[PATCH] c++: Improve static_assert diagnostic [PR97518]
Marek Polacek
polacek@redhat.com
Tue Nov 10 18:59:16 GMT 2020
On Tue, Nov 10, 2020 at 11:32:04AM -0500, Jason Merrill wrote:
> On 11/9/20 7:21 PM, Marek Polacek wrote:
> > Currently, when a static_assert fails, we only say "static assertion failed".
> > It would be more useful if we could also print the expression that
> > evaluated to false; this is especially useful when the condition uses
> > template parameters. Consider the motivating example, in which we have
> > this line:
> >
> > static_assert(is_same<X, Y>::value);
> >
> > if this fails, the user has to play dirty games to get the compiler to
> > print the template arguments. With this patch, we say:
> >
> > static assertion failed due to requirement 'is_same<int*, int>::value'
>
> I'd rather avoid the word "requirement" here, to avoid confusion with
> Concepts.
>
> Maybe have the usual failed error, and if we're showing the expression, have
> a second inform to say e.g. "%qE evaluates to false"?
Works for me.
> Also, if we've narrowed the problem down to a particular subexpression,
> let's only print that one.
Done.
> > which I think is much better. However, always printing the condition that
> > evaluated to 'false' wouldn't be very useful: e.g. noexcept(fn) is
> > always parsed to true/false, so we would say "static assertion failed due
> > to requirement 'false'" which doesn't help. So I wound up only printing
> > the condition when it was instantiation-dependent, that is, we called
> > finish_static_assert from tsubst_expr.
> >
> > Moreover, this patch also improves the diagnostic when the condition
> > consists of a logical AND. Say you have something like this:
> >
> > static_assert(fn1() && fn2() && fn3() && fn4() && fn5());
> >
> > where fn4() evaluates to false and the other ones to true. Highlighting
> > the whole thing is not that helpful because it won't say which clause
> > evaluated to false. With the find_failing_clause tweak in this patch
> > we emit:
> >
> > error: static assertion failed
> > 6 | static_assert(fn1() && fn2() && fn3() && fn4() && fn5());
> > | ~~~^~
> >
> > so you know right away what's going on. Unfortunately, when you combine
> > both things, that is, have an instantiation-dependent expr and && in
> > a static_assert, we can't yet quite point to the clause that failed. It
> > is because when we tsubstitute something like is_same<X, Y>::value, we
> > generate a VAR_DECL that doesn't have any location. It would be awesome
> > if we could wrap it with a location wrapper, but I didn't see anything
> > obvious.
>
> Hmm, I vaguely remember that at first we were using location wrappers less
> in templates, but I thought that was fixed. I don't see anything setting
> suppress_location_wrappers, and it looks like tsubst_copy_and_build should
> preserve a location wrapper.
The problem here is that tsubst_qualified_id produces a VAR_DECL and for those
CAN_HAVE_LOCATION_P is false. If the user writes (bool) is_same<X, Y>::value,
this works well. Maybe we could create an artificial (bool) for clauses of
the logical AND operator; decay_conversion in cp_build_binary_op will likely
do it anyway, it just doesn't have any location: cp_build_binary_op takes
an op_location_t, but that doesn't track locations of the operands. Maybe
some hacks in tsubst_copy_and_build/TRUTH_AND_EXPR would help, I'm not sure.
> > Further tweak could be to print the failing clause of the condition if
> > possible, whether or not it was instantiation-dependent.
>
> If not instantiation-dependent, underlining the clause seems sufficient.
Ack.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Currently, when a static_assert fails, we only say "static assertion failed".
It would be more useful if we could also print the expression that
evaluated to false; this is especially useful when the condition uses
template parameters. Consider the motivating example, in which we have
this line:
static_assert(is_same<X, Y>::value);
if this fails, the user has to play dirty games to get the compiler to
print the template arguments. With this patch, we say:
error: static assertion failed
note: 'is_same<int*, int>::value' evaluates to false
which I think is much better. However, always printing the condition that
evaluated to 'false' wouldn't be very useful: e.g. noexcept(fn) is
always parsed to true/false, so we would say "'false' evaluates to false"
which doesn't help. So I wound up only printing the condition when it was
instantiation-dependent, that is, we called finish_static_assert from
tsubst_expr.
