PR gdb/28480: Improve ambiguous member detection

Basic ambiguity detection assumes that when 2 fields with the same name
have the same byte offset, it must be an unambiguous request. This is not
always correct. Consider the following code:

class empty { };

class A {
public:
  [[no_unique_address]] empty e;
};

class B {
public:
  int e;
};

class C: public A, public B { };

if we tried to use c.e in code, the compiler would warn of an ambiguity,
however, since A::e does not demand an unique address, it gets the same
address (and thus byte offset) of the members, making A::e and B::e have the
same address. however, "print c.e" would fail to report the ambiguity,
and would instead print it as an empty class (first path found).

The new code solves this by checking for other found_fields that have
different m_struct_path.back() (final class that the member was found
in), despite having the same byte offset.

The testcase gdb.cp/ambiguous.exp was also changed to test for this
behavior.

3 files changed, 57 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.cp/ambiguous.cc b/gdb/testsuite/gdb.cp/ambiguous.cc
index a556865..af2198d 100644
--- a/gdb/testsuite/gdb.cp/ambiguous.cc
+++ b/gdb/testsuite/gdb.cp/ambiguous.cc
@@ -1,3 +1,4 @@
+class empty { };
 
 class A1 {
 public:
@@ -17,6 +18,17 @@ public:
   int y;
 };
 
+#if !defined (__GNUC__) || __GNUC__ > 7
+# define NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+# define NO_UNIQUE_ADDRESS
+#endif
+
+class A4 {
+public:
+    NO_UNIQUE_ADDRESS empty x;
+};
+
 class X : public A1, public A2 {
 public:
   int z;
@@ -77,6 +89,10 @@ public:
   int jva1v;
 };
 
+class JE : public A1, public A4 {
+public:
+};
+
 int main()
 {
   A1 a1;
@@ -92,6 +108,7 @@ int main()
   JVA1 jva1;
   JVA2 jva2;
   JVA1V jva1v;
+  JE je;
   
   int i;
 
@@ -173,5 +190,7 @@ int main()
   jva1v.i = 4;
   jva1v.jva1v = 5;
 
+  je.A1::x = 1;
+
   return 0; /* set breakpoint here */
 }
diff --git a/gdb/testsuite/gdb.cp/ambiguous.exp b/gdb/testsuite/gdb.cp/ambiguous.exp
index 008898c..1e63dc0 100644
--- a/gdb/testsuite/gdb.cp/ambiguous.exp
+++ b/gdb/testsuite/gdb.cp/ambiguous.exp
@@ -264,3 +264,13 @@ gdb_test "print (A1)(KV)jva1" " = \{x = 3, y = 4\}"
 # JVA1V is derived from A1; A1 is a virtual base indirectly
 # and also directly; must not report ambiguity when a JVA1V is cast to an A1.
 gdb_test "print (A1)jva1v" " = {x = 1, y = 2}"
+
+# C++20 introduced a way to have ambiguous fields with the same byte offset.
+# This class explicitly tests for that.
+# if this is tested with a compiler that can't handle [[no_unique_address]]
+# the code should still correctly identify the ambiguity because of
+# different byte offsets.
+test_ambiguous "je.x" "x" "JE" {
+    "'int A1::x' (JE -> A1)"
+    "'empty A4::x' (JE -> A4)"
+}
diff --git a/gdb/valops.c b/gdb/valops.c
index 9787cdb..c552e82 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -1962,6 +1962,34 @@ struct_field_searcher::update_result (struct value *v, LONGEST boffset)
 	     space.  */
 	  if (m_fields.empty () || m_last_boffset != boffset)
 	    m_fields.push_back ({m_struct_path, v});
+	  else
+	    {
+	    /*Fields can occupy the same space and have the same name (be
+	      ambiguous).  This can happen when fields in two different base
+	      classes are marked [[no_unique_address]] and have the same name.
+	      The C++ standard says that such fields can only occupy the same
+	      space if they are of different type, but we don't rely on that in
+	      the following code. */
+	      bool ambiguous = false, insert = true;
+	      for (const found_field &field: m_fields)
+		{
+		  if(field.path.back () != m_struct_path.back ())
+		    {
+		    /* Same boffset points to members of different classes.
+		       We have found an ambiguity and should record it.  */
+		      ambiguous = true;
+		    }
+		  else
+		    {
+		    /* We don't need to insert this value again, because a
+		       non-ambiguous path already leads to it.  */
+		      insert = false;
+		      break;
+		    }
+		}
+	      if (ambiguous && insert)
+		m_fields.push_back ({m_struct_path, v});
+	    }
 	}
     }
 }