Prefer more specific path map key

src/Compilers/CSharp/Portable/CommandLine/CSharpCommandLineParser.cs

@@ -1482,6 +1482,8 @@ internal sealed override CommandLineArguments CommonParse(IEnumerable<string> ar
                                                  new CSharpRequiredLanguageVersion(MessageID.IDS_FeatureNullableReferenceTypes.RequiredVersion())), Location.None));
             }
 
+            pathMap = SortPathMap(pathMap);
+
             return new CSharpCommandLineArguments
             {
                 IsScriptRunner = IsScriptCommandLineParser,

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -10740,6 +10740,12 @@ public void PathMapParser()
             parsedArgs.Errors.Verify();
             Assert.Equal(KeyValuePairUtil.Create("a =,b" + s, "1,= 2" + s), parsedArgs.PathMap[0]);
             Assert.Equal(KeyValuePairUtil.Create("x =,y" + s, "3 4" + s), parsedArgs.PathMap[1]);
+
+            parsedArgs = DefaultParse(new[] { @"/pathmap:C:\temp\=/_1/,C:\temp\a\=/_2/,C:\temp\a\b\=/_3/", "a.cs", @"a\b.cs", @"a\b\c.cs" }, WorkingDirectory);
+            parsedArgs.Errors.Verify();
+            Assert.Equal(KeyValuePairUtil.Create(@"C:\temp\a\b\", "/_3/"), parsedArgs.PathMap[0]);
+            Assert.Equal(KeyValuePairUtil.Create(@"C:\temp\a\", "/_2/"), parsedArgs.PathMap[1]);
+            Assert.Equal(KeyValuePairUtil.Create(@"C:\temp\", "/_1/"), parsedArgs.PathMap[2]);
         }
 
         [Theory]

src/Compilers/Core/Portable/CommandLine/CommandLineParser.cs

@@ -1195,5 +1195,13 @@ internal static ImmutableDictionary<string, string> ParseFeatures(List<string> f
             CompilerOptionParseUtilities.ParseFeatures(builder, features);
             return builder.ToImmutable();
         }
+
+        /// <summary>
+        /// Sort so that more specific keys precede less specific.
+        /// When mapping a path we find the first key in the array that is a prefix of the path.
+        /// If multiple keys are prefixes of the path we want to use the longest (more specific) one for the mapping.
+        /// </summary>
+        internal static ImmutableArray<KeyValuePair<string, string>> SortPathMap(ImmutableArray<KeyValuePair<string, string>> pathMap)
+            => pathMap.Sort((x, y) => -x.Key.Length.CompareTo(y.Key.Length));
     }
 }

src/Compilers/Core/Portable/SourceFileResolver.cs

@@ -60,15 +60,13 @@ public SourceFileResolver(
             {
                 var pathMapBuilder = ArrayBuilder<KeyValuePair<string, string>>.GetInstance(pathMap.Length);
 
-                foreach (var kv in pathMap)
+                foreach (var (key, value) in pathMap)
                 {
-                    var key = kv.Key;
                     if (key == null || key.Length == 0)
                     {
                         throw new ArgumentException(CodeAnalysisResources.EmptyKeyInPathMap, nameof(pathMap));
                     }
 
-                    var value = kv.Value;
                     if (value == null)
                     {
                         throw new ArgumentException(CodeAnalysisResources.NullValueInPathMap, nameof(pathMap));

src/Compilers/VisualBasic/Portable/CommandLine/VisualBasicCommandLineParser.vb

@@ -1440,6 +1440,8 @@ lVbRuntimePlus:
             ' If the script is passed without the `\i` option simply execute the script (`vbi script.vbx`).
             interactiveMode = interactiveMode Or (IsScriptCommandLineParser AndAlso sourceFiles.Count = 0)
 
+            pathMap = SortPathMap(pathMap)
+
             Return New VisualBasicCommandLineArguments With
             {
                 .IsScriptRunner = IsScriptCommandLineParser,

src/Compilers/VisualBasic/Test/CommandLine/CommandLineTests.vb

@@ -3483,6 +3483,12 @@ print Goodbye, World"
             parsedArgs.Errors.Verify()
             Assert.Equal(KeyValuePairUtil.Create("a =,b" & s, "1,= 2" & s), parsedArgs.PathMap(0))
             Assert.Equal(KeyValuePairUtil.Create("x =,y" & s, "3 4" & s), parsedArgs.PathMap(1))
+
+            parsedArgs = DefaultParse({"/pathmap:C:\temp\=/_1/,C:\temp\a\=/_2/,C:\temp\a\b\=/_3/", "a.cs", "a\b.cs", "a\b\c.cs"}, _baseDirectory)
+            parsedArgs.Errors.Verify()
+            Assert.Equal(KeyValuePairUtil.Create("C:\temp\a\b\", "/_3/"), parsedArgs.PathMap(0))
+            Assert.Equal(KeyValuePairUtil.Create("C:\temp\a\", "/_2/"), parsedArgs.PathMap(1))
+            Assert.Equal(KeyValuePairUtil.Create("C:\temp\", "/_1/"), parsedArgs.PathMap(2))
         End Sub
 
         ' PathMapKeepsCrossPlatformRoot and PathMapInconsistentSlashes should be in an