[gdb/symtab] Prefer def over decl (inter-CU case, with context)

This is a follow-up patch on "[PATCH][gdb/symtab] Prefer def over decl
(inter-CU case)" (
https://sourceware.org/pipermail/gdb-patches/2020-April/167489.html ).

Consider the test-case from that patch.  It contains a decl and def of var a
in different CUs, and tests whether var a can be printed using the def, even
if the decl is found first.

However, the test-case does this in a contextless environment, so if we add to
the test-case like this to set the context to the CU containing main:
...
 gdb_test "p a" { = \{1, 2\}}
+
+if ![runto_main] then {
+    fail "can't run to main"
+    return 0
+}
+
+gdb_test "p a" { = \{1, 2\}}
...
then the second test fails, because the decl is found in the context.

Fix this by preferring defs over decls in lookup_global_symbol.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-23  Tom de Vries  <[email protected]>

	* symtab.c (lookup_global_symbol): Prefer def over decl.

gdb/testsuite/ChangeLog:

2020-04-23  Tom de Vries  <[email protected]>

	* gdb.base/decl-before-def.exp: Run to main and print a again.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-04-23  Tom de Vries  <[email protected]>
+
+	* symtab.c (lookup_global_symbol): Prefer def over decl.
+
 2020-04-23  Tom de Vries  <[email protected]>
 
 	PR symtab/25807

gdb/symtab.c

@@ -2718,17 +2718,23 @@ lookup_global_symbol (const char *name,
      global block first.  This yields "more expected" behavior, and is
      needed to support 'FILENAME'::VARIABLE lookups.  */
   const struct block *global_block = block_global_block (block);
+  symbol *sym = NULL;
   if (global_block != nullptr)
     {
-      symbol *sym = lookup_symbol_in_block (name,
-					    symbol_name_match_type::FULL,
-					    global_block, domain);
-      if (sym != nullptr)
+      sym = lookup_symbol_in_block (name,
+				    symbol_name_match_type::FULL,
+				    global_block, domain);
+      if (sym != NULL && best_symbol (sym, domain))
 	return { sym, global_block };
     }
 
   struct objfile *objfile = lookup_objfile_from_block (block);
-  return lookup_global_or_static_symbol (name, GLOBAL_BLOCK, objfile, domain);
+  block_symbol bs
+    = lookup_global_or_static_symbol (name, GLOBAL_BLOCK, objfile, domain);
+  if (better_symbol (sym, bs.symbol, domain) == sym)
+    return { sym, global_block };
+  else
+    return bs;
 }
 
 bool

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2020-04-23  Tom de Vries  <[email protected]>
+
+	* gdb.base/decl-before-def.exp: Run to main and print a again.
+
 2020-04-23  Tom de Vries  <[email protected]>
 
 	* gdb.base/decl-before-def-decl.c: New test.

gdb/testsuite/gdb.base/decl-before-def.exp

@@ -24,3 +24,10 @@ if {[prepare_for_testing "failed to prepare" $testfile $sources]} {
 gdb_test "maint expand-symtabs"
 
 gdb_test "p a" { = \{1, 2\}}
+
+if ![runto_main] then {
+    fail "can't run to main"
+    return 0
+}
+
+gdb_test "p a" { = \{1, 2\}}