Bug 21607 – dmd segfaults for forward reference in base class

Reduced example from https://github.com/adamdruppe/arsd/blob/a8ec499f2b24915cb27a660088024a6b23f5b04b/jni.d#L191-L193: class CharSequence : JavaClass!CharSequence { string toString(); } class JavaClass(Class) { alias m = __traits(getMember, Class, "toString"); } DMD crashes because it tries to access the vtable when it's still empty: Stack trace generated by inserting `assert(cd.vtbl.length);`: core.exception.AssertError@src/dmd/dsymbolsem.d(3769): Assertion failure ---------------- ??:? _d_assertp [0x7f31b82062d5] src/dmd/dsymbolsem.d:3769 _ZN22DsymbolSemanticVisitor23funcDeclarationSemanticEP15FuncDeclaration [0x7f31b7f9781e] src/dmd/dsymbolsem.d:4165 _ZN22DsymbolSemanticVisitor5visitEP15FuncDeclaration [0x7f31b7f98c80] src/dmd/func.d:2634 _ZN15FuncDeclaration6acceptEP7Visitor [0x7f31b8033435] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/typesem.d:4356 dmd.expression.Expression dmd.typesem.dotExp(dmd.mtype.Type, dmd.dscope.Scope*, dmd.expression.Expression, dmd.identifier.Identifier, int).visitClass(dmd.mtype.TypeClass) [0x7f31b80ce89c] src/dmd/typesem.d:4412 dmd.expression.Expression dmd.typesem.dotExp(dmd.mtype.Type, dmd.dscope.Scope*, dmd.expression.Expression, dmd.identifier.Identifier, int) [0x7f31b80c9ff6] src/dmd/expressionsem.d:12057 dmd.expression.Expression dmd.expressionsem.semanticY(dmd.expression.DotIdExp, dmd.dscope.Scope*, int) [0x7f31b802b9fa] src/dmd/expressionsem.d:6313 _ZN25ExpressionSemanticVisitor5visitEP8DotIdExp [0x7f31b801a0fd] src/dmd/expression.d:4720 _ZN8DotIdExp6acceptEP7Visitor [0x7f31b7fff651] src/dmd/expressionsem.d:11655 _Z18expressionSemanticP10ExpressionP5Scope [0x7f31b802a2b3] src/dmd/traits.d:1041 dmd.expression.Expression dmd.traits.semanticTraits(dmd.expression.TraitsExp, dmd.dscope.Scope*) [0x7f31b80bc329] src/dmd/typesem.d:1778 dmd.mtype.Type dmd.typesem.typeSemantic(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*).visitTraits(dmd.mtype.TypeTraits) [0x7f31b80c62d2] src/dmd/typesem.d:2002 _Z12typeSemanticP4TypeRK3LocP5Scope [0x7f31b80c2d7b] src/dmd/typesem.d:3114 void dmd.typesem.resolve(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*, out dmd.expression.Expression, out dmd.mtype.Type, out dmd.dsymbol.Dsymbol, bool).visitTraits(dmd.mtype.TypeTraits) [0x7f31b80c9ebc] src/dmd/typesem.d:3134 void dmd.typesem.resolve(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*, out dmd.expression.Expression, out dmd.mtype.Type, out dmd.dsymbol.Dsymbol, bool) [0x7f31b80c86e0] src/dmd/mtype.d:5394 _ZN10TypeTraits9toDsymbolEP5Scope [0x7f31b80638e8] src/dmd/dsymbolsem.d:6661 void dmd.dsymbolsem.aliasSemantic(dmd.declaration.AliasDeclaration, dmd.dscope.Scope*) [0x7f31b7fa0266] src/dmd/dsymbolsem.d:771 _ZN22DsymbolSemanticVisitor5visitEP16AliasDeclaration [0x7f31b7f8df31] src/dmd/declaration.d:989 _ZN16AliasDeclaration6acceptEP7Visitor [0x7f31b7f4a41d] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/dsymbolsem.d:5379 void dmd.dsymbolsem.DsymbolSemanticVisitor.visit(dmd.dclass.ClassDeclaration).__lambda9!(dmd.dsymbol.Dsymbol).__lambda9(dmd.dsymbol.Dsymbol) [0x7f31b7f9d793] src/dmd/dsymbol.d:103 void dmd.dsymbol.foreachDsymbol(dmd.root.array.Array!