This is a serious bug in Clang: it generates incorrect machine code.

The code that Clang generates for the following (gawk/support/dfa.c 
lines 1141-1143):

     ((dfa->syntax.dfaopts & DFA_CONFUSING_BRACKETS_ERROR
       ? dfaerror : dfawarn)
      (_("character class syntax is [[:space:]], not [:space:]")));

is immediately followed by the code generated for the following 
(gawk/support/dfa.c line 1015):

                     dfaerror (_("invalid character class"));

and this is incorrect because the two source code regions are not 
connected with each other.

You can see the bug in the attached (compressed) file dfa.s which 
contains the assembly language output. Here's the dfa.s file starting 
with line 6975:

   6975		testb	$4, 456(%r12)
   6976		movl	$dfawarn, %eax
   6977		movl	$dfaerror, %ebx
   6978		cmoveq	%rax, %rbx
   6979		movl	$.L.str.26, %esi
   6980		xorl	%edi, %edi
   6981		movl	$5, %edx
   6982		callq	dcgettext
   6983		movq	%rax, %rdi
   6984		callq	*%rbx
   6985	.LBB34_144:
   6986		movl	$.L.str.25, %esi
   6987		xorl	%edi, %edi
   6988		movl	$5, %edx
   6989		callq	dcgettext
   6990		movq	%rax, %rdi
   6991		callq	dfaerror

Line 6984, which is source lines 1141-1143 call to either dfaerror or 
dfawarn, is immediately followed by the code for source line 1015. This 
means that at runtime when dfawarn returns the code immediately calls 
dfaerror, which is incorrect.

My guess is that Clang got confused because dfaerror is declared 
_Noreturn, so Clang mistakenly assumed that dfawarn is also _Noreturn, 
which it is not.

I worked around the Clang bug by installed the attached patch into 
Gnulib. Please give it a try with Gawk.

Incorrect code generation is a serious bug in Clang; can you please 
report it to the Clang folks? I am considering using a bigger hammer, 
and doing this:

    #define _Noreturn /*empty*/

whenever Clang is used, until the bug is fixed.

This is because if the bug occurs here it's likely that similar bugs 
will occur elsewhere and this sort of thing can be really subtle and 
hard to catch or work around in general. Clang really needs to get this 
fixed.

Thanks.