[PATCH] c++: Avoid some false positive -Wfloat-conversion warnings with extended precision [PR108285]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

On the following testcase trunk emits a false positive warning on ia32.
convert_like_internal is there called with type of double and
expr EXCESS_PRECISION_EXPR with float type with long double operand
2.L * (long double) x.
Now, for the code generation we do the right thing, cp_convert
to double from that 2.L * (long double) x, but we call even
cp_convert_and_check with that and that emits the -Wfloat-conversion
warning.  Looking at what the C FE does in this case, it calls
convert_and_check with the EXCESS_PRECISION_EXPR expression rather
than its operand, and essentially uses the operand for code generation
and EXCESS_PRECISION_EXPR itself for warnings.

The following patch does that too for the C++ FE.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2023-01-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108285
	* cvt.cc (cp_convert_and_check): For EXCESS_PRECISION_EXPR
	use its operand except that for warning purposes use the original
	EXCESS_PRECISION_EXPR.
	* call.cc (convert_like_internal): Only look through
	EXCESS_PRECISION_EXPR when calling cp_convert, not when calling
	cp_convert_and_check.

	* g++.dg/warn/pr108285.C: New test.

--- gcc/cp/cvt.cc.jj	2022-10-14 09:32:32.403797521 +0200
+++ gcc/cp/cvt.cc	2023-01-10 13:53:00.639130717 +0100
@@ -652,8 +652,10 @@ cp_convert (tree type, tree expr, tsubst
 tree
 cp_convert_and_check (tree type, tree expr, tsubst_flags_t complain)
 {
-  tree result;
+  tree result, expr_for_warning = expr;
 
+  if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
+    expr = TREE_OPERAND (expr, 0);
   if (TREE_TYPE (expr) == type)
     return expr;
   if (expr == error_mark_node)
@@ -663,7 +665,7 @@ cp_convert_and_check (tree type, tree ex
   if ((complain & tf_warning)
       && c_inhibit_evaluation_warnings == 0)
     {
-      tree folded = cp_fully_fold (expr);
+      tree folded = cp_fully_fold (expr_for_warning);
       tree folded_result;
       if (folded == expr)
 	folded_result = result;
--- gcc/cp/call.cc.jj	2023-01-09 23:41:11.135159084 +0100
+++ gcc/cp/call.cc	2023-01-10 13:50:09.277640628 +0100
@@ -8863,12 +8863,14 @@ convert_like_internal (conversion *convs
     return error_mark_node;
 
   warning_sentinel w (warn_zero_as_null_pointer_constant);
-  if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
-    expr = TREE_OPERAND (expr, 0);
   if (issue_conversion_warnings)
     expr = cp_convert_and_check (totype, expr, complain);
   else
-    expr = cp_convert (totype, expr, complain);
+    {
+      if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
+	expr = TREE_OPERAND (expr, 0);
+      expr = cp_convert (totype, expr, complain);
+    }
 
   return expr;
 }
--- gcc/testsuite/g++.dg/warn/pr108285.C.jj	2023-01-10 16:52:06.115345345 +0100
+++ gcc/testsuite/g++.dg/warn/pr108285.C	2023-01-10 16:39:26.646532929 +0100
@@ -0,0 +1,11 @@
+// PR c++/108285
+// { dg-do compile }
+// { dg-options "-fexcess-precision=standard -Wfloat-conversion" }
+
+void bar (double);
+
+void
+foo (float x)
+{
+  bar (2 * x);	// { dg-bogus "conversion from '\[^\n\r]\*' to 'double' may change value" }
+}

	Jakub