Fix Type.GetType for global type names with leading '.'

src/libraries/Common/src/System/Reflection/TypeNameParser.cs

@@ -167,6 +167,8 @@ private TypeNameParser(ReadOnlySpan<char> name)
             if (fullName is null)
                 return null;
 
+            fullName = ApplyLeadingDotCompatQuirk(fullName);
+
             if (Peek == TokenType.Plus)
             {
                 string[] nestedNames = new string[1];
@@ -180,6 +182,8 @@ private TypeNameParser(ReadOnlySpan<char> name)
                     if (nestedName is null)
                         return null;
 
+                    nestedName = ApplyLeadingDotCompatQuirk(nestedName);
+
                     if (nestedNamesCount >= nestedNames.Length)
                         Array.Resize(ref nestedNames, 2 * nestedNamesCount);
                     nestedNames[nestedNamesCount++] = nestedName;
@@ -192,6 +196,19 @@ private TypeNameParser(ReadOnlySpan<char> name)
             {
                 return new NamespaceTypeName(fullName);
             }
+
+            // Compat: Ignore leading '.' for type names without namespace. .NET Framework historically ignored leading '.' here. It is likely
+            // that code out there depends on this behavior. For example, type names formed by concatenating namespace and name, without checking for
+            // empty namespace (bug), are going to have superfluous leading '.'.
+            // This behavior means that types that start with '.' are not round-trippable via type name.
+            static string ApplyLeadingDotCompatQuirk(string typeName)
+            {
+#if NETCOREAPP
+                return (typeName.StartsWith('.') && !typeName.AsSpan(1).Contains('.')) ? typeName.Substring(1) : typeName;
+#else
+                return ((typeName.Length > 0) && (typeName[0] == '.') && typeName.LastIndexOf('.') == 0) ? typeName.Substring(1) : typeName;
+#endif
+            }
         }
 
         //

src/libraries/System.Runtime/tests/System/Type/TypeTests.cs

@@ -507,6 +507,8 @@ public void MakePointerType_ByRef_ThrowsTypeLoadException()
         [InlineData("Outside[,,]", typeof(Outside[,,]))]
         [InlineData("Outside[][]", typeof(Outside[][]))]
         [InlineData("Outside`1[System.Nullable`1[System.Boolean]]", typeof(Outside<bool?>))]
+        [InlineData(".Outside`1", typeof(Outside<>))]
+        [InlineData(".Outside`1+.Inside`1", typeof(Outside<>.Inside<>))]
         public void GetTypeByName_ValidType_ReturnsExpected(string typeName, Type expectedType)
         {
             Assert.Equal(expectedType, Type.GetType(typeName, throwOnError: false, ignoreCase: false));
@@ -520,6 +522,8 @@ public void GetTypeByName_ValidType_ReturnsExpected(string typeName, Type expect
         [InlineData("System.Int32[,*,]", typeof(ArgumentException), false)]
         [InlineData("Outside`2", typeof(TypeLoadException), false)]
         [InlineData("Outside`1[System.Boolean, System.Int32]", typeof(ArgumentException), true)]
+        [InlineData(".System.Int32", typeof(TypeLoadException), false)]
+        [InlineData("..Outside`1", typeof(TypeLoadException), false)]
         public void GetTypeByName_Invalid(string typeName, Type expectedException, bool alwaysThrowsException)
         {
             if (!alwaysThrowsException)
@@ -530,6 +534,16 @@ public void GetTypeByName_Invalid(string typeName, Type expectedException, bool
             Assert.Throws(expectedException, () => Type.GetType(typeName, throwOnError: true, ignoreCase: false));
         }
 
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotBuiltWithAggressiveTrimming))]
+        [InlineData(".GlobalStructStartingWithDot")]
+        [InlineData(" GlobalStructStartingWithSpace")]
+        public void GetTypeByName_NonRoundtrippable(string typeName)
+        {
+            Type type = Assembly.Load("System.TestStructs").GetTypes().Single((t) => t.FullName == typeName);
+            string assemblyQualifiedName = type.AssemblyQualifiedName;
+            Assert.Null(Type.GetType(assemblyQualifiedName));
+        }
+
         [Fact]
         [ActiveIssue("https://github.com/dotnet/runtimelab/issues/155", typeof(PlatformDetection), nameof(PlatformDetection.IsNativeAot))]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/52393", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsMonoAOT))]

src/libraries/System.Runtime/tests/TestStructs/System.TestStructs.il

@@ -70,3 +70,13 @@
         }
     }
 }
+
+.class public sequential sealed '.GlobalStructStartingWithDot'
+    extends [System.Runtime]System.ValueType
+{
+}
+
+.class public sequential sealed ' GlobalStructStartingWithSpace'
+    extends [System.Runtime]System.ValueType
+{
+}