Moreover, this patch also improves the diagnostic when the condition
consists of a logical AND. Say you have something like this:
static_assert(fn1() && fn2() && fn3() && fn4() && fn5());
where fn4() evaluates to false and the other ones to true. Highlighting
the whole thing is not that helpful because it won't say which clause
evaluated to false. With the find_failing_clause tweak in this patch
we emit:
error: static assertion failed
6 | static_assert(fn1() && fn2() && fn3() && fn4() && fn5());
| ~~~^~
so you know right away what's going on. Unfortunately, when you combine
both things, that is, have an instantiation-dependent expr and && in
a static_assert, we can't yet quite point to the clause that failed. It
is because when we tsubstitute something like is_same<X, Y>::value, we
generate a VAR_DECL that doesn't have any location. It would be awesome
if we could wrap it with a location wrapper, but I didn't see anything
obvious.
In passing, I've cleaned up some things:
* use iloc_sentinel when appropriate,
* it's nicer to call contextual_conv_bool instead of the rather verbose
perform_implicit_conversion_flags,
* no need to check for INTEGER_CST before calling integer_zerop.
gcc/cp/ChangeLog:
PR c++/97518
* cp-tree.h (finish_static_assert): Adjust declaration.
* parser.c (cp_parser_static_assert): Pass false to
finish_static_assert.
* pt.c (tsubst_expr): Pass true to finish_static_assert.
* semantics.c (find_failing_clause_r): New function.
(find_failing_clause): New function.
(finish_static_assert): Add a bool parameter. Use
iloc_sentinel. Call contextual_conv_bool instead of
perform_implicit_conversion_flags. Don't check for INTEGER_CST before
calling integer_zerop. Call find_failing_clause and maybe use its
location. Print the original condition or the failing clause if
SHOW_EXPR_P.
gcc/testsuite/ChangeLog:
PR c++/97518
* g++.dg/diagnostic/pr87386.C: Adjust expected output.
* g++.dg/diagnostic/static_assert1.C: New test.
* g++.dg/diagnostic/static_assert2.C: New test.
libcc1/ChangeLog:
PR c++/97518
* libcp1plugin.cc (plugin_add_static_assert): Pass false to
finish_static_assert.
---
gcc/cp/cp-tree.h | 2 +-
gcc/cp/parser.c | 3 +-
gcc/cp/pt.c | 6 +-
gcc/cp/semantics.c | 73 ++++++++++++++++---
gcc/testsuite/g++.dg/diagnostic/pr87386.C | 2 +-
.../g++.dg/diagnostic/static_assert1.C | 30 ++++++++
.../g++.dg/diagnostic/static_assert2.C | 68 +++++++++++++++++
libcc1/libcp1plugin.cc | 2 +-
8 files changed, 167 insertions(+), 19 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/diagnostic/static_assert1.C
create mode 100644 gcc/testsuite/g++.dg/diagnostic/static_assert2.C
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b98d47a702f..230a1525c63 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7234,7 +7234,7 @@ extern bool cxx_omp_create_clause_info (tree, tree, bool, bool,
bool, bool);
extern tree baselink_for_fns (tree);
extern void finish_static_assert (tree, tree, location_t,
- bool);
+ bool, bool);
extern tree finish_decltype_type (tree, bool, tsubst_flags_t);
extern tree finish_trait_expr (location_t, enum cp_trait_kind, tree, tree);
extern tree build_lambda_expr (void);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 9c08c0e46a2..36322812310 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -14886,7 +14886,8 @@ cp_parser_static_assert(cp_parser *parser, bool member_p)
/* Complete the static assertion, which may mean either processing
the static assert now or saving it for template instantiation. */
- finish_static_assert (condition, message, assert_loc, member_p);
+ finish_static_assert (condition, message, assert_loc, member_p,
+ /*show_expr_p=*/false);
}
/* Parse the expression in decltype ( expression ). */
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index a2655a0ff52..6ba114c9da3 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -18492,8 +18492,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
tree condition;
++c_inhibit_evaluation_warnings;
- condition =
- tsubst_expr (STATIC_ASSERT_CONDITION (t),
+ condition =
+ tsubst_expr (STATIC_ASSERT_CONDITION (t),
args,
complain, in_decl,
/*integral_constant_expression_p=*/true);
@@ -18502,7 +18502,7 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
finish_static_assert (condition,
STATIC_ASSERT_MESSAGE (t),
STATIC_ASSERT_SOURCE_LOCATION (t),
- /*member_p=*/false);
+ /*member_p=*/false, /*show_expr_p=*/true);
}
break;
