4197+5453 ICE with CTFE forward reference

Fixes two ice-on-invalid-code bugs:
4197 ICE(glue.c): error in forward-referenced in/out contract
5453 ICE(statement.c): invalid switch statement forward referenced by CTFE

When CTFE finds that it needs to run semantic on a function, it should ungag
errors, unless the function is part of a speculative template.

src/interpret.c

@@ -25,6 +25,10 @@
 #include "id.h"
 #include "utf.h"
 
+#include "template.h"
+TemplateInstance *isSpeculativeFunction(FuncDeclaration *fd);
+
+
 #define LOG     0
 #define LOGASSIGN 0
 
@@ -150,9 +154,23 @@ Expression *FuncDeclaration::interpret(InterState *istate, Expressions *argument
 
     if (semanticRun < PASSsemantic3 && scope)
     {
+        /* Forward reference - we need to run semantic3 on this function.
+         * If errors are gagged, and it's not part of a speculative
+         * template instance, we need to temporarily ungag errors.
+         */
         int olderrors = global.errors;
+        int oldgag = global.gag;
+        TemplateInstance *spec = isSpeculativeFunction(this);
+        if (global.gag && !spec)
+            global.gag = 0;
         semantic3(scope);
-        if (olderrors != global.errors)      // if errors compiling this function
+        global.gag = oldgag;    // regag errors
+
+        // If it is a speculatively-instantiated template, and errors occur,
+        // we need to mark the template as having errors.
+        if (spec && global.errors != olderrors)
+            spec->errors = global.errors - olderrors;
+        if (olderrors != global.errors) // if errors compiling this function
             return NULL;
     }
     if (semanticRun < PASSsemantic3done)