reject decl with incomplete struct/union type in check_global_declaration()

[Prathamesh Kulkarni]

Hi,
For test-case containing only the following declaration:
static struct undefined_struct object;
gcc rejects it at -O0 in assemble_variable() with error "storage size
of <var> is unknown",
however no error is reported when compiled with -O2.
AFAIU that happens because at -O2, analyze_function() removes the
symbol "object" from symbol table and assemble_variable() has no
chance to process it.
g++ rejects it during parsing. I tried similarly in C FE by adding a
check for decl with incomplete struct/union type in finish_decl(),
however that fails to compile the following case:
typedef struct foo foo_t;
foo_t x;
struct foo { int i; };
g++ rejects the above case as well but gcc accepts it.
Do C and C++ standards differ in this regard ?

So instead of finish_decl(), I added check for incomplete struct/union
in check_global_declaration(),
which is called by analyze_function() before removing unused nodes.
The patch regresses Wcxx-compat-8.c and declspecs-1.c because the error
"storage size of <var> is unknown" shows up in these test-cases. I
modified the test-cases
to accept that error.
Does the patch look OK ?

Thank you,
Prathamesh

diff --git a/gcc/cgraphunit.c b/gcc/cgraphunit.c
index 950f6c5..1560a78 100644
--- a/gcc/cgraphunit.c
+++ b/gcc/cgraphunit.c
@@ -974,6 +974,12 @@ check_global_declaration (symtab_node *snode)
                   ? OPT_Wunused_const_variable
                   : OPT_Wunused_variable),
                "%qD defined but not used", decl);
+
+  if (VAR_P (decl) && !DECL_EXTERNAL (decl)
+      && RECORD_OR_UNION_TYPE_P (TREE_TYPE (decl)) && DECL_SIZE (decl) == 0)
+    {
+      error ("storage size of %q+D isn%'t known", decl);
+    }
 }
 
 /* Discover all functions and variables that are trivially needed, analyze
diff --git a/gcc/testsuite/gcc.dg/Wcxx-compat-8.c 
b/gcc/testsuite/gcc.dg/Wcxx-compat-8.c
index f7e8c55..4e9ddc1 100644
--- a/gcc/testsuite/gcc.dg/Wcxx-compat-8.c
+++ b/gcc/testsuite/gcc.dg/Wcxx-compat-8.c
@@ -33,6 +33,7 @@ enum e3
 
 __typeof__ (struct s5 { int i; }) v5; /* { dg-warning "invalid in C\[+\]\[+\]" 
} */
 __typeof__ (struct t5) w5; /* { dg-bogus "invalid in C\[+\]\[+\]" } */
+  /* { dg-error "storage size of 'w5' isn't known" "" { target *-*-* } 35 } */
 
 int
 f1 (struct s1 *p)
@@ -64,4 +65,4 @@ f5 ()
   return &((struct t8) { });  /* { dg-warning "invalid in C\[+\]\[+\]" } */
 }
 
-/* { dg-error "invalid use of undefined type" "" { target *-*-* } 64 } */
+/* { dg-error "invalid use of undefined type" "" { target *-*-* } 65 } */
diff --git a/gcc/testsuite/gcc.dg/declspec-1.c 
b/gcc/testsuite/gcc.dg/declspec-1.c
index c19f107..9113076 100644
--- a/gcc/testsuite/gcc.dg/declspec-1.c
+++ b/gcc/testsuite/gcc.dg/declspec-1.c
@@ -9,7 +9,8 @@ typedef int t;
 /* These should all be diagnosed, but only once, not for every
    identifier declared.  */
 struct s0 int x0, /* { dg-error "two or more data types" } */
-x1;
+/* { dg-error "storage size of 'x0' isn't known" "" { target *-*-* } 11 } */
+x1; /* { dg-error "storage size of 'x1' isn't known" } */ 
 
 char union u0 x2, /* { dg-error "two or more data types" } */
 x3;
diff --git a/gcc/testsuite/gcc.dg/struct-incompl-2.c 
b/gcc/testsuite/gcc.dg/struct-incompl-2.c
new file mode 100644
index 0000000..f3957eb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/struct-incompl-2.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+static struct foo x;  /* { dg-error "storage size of 'x' isn't known" } */
+static union bar y;  /* { dg-error "storage size of 'y' isn't known" } */
+
+typedef struct P p;
+static p p_obj;  /* { dg-error "storage size of 'p_obj' isn't known" } */
+
+extern struct undefined_object object;

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