Panic if asked to generate multi-argument invokes (#1599)

Presently, Java code generation does not support the
`MultiArgumentInputs` field on invokes. When set, this flag instructs
the generation of functions that take multiple independent arguments
instead of a single object containing all arguments. For example,
instead of:

```java
Class.function(FunctionArgs.build().setA(a).setB(b).build());
```

we might generate:

```java
Class.function(a, b);
```

Since it is not supported, the correct behaviour is to panic and abort
code generation, as we do in other languages with this limitation (e.g.
Python). Currently, however, Java does not, proceeding instead to
generate incorrect code. This change fixes that, panicking if
`MultiArgumentInputs` is true. This is technically breaking, but a bug
fix (and in theory, any SDKs we have generated would already be
panicking for Python, so the feeling is that this likely not a highly
used feature).

Fixes #1598

CHANGELOG_PENDING.md

@@ -10,4 +10,7 @@
  - Bump core SDK versions in generated code
  - Emit plugin download URL in default resource options of the generated SDKs
 
-### Bug Fixes
+
+### Bug Fixes
+
+ - Panic when asked to generate SDKs involving multi-argument invokes

pkg/codegen/java/gen_program_expressions.go

@@ -3,6 +3,7 @@
 package java
 
 import (
+	"fmt"
 	"io"
 	"math/big"
 	"strings"
@@ -322,6 +323,12 @@ func (g *generator) GenFunctionCallExpression(w io.Writer, expr *model.FunctionC
 		// Assuming the existence of the following helper method
 		g.Fgenf(w, "computeFileBase64Sha256(%v)", expr.Args[0])
 	case pcl.Invoke:
+		if expr.Signature.MultiArgumentInputs {
+			err := fmt.Errorf("Java program-gen does not implement MultiArgumentInputs for function '%v'",
+				expr.Args[0])
+			panic(err)
+		}
+
 		fullyQualifiedType, funcName := g.functionImportDef(expr.Args[0])
 		if !g.emittedTypeImportSymbols.Has(fullyQualifiedType) {
 			// the fully qualified name isn't emitted