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 33d715edaec..089c83e511f 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -9827,13 +9827,53 @@ init_cp_semantics (void)
{
}
+
+/* If we have a condition in conjunctive normal form (CNF), find the first
+ failing clause. In other words, given an expression like
+
+ true && true && false && true && false
+
+ return the first 'false'. EXPR is the expression. */
+
+static tree
+find_failing_clause_r (tree expr)
+{
+ if (TREE_CODE (expr) == TRUTH_ANDIF_EXPR)
+ {
+ /* First check the left side... */
+ tree e = find_failing_clause_r (TREE_OPERAND (expr, 0));
+ if (e == NULL_TREE)
+ /* ...if we didn't find a false clause, check the right side. */
+ e = find_failing_clause_r (TREE_OPERAND (expr, 1));
+ return e;
+ }
+ tree e = contextual_conv_bool (expr, tf_none);
+ e = fold_non_dependent_expr (e, tf_none, /*manifestly_const_eval=*/true);
+ if (integer_zerop (e))
+ /* This is the failing clause. */
+ return expr;
+ return NULL_TREE;
+}
+
+/* Wrapper for find_failing_clause_r. */
+
+static tree
+find_failing_clause (tree expr)
+{
+ if (TREE_CODE (expr) != TRUTH_ANDIF_EXPR)
+ return NULL_TREE;
+ return find_failing_clause_r (expr);
+}
+
/* Build a STATIC_ASSERT for a static assertion with the condition
CONDITION and the message text MESSAGE. LOCATION is the location
of the static assertion in the source code. When MEMBER_P, this
- static assertion is a member of a class. */
+ static assertion is a member of a class. If SHOW_EXPR_P is true,
+ print the condition (because it was instantiation-dependent). */
+
void
finish_static_assert (tree condition, tree message, location_t location,
- bool member_p)
+ bool member_p, bool show_expr_p)
{
tsubst_flags_t complain = tf_warning_or_error;
@@ -9871,8 +9911,7 @@ finish_static_assert (tree condition, tree message, location_t location,
tree orig_condition = condition;
/* Fold the expression and convert it to a boolean value. */
- condition = perform_implicit_conversion_flags (boolean_type_node, condition,
- complain, LOOKUP_NORMAL);
+ condition = contextual_conv_bool (condition, complain);
condition = fold_non_dependent_expr (condition, complain,
/*manifestly_const_eval=*/true);
@@ -9881,21 +9920,32 @@ finish_static_assert (tree condition, tree message, location_t location,
;
else
{
- location_t saved_loc = input_location;
+ iloc_sentinel ils (location);
- input_location = location;
- if (TREE_CODE (condition) == INTEGER_CST
- && integer_zerop (condition))
+ if (integer_zerop (condition))
{
int sz = TREE_INT_CST_LOW (TYPE_SIZE_UNIT
(TREE_TYPE (TREE_TYPE (message))));
int len = TREE_STRING_LENGTH (message) / sz - 1;
+
+ /* See if we can find which clause was failing (for logical AND). */
+ tree bad = find_failing_clause (orig_condition);
+ /* If not, or its location is unusable, fall back to the previous
+ location. */
+ location_t cloc = location;
+ if (cp_expr_location (bad) != UNKNOWN_LOCATION)
+ cloc = cp_expr_location (bad);
+
/* Report the error. */
if (len == 0)
- error ("static assertion failed");
+ error_at (cloc, "static assertion failed");
else
- error ("static assertion failed: %s",
- TREE_STRING_POINTER (message));
+ error_at (cloc, "static assertion failed: %s",
+ TREE_STRING_POINTER (message));
+ if (show_expr_p)
+ inform (cloc, "%qE evaluates to false",
+ /* Nobody wants to see the artifical (bool) cast. */
+ (bad ? tree_strip_nop_conversions (bad) : orig_condition));
/* Actually explain the failure if this is a concept check or a
requires-expression. */
@@ -9909,7 +9959,6 @@ finish_static_assert (tree condition, tree message, location_t location,
if (require_rvalue_constant_expression (condition))
cxx_constant_value (condition);
}
- input_location = saved_loc;
}
}
diff --git a/gcc/testsuite/g++.dg/diagnostic/pr87386.C b/gcc/testsuite/g++.dg/diagnostic/pr87386.C
index 85726af9f01..679a5177f64 100644
--- a/gcc/testsuite/g++.dg/diagnostic/pr87386.C
+++ b/gcc/testsuite/g++.dg/diagnostic/pr87386.C
@@ -14,5 +14,5 @@ static_assert (foo::test<int>::value, "foo"); // { dg-error "static assertion f
static_assert (foo::test<int>::value && true, "bar"); // { dg-error "static assertion failed: bar" }
/* { dg-begin-multiline-output "" }
static_assert (foo::test<int>::value && true, "bar");
- ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
+ ~~~~~~~~~~~~~~~~^~~~~
{ dg-end-multiline-output "" } */
diff --git a/gcc/testsuite/g++.dg/diagnostic/static_assert1.C b/gcc/testsuite/g++.dg/diagnostic/static_assert1.C
