TypeSystemAstBuilder.FractionApprox() will overflow for special double value on debug builds of ILSpy

[yyjdelete]

Not sure if it's an actual issue, it only happen on debug builds of ILSpy.

Steps to reproduce

double Z => 1D / long.MaxValue;

Error message shown

For debug builds only

System.OverflowException: 
   at ICSharpCode.Decompiler.CSharp.Syntax.TypeSystemAstBuilder.FractionApprox(Double value, Int32 maxDenominator) in TypeSystemAstBuilder.cs:line 1585
   at ICSharpCode.Decompiler.CSharp.Syntax.TypeSystemAstBuilder.ConvertFloatingPointLiteral(IType type, Object constantValue) in TypeSystemAstBuilder.cs:line 1379
   at ICSharpCode.Decompiler.CSharp.Syntax.TypeSystemAstBuilder.ConvertConstantValue(IType expectedType, IType type, Object constantValue) in TypeSystemAstBuilder.cs:line 1038
   at ICSharpCode.Decompiler.CSharp.Syntax.TypeSystemAstBuilder.ConvertConstantValue(IType type, Object constantValue) in TypeSystemAstBuilder.cs:line 975
   at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.VisitLdcF8(LdcF8 inst, TranslationContext context) in ExpressionBuilder.cs:line 599
   at ICSharpCode.Decompiler.IL.LdcF8.AcceptVisitor[C,T](ILVisitor`2 visitor, C context) in Instructions.cs:line 2979
   at ICSharpCode.Decompiler.CSharp.ExpressionBuilder.Translate(ILInstruction inst, IType typeHint) in ExpressionBuilder.cs:line 161
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.VisitLeave(Leave inst) in StatementBuilder.cs:line 383
   at ICSharpCode.Decompiler.IL.Leave.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 1316
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 78
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockStatement blockStatement, BlockContainer container, IEnumerable`1 blocks, Boolean isLoop) in StatementBuilder.cs:line 1398
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertBlockContainer(BlockContainer container, Boolean isLoop) in StatementBuilder.cs:line 1328
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.VisitBlockContainer(BlockContainer container) in StatementBuilder.cs:line 1214
   at ICSharpCode.Decompiler.IL.BlockContainer.AcceptVisitor[T](ILVisitor`1 visitor) in Instructions.cs:line 847
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.Convert(ILInstruction inst) in StatementBuilder.cs:line 78
   at ICSharpCode.Decompiler.CSharp.StatementBuilder.ConvertAsBlock(ILInstruction inst) in StatementBuilder.cs:line 83
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.DecompileBody(IMethod method, EntityDeclaration entityDecl, DecompileRun decompileRun, ITypeResolveContext decompilationContext) in CSharpDecompiler.cs:line 1574

Details

• Product in use: ILSpy
• Version in use: 7.2.0.6825-preview4(Debug)

[dgrunwald]

Ah, that's an interesting constant: 1 / (double)long.MaxValue
The (double) cast involves loss of precision: long.MaxValue is rounded up to long.MaxValue+1, which is a power of two.
The division then is exact, resulting in the double 0x3c00000000000000.

When trying to represent this as a fraction, we get long.MaxValue+1 as divisor, which does not fit type long, but compares equal to (not greater than!) long.MaxValue because the long->double conversion involved in a some_double > long.MaxValue comparison has the same loss of precision.