Instrument.C: fix issue related to metaprogramming instances

Do not instrument decisions in sources of interest whose parent
statement does not belong to a source of interest. This can happen with
metaprogramming instances in C/C++.

Author: Jugst3r

testsuite/tests/instr-cov/c_metaprog/main.c

@@ -0,0 +1,8 @@
+int
+main ()
+{
+  int a = 0;
+#define DECISION a
+#include "meta_print_msg.h"
+  return 0;
+}

testsuite/tests/instr-cov/c_metaprog/meta_print_msg.h

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+#ifdef DECISION
+if (DECISION)
+  printf("Hello world!\n");
+#endif

testsuite/tests/instr-cov/c_metaprog/test.opt

@@ -0,0 +1,2 @@
+bin-traces DEAD Check instrumentation-specific behavior
+

testsuite/tests/instr-cov/c_metaprog/test.py

@@ -0,0 +1,27 @@
+"""
+Checks that gnatcov does not instrument decisions in sources of interest whose
+parent statement does not belong to a source of interest. This can happen with
+metaprogramming instances in C/C++.
+"""
+
+from SCOV.minicheck import build_run_and_coverage, check_xcov_reports
+from SUITE.context import thistest
+from SUITE.cutils import Wdir
+from SUITE.gprutils import GPRswitches
+from SUITE.tutils import gprfor
+
+
+tmp = Wdir("tmp_")
+
+build_run_and_coverage(
+    gprsw=GPRswitches(root_project=gprfor(srcdirs=[".."], mains=["main.c"])),
+    covlevel="stmt+decision",
+    mains=["main"],
+    extra_instr_args=["--ignore-source-files=meta_print_msg.h"],
+    extra_coverage_args=["-axcov"],
+    trace_mode="src",
+)
+
+check_xcov_reports("obj/*.xcov", {"obj/main.c.xcov": {"+": {4, 7}}})
+
+thistest.result()

tools/gnatcov/instrument-c.adb

@@ -1951,6 +1951,13 @@ package body Instrument.C is
             Append (Trailing_Braces, '}');
          end if;
 
+         --  If this statement does not belong to a source of interest, skip
+         --  it altogether.
+
+         if Is_Null (N) or not Is_Source_Of_Interest (UIC, N) then
+            return;
+         end if;
+
          --  Initialize or extend current statement sequence. Note that for
          --  special cases such as IF and SWITCH statements we will modify
          --  the range to exclude internal statements that should not be
@@ -2248,7 +2255,7 @@ package body Instrument.C is
          --  statement source location range.
 
       begin
-         if Is_Null (N) or else not Is_Source_Of_Interest (UIC, N) then
+         if Is_Null (N) then
             return;
          end if;