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copied!<p>In the following, I'll explain using the complete example below (which you can compile and link to try things):</p> <pre><code>module mymodule contains integer function foo () foo = 1 end function integer function bar () integer :: foo bar = foo() end function end module program test use mymodule print *, bar() end </code></pre> <p>In the code of function <code>bar</code>, the declaration <code>integer :: foo</code> is strictly equivalent to:</p> <pre><code>integer, external :: foo </code></pre> <p>Thus, in the code of <code>bar</code>, you are explicitly stating:</p> <blockquote> <p><em>"there may already be a symbol of name <code>foo</code> accessible to you, but from now on, when I use it I mean it to be an external function of this name"</em></p> </blockquote> <p>So, this is valid code, and the compiler just expect you to provide an <code>external</code> function named <code>foo</code>. Because you don't (the module function isn't external), it fails to link. You can provide an external <code>foo</code> function by adding the following code (not in the module, just at the end of the same file):</p> <pre><code>integer function foo () foo = 42 end function </code></pre> <p>If you add this function body, then your code will compile, and the output will be <code>42</code> (as the external function is called, not the module function).</p> <p>Also worth noting, if you comment out <code>integer :: foo</code> line in the code of <code>bar</code>, symbol <code>foo</code> will resolve to the module function, which will then be called whether or not you provide an external function named <code>foo</code> (thus, the output will be <code>1</code>).</p> <p><strong>Conclusion:</strong> not a compiler bug, but misuse of an old feature of the language (external declarations). To be honest, I think it's better to explicitly mark your <code>external</code> declarations as such, which would at least have highlighted the issue here.</p>

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