improved documentation and added new test cases

Author: aman-godara

doc/specs/stdlib_string_type.md

@@ -1981,7 +1981,7 @@ end program demo
 
 Moves the allocation from `from` to `to`, consequently deallocating `from` in this process.  
 If `from` is not allocated before execution, `to` gets deallocated by the process.  
-An unallocated string is equivalent to an empty string.
+An unallocated `string_type` instance is equivalent to an empty string.
 
 #### Syntax
 
@@ -2005,25 +2005,25 @@ Pure Subroutine.
 #### Example
 
 ```fortran
-program demo
+program demo_move
   use stdlib_string_type, only : string_type, assignment(=), move
   implicit none
-  type(string_type) :: from_string, to_string
-  character(len=:), allocatable :: from_char
+  type(string_type) :: from_string
+  character(len=:), allocatable :: from_char, to_char
 
   from_string = "move this string"
   from_char = "move this char"
   ! from_string <-- "move this string"
   ! from_char   <-- "move this char"
-  ! to_string   <-- "" (unallocated)
+  ! to_char   <-- (unallocated)
 
-  call move(from_string, to_string)
-  ! from_string <-- "" (unallocated)
-  ! to_string   <-- "move this string"
+  call move(from_string, to_char)
+  ! from_string <-- ""
+  ! to_char   <-- "move this string"
 
-  call move(from_char, to_string)
+  call move(from_char, to_char)
   ! from_char <-- (unallocated)
   ! to_string <-- "move this char"
 
-end program demo
+end program demo_move
 ```

src/tests/string/test_string_intrinsic.f90

@@ -464,14 +464,14 @@ subroutine test_iachar
     end subroutine test_iachar
 
     subroutine test_move
-        type(string_type) :: from_string
-        type(string_type) :: to_string
-        character(len=:), allocatable :: from_char
+        type(string_type) :: from_string, to_string
+        character(len=:), allocatable :: from_char, to_char
 
         from_string = "Move This String"
         from_char = "Move This Char"
         call check(from_string == "Move This String" .and. to_string == "" .and. &
-                    & from_char == "Move This Char", "move: test_case 1")
+                    & from_char == "Move This Char" .and. .not. allocated(to_char), &
+                    & "move: test_case 1")
 
         ! string_type (allocated) --> string_type (not allocated)
         call move(from_string, to_string)
@@ -483,17 +483,27 @@ subroutine test_move
                     & "move: test_case 3")
 
         ! string_type (allocated) --> character (not allocated)
-        call move(to_string, from_char)
-        call check(to_string == "" .and. from_char == "Move This String", "move: test_case 4")
+        call move(to_string, to_char)
+        call check(to_string == "" .and. to_char == "Move This String", "move: test_case 4")
 
         ! character (allocated) --> string_type (allocated)
-        call move(from_char, from_string)
-        call check(.not. allocated(from_char) .and. from_string == "Move This String", &
+        call move(to_char, from_string)
+        call check(.not. allocated(to_char) .and. from_string == "Move This String", &
                     & "move: test_case 5")
 
+        from_char = "new char"
         ! character (allocated) --> string_type (allocated)
         call move(from_char, from_string)
-        call check(.not. allocated(from_char) .and. from_string == "", "move: test_case 6")
+        call check(.not. allocated(from_char) .and. from_string == "new char", "move: test_case 6")
+
+        ! character (unallocated) --> string_type (allocated)
+        call move(from_char, from_string)
+        call check(from_string == "", "move: test_case 7")
+
+        from_string = "moving to self"
+        ! string_type (allocated) --> string_type (allocated)
+        call move(from_string, from_string)
+        call check(from_string == "", "move: test_case 8")
 
     end subroutine test_move