Fixed all build warnings

src/Directory.Build.targets

@@ -6,4 +6,9 @@
     <DocumentationFile>bin\$(Configuration)\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile>
   </PropertyGroup>
 
+  <PropertyGroup Label="Disabled Warnings">
+    <!-- We get this meaningless warning because we have the same files shared in multiple projects -->
+    <NoWarn Label="Type conflicts with imported type">$(NoWarn);CS0436</NoWarn>
+  </PropertyGroup>
+
 </Project>

src/ICU4N.Collation/Text/RuleBasedCollator.cs

@@ -28,7 +28,7 @@ namespace ICU4N.Text
     /// <remarks>
     /// Create a <see cref="RuleBasedCollator"/> from a locale by calling the <see cref="Collator.GetInstance(System.Globalization.CultureInfo)"/> factory method in the base class
     /// <see cref="Collator"/>. <see cref="Collator.GetInstance(System.Globalization.CultureInfo)"/> creates a <see cref="RuleBasedCollator"/> object based on the collation rules defined by the
-    /// argument locale. If a customized collation ordering or attributes is required, use the <see cref="RuleBasedCollator(string)"/>
+    /// argument locale. If a customized collation ordering or attributes is required, use the <see cref="RuleBasedCollator.RuleBasedCollator(string)"/>
     /// constructor with the appropriate rules. The customized <see cref="RuleBasedCollator"/> will base its ordering on the CLDR root collation, while
     /// re-adjusting the attributes and orders of the characters in the specified rule accordingly.
     /// <para/>

src/ICU4N/Impl/PluralRulesLoader.cs

@@ -80,13 +80,13 @@ public override UCultureInfo GetFunctionalEquivalent(UCultureInfo locale, out bo
         public override UCultureInfo GetFunctionalEquivalent(UCultureInfo locale) // ICU4N specific: Added overload so we don't need to pass a null parameter
 #pragma warning restore CS0672 // Type or member is obsolete
         {
-            string rulesId = GetRulesIdForLocale(locale, PluralType.Cardinal);
+            string? rulesId = GetRulesIdForLocale(locale, PluralType.Cardinal);
             if (rulesId is null || rulesId.Trim().Length == 0)
             {
                 return UCultureInfo.InvariantCulture; // ultimate fallback
             }
 
-            if (!RulesIdToEquivalentULocaleMap.TryGetValue(rulesId, out UCultureInfo result) || result is null)
+            if (!RulesIdToEquivalentULocaleMap.TryGetValue(rulesId, out UCultureInfo? result) || result is null)
             {
                 return UCultureInfo.InvariantCulture; // ultimate fallback
             }
@@ -100,7 +100,7 @@ public override UCultureInfo GetFunctionalEquivalent(UCultureInfo locale) // ICU
         private IDictionary<string, string> GetLocaleIdToRulesIdMap(PluralType type)
         {
             CheckBuildRulesIdMaps();
-            return (type == PluralType.Cardinal) ? localeIdToCardinalRulesId : localeIdToOrdinalRulesId;
+            return (type == PluralType.Cardinal) ? localeIdToCardinalRulesId! : localeIdToOrdinalRulesId!;
         }
 
         /// <summary>
@@ -111,7 +111,7 @@ private IDictionary<string, UCultureInfo> RulesIdToEquivalentULocaleMap
             get
             {
                 CheckBuildRulesIdMaps();
-                return rulesIdToEquivalentULocale;
+                return rulesIdToEquivalentULocale!;
             }
         }
 
@@ -551,8 +551,8 @@ private static IDictionary<string, PluralRanges> LoadLocaleIdToPluralRanges()
                 new string[] {"other", "other", "other"},
             };
 #pragma warning disable 612, 618
-            PluralRanges pr = null;
-            string[] locales = null;
+            PluralRanges? pr = null;
+            string[]? locales = null;
             IDictionary<string, PluralRanges> tempLocaleIdToPluralRanges = new Dictionary<string, PluralRanges>();
 #pragma warning restore 612, 618
             foreach (string[] row in pluralRangeData)
@@ -580,14 +580,14 @@ private static IDictionary<string, PluralRanges> LoadLocaleIdToPluralRanges()
                     StandardPluralUtil.TryFromString(row[1], out StandardPlural end);
                     StandardPluralUtil.TryFromString(row[2], out StandardPlural result);
 #pragma warning disable 612, 618
-                    pr.Add(start, end, result);
+                    pr!.Add(start, end, result);
 #pragma warning restore 612, 618
                 }
             }
             // do last one
             foreach (string locale in locales!)
             {
-                tempLocaleIdToPluralRanges[locale] = pr;
+                tempLocaleIdToPluralRanges[locale] = pr!;
             }
             // now make whole thing immutable
 #if FEATURE_IDICTIONARY_ASREADONLY

