Fix segfault in DComputeSemanticAnalyser::visit(VarDeclaration)

Guard against decl->type being null when semantic analysis
encounters VarDeclarations from failed template instantiations
in imported modules.

When a @compute module imports a template via -I and uses
__traits(compiles, ...) to test it, the recursive walker may
visit VarDeclaration nodes whose type field is null. The
unconditional dereference of decl->type on the check for
Taarray/Tclass causes a segmentation fault.

Fixes #5027.

Author: Codingisinmyblud

gen/semantic-dcompute.cpp

@@ -92,6 +92,9 @@ struct DComputeSemanticAnalyser : public StoppableVisitor {
       return;
     }
 
+    if (!decl->type)
+      return;
+
     if (decl->type->ty == TY::Taarray) {
       error(decl->loc, "associative arrays not allowed in `@compute` code");
       stop = true;

tests/compilable/dcompute_template_import.d

@@ -0,0 +1,14 @@
+// Regression test: importing a module with a recursive template via -I and using
+// __traits(compiles, ...) on it should not crash the compiler during dcompute
+// semantic analysis. Previously, the VarDeclaration's type field could be null
+// for error'd template instantiations, causing a segfault in
+// DComputeSemanticAnalyser::visit(VarDeclaration*).
+
+// REQUIRES: target_NVPTX
+// RUN: %ldc -mdcompute-targets=cuda-350 -o- -I%S/inputs %s
+
+@compute(CompileFor.deviceOnly) module tests.compilable.dcompute_template_import;
+import ldc.dcompute;
+import dcompute_testmod;
+
+static assert(!__traits(compiles, { imported!"dcompute_testmod".hasIndirections!(int[]); }));

tests/compilable/inputs/dcompute_testmod.d

@@ -0,0 +1,5 @@
+template hasIndirections(T)
+{
+    static if (is(T == enum))
+        enum hasIndirections = hasIndirections;
+}