libstdc++: Fix std::format alternate form for floating-point [PR108046]

A decimal point was being added to the end of the string for {:#.0}
because the __expc character was not being set, for the _Pres_none
presentation type, so __s.find(__expc) didn't the 'e' in "1e+01" and so
we created "1e+01." by appending the radix char to the end.

This can be fixed by ensuring that __expc='e' is set for the _Pres_none
case. I realized we can also set __expc='P' and __expc='E' when needed,
to save a call to std::toupper later.

For the {:#.0g} format, __expc='e' was being set and so the 'e' was
found in "1e+10" but then __z = __prec - __sigfigs would wraparound to
SIZE_MAX. That meant we would decide not to add a radix char because the
number of extra characters to insert would be 1+SIZE_MAX i.e. zero.

This can be fixed by using __z == 0 when __prec == 0.

libstdc++-v3/ChangeLog:

	PR libstdc++/108046
	* include/std/format (__formatter_fp::format): Ensure __expc is
	always set for all presentation types. Set __z correctly for
	zero precision.
	* testsuite/std/format/functions/format.cc: Check problem cases.

2 files changed, 13 insertions, 8 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 0c6069b..1e0ef61 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1430,22 +1430,24 @@ namespace __format
 	  chars_format __fmt{};
 	  bool __upper = false;
 	  bool __trailing_zeros = false;
-	  char __expc = 0;
+	  char __expc = 'e';
 
 	  switch (_M_spec._M_type)
 	  {
 	    case _Pres_A:
 	      __upper = true;
+	      __expc = 'P';
 	      [[fallthrough]];
 	    case _Pres_a:
-	      __expc = 'p';
+	      if (_M_spec._M_type != _Pres_A)
+		__expc = 'p';
 	      __fmt = chars_format::hex;
 	      break;
 	    case _Pres_E:
 	      __upper = true;
+	      __expc = 'E';
 	      [[fallthrough]];
 	    case _Pres_e:
-	      __expc = 'e';
 	      __use_prec = true;
 	      __fmt = chars_format::scientific;
 	      break;
@@ -1455,10 +1457,10 @@ namespace __format
 	      break;
 	    case _Pres_G:
 	      __upper = true;
+	      __expc = 'E';
 	      [[fallthrough]];
 	    case _Pres_g:
 	      __trailing_zeros = true;
-	      __expc = 'e';
 	      __use_prec = true;
 	      __fmt = chars_format::general;
 	      break;
@@ -1511,7 +1513,6 @@ namespace __format
 	    {
 	      for (char* __p = __start; __p != __res.ptr; ++__p)
 		*__p = std::toupper(*__p);
-	      __expc = std::toupper(__expc);
 	    }
 
 	  // Add sign for non-negative values.
@@ -1545,15 +1546,15 @@ namespace __format
 		  __p = __s.find(__expc);
 		  if (__p == __s.npos)
 		    __p = __s.size();
-		  __d = __p;
+		  __d = __p; // Position where '.' should be inserted.
 		  __sigfigs = __d;
 		}
 
-	      if (__trailing_zeros)
+	      if (__trailing_zeros && __prec != 0)
 		{
 		  if (!__format::__is_xdigit(__s[0]))
 		    --__sigfigs;
-		  __z = __prec - __sigfigs;
+		  __z = __prec - __sigfigs; // Number of zeros to insert.
 		}
 
 	      if (size_t __extras = int(__d == __p) + __z)
diff --git a/libstdc++-v3/testsuite/std/format/functions/format.cc b/libstdc++-v3/testsuite/std/format/functions/format.cc
index 3485535..bd914df 100644
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -152,6 +152,10 @@ test_alternate_forms()
 
   s = std::format("{:#.2g}", -0.0);
   VERIFY( s == "-0.0" );
+
+  // PR libstdc++/108046
+  s = std::format("{0:#.0} {0:#.1} {0:#.0g}", 10.0);
+  VERIFY( s == "1.e+01 1.e+01 1.e+01" );
 }
 
 struct euro_punc : std::numpunct<char>