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

[Patrick Palka via Gcc-patches]

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