Initial support for explicit default initializers (#1235)

This change makes it so that for a suitable type `MyType`, a variable declaration like:

    MyType v;

is treated as if it were written:

    MyType v = MyType();

The definition of "suitable" here is that `MyType` needs to have an available `__init` declaration that can be invoked with zero arguments. I've added a test to confirm that the new behavior works in this specific case.

There are a bunch of caveats to the feature as it stands today:

* Just because `MyType` has a zero-parameter `__init`, that doesn't mean an array type like `MyType[10]` does, so arrays currently remain uninitialized by default. Fixing this gap requires careful consideration because some, but not all, array types should be default-initializable.

* The change here should mean that a `struct` type with a field like `MyType f;` should count as having a default initial-value expression for that field, but I haven't confirmed that.

* Even if a `struct` provides initial values for all its fields (e.g., `struct S { float f = 0; }`), that doesn't mean it has a default `__init` right now, so those `struct` types will still be left uninitialized by default. Converging all this behavior is still TBD.

Just to be clear: there is no provision or plan in Slang to support destructors, RAII, copy constructors, move constructors, overloaded assignment operations, or any other features that buy heavily into the C++ model of how construction and destruction of values gets done.

In fact, I'm not even 100% sure I like having this change in place at all, and I think we should reserve the right to revert it and say that only specific stdlib types get to opt in to default initialization along these lines.

2 files changed, 69 insertions, 0 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 99400c17c..c2b356eff 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -829,10 +829,77 @@ namespace Slang
     {
         if (auto initExpr = varDecl->initExpr)
         {
+            // If the variable has an explicit initial-value expression,
+            // then we simply need to check that expression and coerce
+            // it to the tyep of the variable.
+            //
             initExpr = CheckTerm(initExpr);
             initExpr = coerce(varDecl->type.Ptr(), initExpr);
             varDecl->initExpr = initExpr;
         }
+        else
+        {
+            // If a variable doesn't have an explicit initial-value
+            // expression, it is still possible that it should
+            // be initialized implicitly, because the type of the
+            // variable has a default (zero parameter) initializer.
+            // That is, for types where it is possible, we will
+            // treat a variable declared like this:
+            //
+            //      MyType myVar;
+            //
+            // as if it were declared as:
+            //
+            //      MyType myVar = MyType();
+            //
+            // Rather than try to code up an ad hoc search for an
+            // appropriate initializer here, we will instead fall
+            // back on the general-purpose overload-resolution
+            // machinery, which can handle looking up initializers
+            // and filtering them to ones that are applicable
+            // to our "call site" with zero arguments.
+            //
+            auto type = varDecl->getType();
+
+            OverloadResolveContext overloadContext;
+            overloadContext.loc = varDecl->nameAndLoc.loc;
+            overloadContext.mode = OverloadResolveContext::Mode::JustTrying;
+            AddTypeOverloadCandidates(type, overloadContext);
+
+            if(overloadContext.bestCandidates.getCount() != 0)
+            {
+                // If there were multiple equally-good candidates to call,
+                // then might have an ambiguity.
+                //
+                // Before issuing any kind of diagnostic we need to check
+                // if any of those candidates are actually applicable,
+                // because if they aren't then we actually just have
+                // an uninitialized varaible.
+                //
+                if(overloadContext.bestCandidates[0].status != OverloadCandidate::Status::Applicable)
+                    return;
+
+                getSink()->diagnose(varDecl, Diagnostics::ambiguousDefaultInitializerForType, type);
+            }
+            else if(overloadContext.bestCandidate)
+            {
+                // If we are in the single-candidate case, then we again
+                // want to ignore the case where that candidate wasn't
+                // actually applicable, because declaring a variable
+                // of a type that *doesn't* have a default initializer
+                // isn't actually an error.
+                //
+                if(overloadContext.bestCandidate->status != OverloadCandidate::Status::Applicable)
+                    return;
+
+                // If we had a single best candidate *and* it was applicable,
+                // then we use it to construct a new initial-value expression
+                // for the variable, that will be used for all downstream
+                // code generation.
+                //
+                varDecl->initExpr = CompleteOverloadCandidate(overloadContext, *overloadContext.bestCandidate);
+            }
+        }
     }
 
     // Fill in default substitutions for the 'subtype' part of a type constraint decl
diff --git a/source/slang/slang-diagnostic-defs.h b/source/slang/slang-diagnostic-defs.h
index e05825aa0..06031c089 100644
--- a/source/slang/slang-diagnostic-defs.h
+++ b/source/slang/slang-diagnostic-defs.h
@@ -314,6 +314,8 @@ DIAGNOSTIC(30504, Error, cannotUseInitializerListForType, "cannot use initialize
 // 306xx: variables
 DIAGNOSTIC(30600, Error, varWithoutTypeMustHaveInitializer, "a variable declaration without an initial-value expression must be given an explicit type");
 
+DIAGNOSTIC(30610, Error, ambiguousDefaultInitializerForType, "more than one default initializer was found for type '$0'")
+
 // 307xx: parameters
 DIAGNOSTIC(30700, Error, outputParameterCannotHaveDefaultValue, "an 'out' or 'inout' parameter cannot have a default-value expression");