new file mode 100644
index 00000000000..fecf3cc51a0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/static_assert1.C
@@ -0,0 +1,30 @@
+// PR c++/97518
+// { dg-do compile { target c++17 } }
+
+template <typename T, typename U> struct is_same { static constexpr bool value = false; };
+template <typename T> struct is_same<T, T> { static constexpr bool value = true; };
+
+template <typename T> using some_metafunction_t = T;
+
+template <typename T>
+void foo(T ) {
+ using X = T*;
+ using Y = some_metafunction_t<T>;
+
+ static_assert(is_same<X, Y>::value); // { dg-error "static assertion failed" }
+ // { dg-message {.is_same<int\*, int>::value. evaluates to false} "" { target *-*-* } .-1 }
+ static_assert(is_same<X, Y>::value, "foo"); // { dg-error "static assertion failed: foo" }
+ // { dg-message {.is_same<int\*, int>::value. evaluates to false} "" { target *-*-* } .-1 }
+ static_assert(is_same<X, X>::value && is_same<X, Y>::value); // { dg-error "static assertion failed" }
+ // { dg-message {.is_same<int\*, int>::value. evaluates to false} "" { target *-*-* } .-1 }
+ static_assert(is_same<X, Y>::value && is_same<X, X>::value); // { dg-error "static assertion failed" }
+ // { dg-message {.is_same<int\*, int>::value. evaluates to false} "" { target *-*-* } .-1 }
+ static_assert(is_same<X, X>::value
+ && is_same<Y, Y>::value
+ && is_same<X, Y>::value); // { dg-error "static assertion failed" }
+ // { dg-message {.is_same<int\*, int>::value. evaluates to false} "" { target *-*-* } .-1 }
+}
+
+void bar() {
+ foo(0);
+}
diff --git a/gcc/testsuite/g++.dg/diagnostic/static_assert2.C b/gcc/testsuite/g++.dg/diagnostic/static_assert2.C
new file mode 100644
index 00000000000..542697f99de
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/static_assert2.C
@@ -0,0 +1,68 @@
+// PR c++/97518
+// { dg-do compile { target c++11 } }
+// { dg-options "-fdiagnostics-show-caret" }
+
+constexpr bool yes () { return true; }
+constexpr bool no () { return false; }
+constexpr bool yay = true;
+constexpr bool nay = false;
+
+void
+bar ()
+{
+ static_assert (true && true && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (true && true && no(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert (yay && nay, ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (yay && nay, "");
+ ^~~
+ { dg-end-multiline-output "" } */
+ static_assert (yes() && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (yes() && no(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert (no() && yes(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (no() && yes(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert (no() && no() && yes(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (no() && no() && yes(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert (yes() && yes() && yes () && no() && yes(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (yes() && yes() && yes () && no() && yes(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert (yes() && yes() && yes () && (no() && yes()), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert (yes() && yes() && yes () && (no() && yes()), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert ((yes() && no()) && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert ((yes() && no()) && no(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert ((yes() && no()) && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert ((yes() && no()) && no(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+ static_assert ((no() || no()) && yes(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert ((no() || no()) && yes(), "");
+ ~~~~~~^~~~~~~~
+ { dg-end-multiline-output "" } */
+ static_assert ((yes() || no()) && no(), ""); // { dg-error "static assertion failed" }
+/* { dg-begin-multiline-output "" }
+ static_assert ((yes() || no()) && no(), "");
+ ~~^~
+ { dg-end-multiline-output "" } */
+}
diff --git a/libcc1/libcp1plugin.cc b/libcc1/libcp1plugin.cc
index bab2751a5ce..67a235f9095 100644
--- a/libcc1/libcp1plugin.cc
+++ b/libcc1/libcp1plugin.cc
@@ -3642,7 +3642,7 @@ plugin_add_static_assert (cc1_plugin::connection *self,
bool member_p = at_class_scope_p ();
- finish_static_assert (condition, message, loc, member_p);
+ finish_static_assert (condition, message, loc, member_p, false);
return 1;
}
base-commit: 831f24a778a016c6ce1ae739235e3f7e1f28ed8c
--
2.28.0
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