optinal value should check before use

Optional argument value has been passed for the first argument of merge
routine. some compiler can't allow such situation.

Author: sakamoti

src/stdlib_string_type.fypp

@@ -421,7 +421,7 @@ contains
         type(string_type), intent(in) :: string
         integer :: ich
 
-        ich = merge(ichar(string%raw), 0, allocated(string%raw))
+        ich = merge(ichar(string%raw(1:1)), 0, allocated(string%raw))
 
     end function ichar_string
 
@@ -431,7 +431,7 @@ contains
         type(string_type), intent(in) :: string
         integer :: ich
 
-        ich = merge(iachar(string%raw), 0, allocated(string%raw))
+        ich = merge(iachar(string%raw(1:1)), 0, allocated(string%raw))
 
     end function iachar_string
 
@@ -571,8 +571,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = index(maybe(string), maybe(substring), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = index(maybe(string), maybe(substring),back)
+        else
+          pos = index(maybe(string), maybe(substring),.false.)
+        endif
 
     end function index_string_string
 
@@ -584,8 +587,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = index(maybe(string), substring, &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = index(maybe(string), substring,back)
+        else
+          pos = index(maybe(string), substring,.false.)
+        endif
 
     end function index_string_char
 
@@ -597,8 +603,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = index(string, maybe(substring), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = index(string, maybe(substring),back)
+        else
+          pos = index(string, maybe(substring),.false.)
+        endif
 
     end function index_char_string
 
@@ -612,8 +621,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = scan(maybe(string), maybe(set), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = scan(maybe(string), maybe(set),back)
+        else
+          pos = scan(maybe(string), maybe(set),.false.)
+        endif
 
     end function scan_string_string
 
@@ -625,8 +637,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = scan(maybe(string), set, &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = scan(maybe(string), set,back)
+        else
+          pos = scan(maybe(string), set,.false.)
+        endif
 
     end function scan_string_char
 
@@ -638,8 +653,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = scan(string, maybe(set), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = scan(string, maybe(set),back)
+        else
+          pos = scan(string, maybe(set),.false.)
+        endif
 
     end function scan_char_string
 
@@ -653,8 +671,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = verify(maybe(string), maybe(set), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = verify(maybe(string), maybe(set),back)
+        else
+          pos = verify(maybe(string), maybe(set),.false.)
+        endif
 
     end function verify_string_string
 
@@ -667,8 +688,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = verify(maybe(string), set, &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = verify(maybe(string), set,back)
+        else
+          pos = verify(maybe(string), set,.false.)
+        endif
 
     end function verify_string_char
 
@@ -681,8 +705,11 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        pos = verify(string, maybe(set), &
-            merge(back, .false., present(back)))
+        if(present(back))then
+          pos = verify(string, maybe(set),back)
+        else
+          pos = verify(string, maybe(set),.false.)
+        endif
 
     end function verify_char_string