[PATCH] c++: diagnosing if-stmt with non-constant branches [PR105050]

[PATCH] c++: diagnosing if-stmt with non-constant branches [PR105050]

[Patrick Palka]

When an if-stmt is deemed non-constant because both of its branches are
non-constant, we issue a rather generic error which, given that it points
to the 'if' token, misleadingly suggests the condition is at fault:

  constexpr-105050.C:8:3: error: expression ‘<statement>’ is not a constant expression
      8 |   if (p != q && *p < 0)
        |   ^~

This patch clarifies the error message to read:

  constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant expression
      8 |   if (p != q && *p < 0)
        |   ^~

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

	PR c++/105050

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case IF_STMT>:
	Clarify error message when a if-stmt is non-constant because its
	branches are non-constant.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-105050.C: New test.
---
 gcc/cp/constexpr.cc                           |  7 ++++++-
 gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C | 12 ++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 778680b8270..9c40b051574 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9439,7 +9439,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	    }
 	}
       if (flags & tf_error)
-	error_at (loc, "expression %qE is not a constant expression", t);
+	{
+	  if (TREE_CODE (t) == IF_STMT)
+	    error_at (loc, "neither branch of %<if%> is a constant expression");
+	  else
+	    error_at (loc, "expression %qE is not a constant expression", t);
+	}
       return false;
 
     case VEC_INIT_EXPR:
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
new file mode 100644
index 00000000000..99d5c9960ac
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
@@ -0,0 +1,12 @@
+// PR c++/105050
+// { dg-do compile { target c++14 } }
+
+void g();
+void h();
+
+constexpr void f(int* p, int *q) {
+  if (p != q && *p < 0) // { dg-error "neither branch of 'if' is a constant expression" }
+    g();
+  else
+    h();
+}
-- 
2.35.1.655.ga68dfadae5

Re: [PATCH] c++: diagnosing if-stmt with non-constant branches [PR105050]

[Marek Polacek]

On Fri, Mar 25, 2022 at 12:07:31PM -0400, Patrick Palka via Gcc-patches wrote:
> When an if-stmt is deemed non-constant because both of its branches are
> non-constant, we issue a rather generic error which, given that it points
> to the 'if' token, misleadingly suggests the condition is at fault:
> 
>   constexpr-105050.C:8:3: error: expression ‘<statement>’ is not a constant expression
>       8 |   if (p != q && *p < 0)
>         |   ^~
> 
> This patch clarifies the error message to read:
> 
>   constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant expression
>       8 |   if (p != q && *p < 0)
>         |   ^~
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

LGTM.
 
> 	PR c++/105050
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (potential_constant_expression_1) <case IF_STMT>:
> 	Clarify error message when a if-stmt is non-constant because its
> 	branches are non-constant.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-105050.C: New test.
> ---
>  gcc/cp/constexpr.cc                           |  7 ++++++-
>  gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C | 12 ++++++++++++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 778680b8270..9c40b051574 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -9439,7 +9439,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>  	    }
>  	}
>        if (flags & tf_error)
> -	error_at (loc, "expression %qE is not a constant expression", t);
> +	{
> +	  if (TREE_CODE (t) == IF_STMT)
> +	    error_at (loc, "neither branch of %<if%> is a constant expression");
> +	  else
> +	    error_at (loc, "expression %qE is not a constant expression", t);
> +	}
>        return false;
>  
>      case VEC_INIT_EXPR:
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> new file mode 100644
> index 00000000000..99d5c9960ac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> @@ -0,0 +1,12 @@
> +// PR c++/105050
> +// { dg-do compile { target c++14 } }
> +
> +void g();
> +void h();
> +
> +constexpr void f(int* p, int *q) {
> +  if (p != q && *p < 0) // { dg-error "neither branch of 'if' is a constant expression" }
> +    g();
> +  else
> +    h();
> +}
> -- 
> 2.35.1.655.ga68dfadae5
> 

Marek

Re: [PATCH] c++: diagnosing if-stmt with non-constant branches [PR105050]

[Jason Merrill]

On 3/25/22 12:07, Patrick Palka wrote:
> When an if-stmt is deemed non-constant because both of its branches are
> non-constant, we issue a rather generic error which, given that it points
> to the 'if' token, misleadingly suggests the condition is at fault:
> 
>    constexpr-105050.C:8:3: error: expression ‘<statement>’ is not a constant expression
>        8 |   if (p != q && *p < 0)
>          |   ^~
> 
> This patch clarifies the error message to read:
> 
>    constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant expression
>        8 |   if (p != q && *p < 0)
>          |   ^~
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.  I wonder if we want to then diagnose the branches individually, but 
let's leave that for stage 1, if at all.

> 	PR c++/105050
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (potential_constant_expression_1) <case IF_STMT>:
> 	Clarify error message when a if-stmt is non-constant because its
> 	branches are non-constant.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-105050.C: New test.
> ---
>   gcc/cp/constexpr.cc                           |  7 ++++++-
>   gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C | 12 ++++++++++++
>   2 files changed, 18 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index 778680b8270..9c40b051574 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -9439,7 +9439,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
>   	    }
>   	}
>         if (flags & tf_error)
> -	error_at (loc, "expression %qE is not a constant expression", t);
> +	{
> +	  if (TREE_CODE (t) == IF_STMT)
> +	    error_at (loc, "neither branch of %<if%> is a constant expression");
> +	  else
> +	    error_at (loc, "expression %qE is not a constant expression", t);
> +	}
>         return false;
>   
>       case VEC_INIT_EXPR:
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> new file mode 100644
> index 00000000000..99d5c9960ac
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105050.C
> @@ -0,0 +1,12 @@
> +// PR c++/105050
> +// { dg-do compile { target c++14 } }
> +
> +void g();
> +void h();
> +
> +constexpr void f(int* p, int *q) {
> +  if (p != q && *p < 0) // { dg-error "neither branch of 'if' is a constant expression" }
> +    g();
> +  else
> +    h();
> +}