src/ICU4N/Impl/RuleCharacterIterator.cs

@@ -16,11 +16,19 @@ public enum RuleCharacterIteratorOptions
         /// its value.  Variables are parsed using the <see cref="SymbolTable"/> API.
         /// </summary>
         ParseVariables = 1,
+#if FEATURE_SPAN
         /// <summary>
         /// Bitmask option to enable parsing of escape sequences.  If (options &amp;
         /// <see cref="ParseEscapes"/> != 0, then an embedded escape sequence will be expanded
-        /// to its value.  Escapes are parsed using <see cref="Utility.UnescapeAt(string, int[])"/>.
+        /// to its value.  Escapes are parsed using <see cref="Utility.UnescapeAt(ReadOnlySpan{char}, ref int)"/>.
         /// </summary>
+#else
+        /// <summary>
+        /// Bitmask option to enable parsing of escape sequences.  If (options &amp;
+        /// <see cref="ParseEscapes"/> != 0, then an embedded escape sequence will be expanded
+        /// to its value.  Escapes are parsed using <see cref="Utility.UnescapeAt(string, ref int)"/>.
+        /// </summary>
+#endif
         ParseEscapes = 2,
         /// <summary>
         /// Bitmask option to enable skipping of whitespace.  If (options &amp;

src/ICU4N/Support/Globalization/UNumberFormatInfo.RuleBasedNumberFormat.cs

@@ -43,15 +43,15 @@ public Capitalization Capitalization
         /// </summary>
         /// <draft>ICU 60.1</draft>
         public NumberFormatRules SpellOut
-            => LazyInitializer.EnsureInitialized(ref spellOut, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.SpellOut));
+            => LazyInitializer.EnsureInitialized(ref spellOut, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.SpellOut))!;
 
         /// <summary>
         /// Gets the ordinal <see cref="NumberFormatRules"/> instance for the rule-based number formatter
         /// that attaches an ordinal suffix from the desired language to the end of the number (e.g. "123rd").
         /// </summary>
         /// <draft>ICU 60.1</draft>
         public NumberFormatRules Ordinal
-            => LazyInitializer.EnsureInitialized(ref ordinal, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.Ordinal));
+            => LazyInitializer.EnsureInitialized(ref ordinal, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.Ordinal))!;
 
         /// <summary>
         /// Gets the duration <see cref="NumberFormatRules"/> instance for the rule-based number formatter
@@ -61,7 +61,7 @@ public NumberFormatRules Ordinal
         [Browsable(false)]
         [EditorBrowsable(EditorBrowsableState.Never)] // ICU4N TODO: Do we need this? We have TimeSpan.ToString() which seems to cover. Need research.
         public NumberFormatRules Duration
-            => LazyInitializer.EnsureInitialized(ref duration, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.Duration));
+            => LazyInitializer.EnsureInitialized(ref duration, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.Duration))!;
 
         /// <summary>
         /// Gets the numbering system <see cref="NumberFormatRules"/> instance for the rule-based number formatter
@@ -70,7 +70,7 @@ public NumberFormatRules Duration
         /// </summary>
         /// <draft>ICU 60.1</draft>
         public NumberFormatRules NumberingSystem
-            => LazyInitializer.EnsureInitialized(ref numberingSystem, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.NumberingSystem));
+            => LazyInitializer.EnsureInitialized(ref numberingSystem, () => NumberFormatRules.GetInstance(CultureData.name, NumberPresentation.NumberingSystem))!;
 #endif
 
     }

src/ICU4N/Support/Globalization/UNumberFormatInfo.cs

@@ -41,7 +41,7 @@ public UNumberFormatInfo()
         internal UNumberFormatInfo(UCultureData cultureData)
         {
             Debug.Assert(cultureData != null);
-            cultureData.GetNFIValues(this);
+            cultureData!.GetNFIValues(this);
             this.cultureData = cultureData;
         }