<format>: fix std::format("{:#.precision}", floating)

stl/inc/format

@@ -2864,7 +2864,7 @@ _NODISCARD _OutputIt _Fmt_write(
 
     auto _To_upper  = false;
     auto _Format    = chars_format::general;
-    auto _Exponent  = '\0';
+    auto _Exponent  = 'e';
     auto _Precision = _Specs._Precision;
 
     switch (_Specs._Type) {
@@ -3015,7 +3015,7 @@ _NODISCARD _OutputIt _Fmt_write(
             _Zeroes_to_append = _Extra_precision;
             break;
         case chars_format::general:
-            if (_Specs._Alt) {
+            if (_Specs._Alt && (_Specs._Type == 'g' || _Specs._Type == 'G')) {
                 auto _Digits = static_cast<int>(_Exponent_start - _Buffer_start);
 
                 if (!_Append_decimal) {

tests/std/tests/GH_003003_format_fix_decimal_point_and_trailing_zero/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst

tests/std/tests/GH_003003_format_fix_decimal_point_and_trailing_zero/test.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <format>
+
+int main() {
+    assert(std::format("{:#.0}", 0.0) == "0.");
+    assert(std::format("{:#.1}", 0.0) == "0.");
+    assert(std::format("{:#.2}", 0.0) == "0.");
+
+    assert(std::format("{:#.0}", 1200.0) == "1.e+03");
+    assert(std::format("{:#.1}", 1200.0) == "1.e+03");
+    assert(std::format("{:#.2}", 1200.0) == "1.2e+03");
+    assert(std::format("{:#.3}", 1200.0) == "1.2e+03");
+    assert(std::format("{:#.4}", 1200.0) == "1200.");
+    assert(std::format("{:#.5}", 1200.0) == "1200.");
+    assert(std::format("{:#.6}", 1200.0) == "1200.");
+
+    assert(std::format("{:#.0}", 0.123) == "0.1");
+    assert(std::format("{:#.1}", 0.123) == "0.1");
+    assert(std::format("{:#.2}", 0.123) == "0.12");
+    assert(std::format("{:#.3}", 0.123) == "0.123");
+    assert(std::format("{:#.4}", 0.123) == "0.123");
+    assert(std::format("{:#.5}", 0.123) == "0.123");
+
+    assert(std::format("{:#.0}", 10.1) == "1.e+01");
+    assert(std::format("{:#.1}", 10.1) == "1.e+01");
+    assert(std::format("{:#.2}", 10.1) == "10.");
+    assert(std::format("{:#.3}", 10.1) == "10.1");
+    assert(std::format("{:#.4}", 10.1) == "10.1");
+    assert(std::format("{:#.5}", 10.1) == "10.1");
+}