Hi all,

as the PR (and the attached testcase) shows, gfortran doesn't handle 
noncontiguous access with substrings. Namely, "str_array(:)(1:5)" is 
mishandled (segfault while building the "tree" as expr == NULL).

As Jerry has dug up in the standard (F2008, R610 alias F2018, R908), 
substrings are only permitted on scalars (or array elements). – 
[gfortran has some partial support for noncontiguous memory as actual 
argument the now rejected test cases show.]

In words of the standard (here: F2018):

9.4.1  Substrings

A substring is a contiguous portion of a character string (7.4.4).

R908   substring   is   parent-string  ( substring-range )

R909   parent-string
             is   scalar-variable-name
             or  array-element
             or  coindexed-named-object
             or  scalar-structure-component
             or  scalar-constant

The patch adds a check to the variable resolving – it comes too late to 
print a nice error message for "string(:)(:)"; that statement gets 
matched unsuccessfully and at the end the buffered error message of 
match_variable (unexpected character "(") gets printed. – One could 
reject it earlier but the code doesn't seem to be that nice. (See PR for 
a variant.)

Build and regtested on x86_64-linux.
OK for the trunk?

Cheers

Tobias