try to cover all cases

Author: lbargaoanu

src/AutoMapper/MemberMap.cs

@@ -39,9 +39,20 @@ public class MemberMap
         public virtual ValueResolverConfiguration ValueResolverConfig { get => default; set { } }
         public virtual ValueResolverConfiguration ValueConverterConfig { get => default; set { } }
         public virtual IReadOnlyCollection<ValueTransformerConfiguration> ValueTransformers => Array.Empty<ValueTransformerConfiguration>();
-        public MemberInfo SourceMember => CustomMapExpression.GetMember() ?? SourceMembers.FirstOrDefault();
-        public string GetSourceMemberName() =>
-            ValueConverterConfig?.GetSourceMemberName() ?? ValueResolverConfig?.GetSourceMemberName() ?? SourceMember?.Name;
+        public MemberInfo SourceMember => this switch
+        {
+            { ValueConverterConfig: ValueResolverConfiguration converter } => converter switch
+                {
+                    { SourceMember: LambdaExpression sourceExpression } => sourceExpression.GetMember(),
+                    { SourceMemberName: { } } => null,
+                    _ => SourceMembers.Length == 1 ? SourceMembers[0] : null
+                },
+            { ValueResolverConfig: ValueResolverConfiguration resolver } => resolver.SourceMember?.GetMember(),
+            { CustomMapFunction: { } } => null,
+            { CustomMapExpression: LambdaExpression mapFrom } => mapFrom.GetMember(),
+            _ => SourceMembers.FirstOrDefault(),
+        };
+        public string GetSourceMemberName() => (ValueConverterConfig ?? ValueResolverConfig)?.SourceMemberName ?? SourceMember?.Name;
         public bool MustUseDestination => UseDestinationValue is true || !CanBeSet;
         public void MapFrom(LambdaExpression sourceMember)
         {
@@ -94,7 +105,6 @@ public ValueResolverConfiguration(object instance, Type interfaceType)
             InterfaceType = interfaceType;
         }
         public Type ResolvedType => InterfaceType.GenericTypeArguments.Last();
-        public string GetSourceMemberName() => SourceMember == null ? SourceMemberName : SourceMember.GetMember()?.Name;
     }
     public readonly struct ValueTransformerConfiguration
     {

src/UnitTests/ConfigurationValidation.cs

@@ -348,6 +348,23 @@ class Destination
         public void Should_be_ignored() => new Action(AssertConfigurationIsValid)
             .ShouldThrow<AutoMapperConfigurationException>().Errors[0].UnmappedPropertyNames[0].ShouldBe(nameof(Source.Value));
     }
+
+    public class MatchingNonMemberExpressionWithSourceValidation : NonValidatingSpecBase
+    {
+        class Source
+        {
+            public string Value { get; set; }
+        }
+        class Destination
+        {
+            public string Value { get; set; }
+        }
+        protected override MapperConfiguration CreateConfiguration() => new(c => c.CreateMap<Source, Destination>(MemberList.Source)
+            .ForMember(d => d.Value, o => o.MapFrom(s => s.Value ?? "")));
+        [Fact]
+        public void Should_be_ignored() => new Action(AssertConfigurationIsValid)
+            .ShouldThrow<AutoMapperConfigurationException>().Errors[0].UnmappedPropertyNames[0].ShouldBe(nameof(Source.Value));
+    }
 
     public class When_testing_a_dto_with_fully_mapped_and_custom_matchers : AutoMapperSpecBase
     {