv,M.ev)
-    elseif k <= -1
+    else # if k <= -1
         return Tridiagonal(M.ev,M.dv,copy(M.ev))
     end
 end
diff --git a/stdlib/LinearAlgebra/test/tridiag.jl b/stdlib/LinearAlgebra/test/tridiag.jl
index 759d692f8bc68..3330fa682fe5e 100644
--- a/stdlib/LinearAlgebra/test/tridiag.jl
+++ b/stdlib/LinearAlgebra/test/tridiag.jl
@@ -135,27 +135,43 @@ end
         @test_throws ArgumentError tril!(SymTridiagonal(d, dl), n)
         @test_throws ArgumentError tril!(Tridiagonal(dl, d, du), -n - 2)
         @test_throws ArgumentError tril!(Tridiagonal(dl, d, du), n)
-        @test tril(SymTridiagonal(d,dl))    == Tridiagonal(dl,d,zerosdl)
-        @test tril(SymTridiagonal(d,dl),1)  == Tridiagonal(dl,d,dl)
-        @test tril(SymTridiagonal(d,dl),-1) == Tridiagonal(dl,zerosd,zerosdl)
-        @test tril(SymTridiagonal(d,dl),-2) == Tridiagonal(zerosdl,zerosd,zerosdl)
-        @test tril(Tridiagonal(dl,d,du))    == Tridiagonal(dl,d,zerosdu)
-        @test tril(Tridiagonal(dl,d,du),1)  == Tridiagonal(dl,d,du)
-        @test tril(Tridiagonal(dl,d,du),-1) == Tridiagonal(dl,zerosd,zerosdu)
-        @test tril(Tridiagonal(dl,d,du),-2) == Tridiagonal(zerosdl,zerosd,zerosdu)
+        @test @inferred(tril(SymTridiagonal(d,dl)))    == Tridiagonal(dl,d,zerosdl)
+        @test @inferred(tril(SymTridiagonal(d,dl),1))  == Tridiagonal(dl,d,dl)
+        @test @inferred(tril(SymTridiagonal(d,dl),-1)) == Tridiagonal(dl,zerosd,zerosdl)
+        @test @inferred(tril(SymTridiagonal(d,dl),-2)) == Tridiagonal(zerosdl,zerosd,zerosdl)
+        @test @inferred(tril(Tridiagonal(dl,d,du)))    == Tridiagonal(dl,d,zerosdu)
+        @test @inferred(tril(Tridiagonal(dl,d,du),1))  == Tridiagonal(dl,d,du)
+        @test @inferred(tril(Tridiagonal(dl,d,du),-1)) == Tridiagonal(dl,zerosd,zerosdu)
+        @test @inferred(tril(Tridiagonal(dl,d,du),-2)) == Tridiagonal(zerosdl,zerosd,zerosdu)
+        @test @inferred(tril!(copy(SymTridiagonal(d,dl))))    == Tridiagonal(dl,d,zerosdl)
+        @test @inferred(tril!(copy(SymTridiagonal(d,dl)),1))  == Tridiagonal(dl,d,dl)
+        @test @inferred(tril!(copy(SymTridiagonal(d,dl)),-1)) == Tridiagonal(dl,zerosd,zerosdl)
+        @test @inferred(tril!(copy(SymTridiagonal(d,dl)),-2)) == Tridiagonal(zerosdl,zerosd,zerosdl)
+        @test @inferred(tril!(copy(Tridiagonal(dl,d,du))))    == Tridiagonal(dl,d,zerosdu)
+        @test @inferred(tril!(copy(Tridiagonal(dl,d,du)),1))  == Tridiagonal(dl,d,du)
+        @test @inferred(tril!(copy(Tridiagonal(dl,d,du)),-1)) == Tridiagonal(dl,zerosd,zerosdu)
+        @test @inferred(tril!(copy(Tridiagonal(dl,d,du)),-2)) == Tridiagonal(zerosdl,zerosd,zerosdu)
 
         @test_throws ArgumentError triu!(SymTridiagonal(d, dl), -n)
         @test_throws ArgumentError triu!(SymTridiagonal(d, dl), n + 2)
         @test_throws ArgumentError triu!(Tridiagonal(dl, d, du), -n)
         @test_throws ArgumentError triu!(Tridiagonal(dl, d, du), n + 2)
-        @test triu(SymTridiagonal(d,dl))    == Tridiagonal(zerosdl,d,dl)
-        @test triu(SymTridiagonal(d,dl),-1) == Tridiagonal(dl,d,dl)
-        @test triu(SymTridiagonal(d,dl),1)  == Tridiagonal(zerosdl,zerosd,dl)
-        @test triu(SymTridiagonal(d,dl),2)  == Tridiagonal(zerosdl,zerosd,zerosdl)
-        @test triu(Tridiagonal(dl,d,du))    == Tridiagonal(zerosdl,d,du)
-        @test triu(Tridiagonal(dl,d,du),-1) == Tridiagonal(dl,d,du)
-        @test triu(Tridiagonal(dl,d,du),1)  == Tridiagonal(zerosdl,zerosd,du)
-        @test triu(Tridiagonal(dl,d,du),2)  == Tridiagonal(zerosdl,zerosd,zerosdu)
+        @test @inferred(triu(SymTridiagonal(d,dl)))    == Tridiagonal(zerosdl,d,dl)
+        @test @inferred(triu(SymTridiagonal(d,dl),-1)) == Tridiagonal(dl,d,dl)
+        @test @inferred(triu(SymTridiagonal(d,dl),1))  == Tridiagonal(zerosdl,zerosd,dl)
+        @test @inferred(triu(SymTridiagonal(d,dl),2))  == Tridiagonal(zerosdl,zerosd,zerosdl)
+        @test @inferred(triu(Tridiagonal(dl,d,du)))    == Tridiagonal(zerosdl,d,du)
+        @test @inferred(triu(Tridiagonal(dl,d,du),-1)) == Tridiagonal(dl,d,du)
+        @test @inferred(triu(Tridiagonal(dl,d,du),1))  == Tridiagonal(zerosdl,zerosd,du)
+        @test @inferred(triu(Tridiagonal(dl,d,du),2))  == Tridiagonal(zerosdl,zerosd,zerosdu)
+        @test @inferred(triu!(copy(SymTridiagonal(d,dl))))    == Tridiagonal(zerosdl,d,dl)
+        @test @inferred(triu!(copy(SymTridiagonal(d,dl)),-1)) == Tridiagonal(dl,d,dl)
+        @test @inferred(triu!(copy(SymTridiagonal(d,dl)),1))  == Tridiagonal(zerosdl,zerosd,dl)
+        @test @inferred(triu!(copy(SymTridiagonal(d,dl)),2))  == Tridiagonal(zerosdl,zerosd,zerosdl)
+        @test @inferred(triu!(copy(Tridiagonal(dl,d,du))))    == Tridiagonal(zerosdl,d,du)
+        @test @inferred(triu!(copy(Tridiagonal(dl,d,du)),-1)) == Tridiagonal(dl,d,du)
+        @test @inferred(triu!(copy(Tridiagonal(dl,d,du)),1))  == Tridiagonal(zerosdl,zerosd,du)
+        @test @inferred(triu!(copy(Tridiagonal(dl,d,du)),2))  == Tridiagonal(zerosdl,zerosd,zerosdu)
 
         @test !istril(SymTridiagonal(d,dl))
         @test istril(SymTridiagonal(d,zerosdl))