fix Issue 19473 - DMD Segfault on circular struct reference

src/dmd/aggregate.d

@@ -39,6 +39,7 @@ enum Sizeok : int
 {
     none,           // size of aggregate is not yet able to compute
     fwd,            // size of aggregate is ready to compute
+    inProcess,      // in the midst of computing the size
     done,           // size of aggregate is set correctly
 }
 
@@ -60,7 +61,7 @@ extern (C++) abstract class AggregateDeclaration : ScopeDsymbol
     uint structsize;        // size of struct
     uint alignsize;         // size of struct for alignment purposes
     VarDeclarations fields; // VarDeclaration fields
-    Sizeok sizeok;          // set when structsize contains valid data
+    Sizeok sizeok = Sizeok.none;  // set when structsize contains valid data
     Dsymbol deferred;       // any deferred semantic2() or semantic3() symbol
     bool isdeprecated;      // true if deprecated
 

src/dmd/aggregate.h

@@ -39,6 +39,7 @@ enum Sizeok
 {
     SIZEOKnone,         // size of aggregate is not yet able to compute
     SIZEOKfwd,          // size of aggregate is ready to compute
+    SIZEOKinProcess,    // in the midst of computing the size
     SIZEOKdone,         // size of aggregate is set correctly
 };
 

src/dmd/dstruct.d

@@ -355,6 +355,12 @@ extern (C++) class StructDeclaration : AggregateDeclaration
         //printf("StructDeclaration::finalizeSize() %s, sizeok = %d\n", toChars(), sizeok);
         assert(sizeok != Sizeok.done);
 
+        if (sizeok == Sizeok.inProcess)
+        {
+            return;
+        }
+        sizeok = Sizeok.inProcess;
+
         //printf("+StructDeclaration::finalizeSize() %s, fields.dim = %d, sizeok = %d\n", toChars(), fields.dim, sizeok);
 
         fields.setDim(0);   // workaround
@@ -368,7 +374,10 @@ extern (C++) class StructDeclaration : AggregateDeclaration
             s.setFieldOffset(this, &offset, isunion);
         }
         if (type.ty == Terror)
+        {
+            errors = true;
             return;
+        }
 
         // 0 sized struct's are set to 1 byte
         if (structsize == 0)

test/fail_compilation/test19473.d

@@ -0,0 +1,31 @@
+/* TEST_OUTPUT:
+---
+fail_compilation/test19473.d(14): Error: union `test19473.P` no size because of forward reference
+---
+ */
+
+// https://issues.dlang.org/show_bug.cgi?id=19473
+
+struct A {
+        P p;
+
+        struct UTpl() {
+                union {
+                        P p;
+                }
+        }
+
+        alias U = UTpl!();
+}
+
+alias B = A.U;
+
+struct C {
+        union D {
+                B b;
+        }
+}
+
+union P {
+        C.D p;
+}