(dmd.dsymbol.Dsymbol).Array*, void delegate(dmd.dsymbol.Dsymbol)) [0x7f31b7f856c3] src/dmd/dsymbolsem.d:5379 _ZN22DsymbolSemanticVisitor5visitEP16ClassDeclaration [0x7f31b7f9cc2d] src/dmd/dclass.d:993 _ZN16ClassDeclaration6acceptEP7Visitor [0x7f31b7f47f61] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/dtemplate.d:7402 void dmd.dtemplate.TemplateInstance.expandMembers(dmd.dscope.Scope*).symbolDg(dmd.dsymbol.Dsymbol) [0x7f31b7fb3104] src/dmd/dsymbol.d:103 void dmd.dsymbol.foreachDsymbol(dmd.root.array.Array!(dmd.dsymbol.Dsymbol).Array*, void delegate(dmd.dsymbol.Dsymbol)) [0x7f31b7f856c3] src/dmd/dtemplate.d:7407 void dmd.dtemplate.TemplateInstance.expandMembers(dmd.dscope.Scope*) [0x7f31b7fb3091] src/dmd/dtemplate.d:7422 void dmd.dtemplate.TemplateInstance.tryExpandMembers(dmd.dscope.Scope*) [0x7f31b7fb316e] src/dmd/dsymbolsem.d:6262 void dmd.dsymbolsem.templateInstanceSemantic(dmd.dtemplate.TemplateInstance, dmd.dscope.Scope*, dmd.root.array.Array!(dmd.expression.Expression).Array*) [0x7f31b7f9f258] src/dmd/dsymbolsem.d:2870 _ZN22DsymbolSemanticVisitor5visitEP16TemplateInstance [0x7f31b7f94e5e] src/dmd/dtemplate.d:7451 _ZN16TemplateInstance6acceptEP7Visitor [0x7f31b7fb3229] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/typesem.d:2854 void dmd.typesem.resolve(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*, out dmd.expression.Expression, out dmd.mtype.Type, out dmd.dsymbol.Dsymbol, bool).visitInstance(dmd.mtype.TypeInstance) [0x7f31b80c922f] src/dmd/typesem.d:3129 void dmd.typesem.resolve(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*, out dmd.expression.Expression, out dmd.mtype.Type, out dmd.dsymbol.Dsymbol, bool) [0x7f31b80c869a] src/dmd/typesem.d:1712 dmd.mtype.Type dmd.typesem.typeSemantic(dmd.mtype.Type, ref const(dmd.globals.Loc), dmd.dscope.Scope*).visitInstance(dmd.mtype.TypeInstance) [0x7f31b80c5fc8] src/dmd/typesem.d:2000 _Z12typeSemanticP4TypeRK3LocP5Scope [0x7f31b80c2d57] src/dmd/dsymbolsem.d:5020 pure @nogc @safe dmd.mtype.Type dmd.dsymbolsem.DsymbolSemanticVisitor.visit(dmd.dclass.ClassDeclaration).__dgliteral2() [0x7f31b7f9d679] src/dmd/dsymbolsem.d:5002 dmd.mtype.Type dmd.dsymbolsem.DsymbolSemanticVisitor.visit(dmd.dclass.ClassDeclaration).resolveBase!(dmd.mtype.Type).resolveBase(lazy dmd.mtype.Type) [0x7f31b7fa0e6c] src/dmd/dsymbolsem.d:5020 _ZN22DsymbolSemanticVisitor5visitEP16ClassDeclaration [0x7f31b7f9b7b5] src/dmd/dclass.d:993 _ZN16ClassDeclaration6acceptEP7Visitor [0x7f31b7f47f61] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/dsymbolsem.d:2289 void dmd.dsymbolsem.DsymbolSemanticVisitor.visit(dmd.dmodule.Module).__lambda2!(dmd.dsymbol.Dsymbol).__lambda2(dmd.dsymbol.Dsymbol) [0x7f31b7f931a0] src/dmd/dsymbol.d:103 void dmd.dsymbol.foreachDsymbol(dmd.root.array.Array!(dmd.dsymbol.Dsymbol).Array*, void delegate(dmd.dsymbol.Dsymbol)) [0x7f31b7f856c3] src/dmd/dsymbolsem.d:2286 _ZN22DsymbolSemanticVisitor5visitEP6Module [0x7f31b7f93132] src/dmd/dmodule.d:1498 _ZN6Module6acceptEP7Visitor [0x7f31b7f6d5fd] src/dmd/dsymbolsem.d:581 _Z15dsymbolSemanticP7DsymbolP5Scope [0x7f31b7f8d6d9] src/dmd/mars.d:573 int dmd.mars.tryMain(ulong, const(char)**, ref dmd.globals.Param) [0x7f31b7efbee5] src/dmd/mars.d:1005 _Dmain [0x7f31b7efd8f0]

Compiling this code with the latest version of master yields: test.d(3): Error: function `test.CharSequence.toString` circular reference to class `CharSequence` test.d(1): Error: template instance `test.JavaClass!(CharSequence)` error instantiating test.d(1): Error: class `test.CharSequence` no size because of forward reference The first error message is confusing, but the rest are correct.