[gcc r12-7831] c++: diagnosing if-stmt with non-constant branches [PR105050]

[gcc r12-7831] c++: diagnosing if-stmt with non-constant branches [PR105050]

[Patrick Palka]

https://gcc.gnu.org/g:ff465bd8a0f0f96a00d3067018442917b194b7af

commit r12-7831-gff465bd8a0f0f96a00d3067018442917b194b7af
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Mar 26 10:20:18 2022 -0400

    c++: diagnosing if-stmt with non-constant branches [PR105050]
    
    When an if-stmt is determined to be non-constant because both of its
    branches are non-constant, we issue a somewhat generic error which,
    since the error also 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 instead read:
    
      constexpr-105050.C:8:3: error: neither branch of ‘if’ is a constant expression
          8 |   if (p != q && *p < 0)
            |   ^~
    
            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.

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

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..e0688fbd38e
--- /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();
+}