Code:

------
mixin template Templ(T) {
    this() { }
}

class C {
    mixin Templ!int;
    mixin Templ!float;
}

void main() {
    auto c = new C;
}
------


Compiler output:

------
test.d: Error: overloadset test.C.__ctor is aliased to a function
core.exception.AssertError@ddmd/func.d(4136): Assertion failure
----------------
dmd() [0x5a58e5]
dmd(_Z15resolveFuncCall3LocP5ScopeP7DsymbolP5ArrayIP10RootObjectEP4TypePS4_IP10ExpressionEi+0x42b) [0x5a2073]
dmd(_ZN6NewExp8semanticEP5Scope+0xcc1) [0x57a721]
dmd(_ZN14ExpInitializer9inferTypeEP5Scope+0x26) [0x5b65f6]
dmd(_ZN14VarDeclaration8semanticEP5Scope+0x150) [0x526858]
dmd(_ZN14DeclarationExp8semanticEP5Scope+0xa9) [0x57d101]
dmd(_ZN24StatementSemanticVisitor5visitEP12ExpStatement+0x52) [0x60c1b2]
dmd(_ZN12ExpStatement6acceptEP7Visitor+0x1a) [0x6004da]
dmd(ddmd.statement.Statement ddmd.statementsem.semantic(ddmd.statement.Statement, ddmd.dscope.Scope*)+0x41) [0x619531]
dmd(_ZN24StatementSemanticVisitor5visitEP17CompoundStatement+0xe8) [0x60c448]
dmd(_ZN17CompoundStatement6acceptEP7Visitor+0x1a) [0x600c42]
dmd(ddmd.statement.Statement ddmd.statementsem.semantic(ddmd.statement.Statement, ddmd.dscope.Scope*)+0x41) [0x619531]
dmd(_ZN15FuncDeclaration9semantic3EP5Scope+0x120c) [0x59b2b4]
dmd(_ZN6Module9semantic3EP5Scope+0x6c) [0x54664c]
dmd(int ddmd.mars.tryMain(ulong, const(char)**)+0x2f43) [0x5c90a3]
dmd(_Dmain+0x27) [0x5c9cd7]
dmd(_D2rt6dmain211_d_run_mainUiPPaPUAAaZiZ6runAllMFZ9__lambda1MFZv+0x1f) [0x70652f]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate())+0x2a) [0x706482]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).runAll()+0x30) [0x7064e8]
dmd(void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate())+0x2a) [0x706482]
dmd(_d_run_main+0x1dc) [0x7063fc]
dmd(main+0x12) [0x5ca3c2]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1) [0x7fd3820fd2b1]
------


Arguably, the code is invalid (but see below), but the compiler should not ICE just because of that.

I said "arguably" because changing "this() {}" to "~this() {}" not only makes the error go away, but also causes the multiple dtors to get aggregated and even merged with any dtors defined explicitly in class C, so that upon class destruction all the instances of ~this() get invoked. This causes one to wonder if this is an undocumented feature.  Which then raises the question of why the asymmetry with ctors, that multiple definitions of the default ctor is rejected (and leads to an ICE).